LEcture_Notes

Non-silicates:

–Carbonates     Calcite     CaCO3

                                                              Dolomite CaMg (CO3)2

–Oxides Magnetite     Fe3O4

                                                              Corundum         Al2O3

–Sulfides    Pyrite FeS2

                                                              Galena     PbS

–Sulfates   Gypsum     CaSO4

                                                               Barite     BaSO4

–

Igneous rocks form from crystallizing magma and have a crystalline texture. They are composed of aluminum silicate minerals. These minerals range in composition from ultramafic to felsic in which iron, calcium, and magnesium-rich minerals are mafic minerals and silicon, potassium, and sodium-rich minerals are felsic minerals.

Intrusive rocks (plutonic) form below the earth's surface. Intrusive rocks form under the earth's surface and have large crystals due to slower rate of crystallization. Extrusive rocks (volcanic) form on the earth's surface on land or under water and have small crystals due to rapid crystallization from the faster loss of heat. If the magma cools very fast, a volcanic glass forms. Intrusive and extrusive igneous rocks of the same composition (same minerals) have different names. There are three important intrusive and extrusive rock pairs occurring in the earth's crust: basalt and gabbro forming mafic rocks; andesite and diorite forming intermediate rocks; and rhyolite and granite forming felsic rocks. The viscosity of the magma increases going from mafic to felsic while its temperature of crystallization (rock melting temperature) decreases. Because, the more felsic minerals melt at lower temperatures than mafic minerals, felsic rocks can be derived from mafic rocks by partial melting the mafic rock and then crystallizing the magma. The minerals that form in the crystallized rock will cover a more felsic range (than the parent rock) due to the more felsic composition. The process can be repeated several times to make an even more felsic rock. This is how peridotites are used to generate magmas forming basalt and gabbro at diverging ridges and hot spots, and how basalts and gabbros are used to generate magmas forming andesite, diorite, and granite at subduction zones.

Oceanic crust forms at diverging ridges and over hot spots from magma produced by partial melting of ultramafic rock in the mantle and is composed of gabbro underlying basalt. Continental crust is composed mostly of granite. Diorites and andesites form at subduction zones by partial melting of oceanic crust. The earth's mantle is composed of peridotite which is an ultramafic intrusive igneous rocks.

An intrusive rock forming a tabular layer parallel to the surrounding rock structure is called a sill. If the tabular layer cuts the structure of the surrounding rock, it is a dike. A massive intrusion is a pluton. If the pluton has a large surface exposure (> 100 square kms), it is a batholith. A stock is a plution with a smaller surface exposure.

Lava is magma on the earth's surface, and it can be emitted from a fissure or a vent on a volcano. Flat sheets of basaltic lava can occur forming flood basalts which have columnar jointing on land and pillow structures under water. Volcanic eruptions often contain partly consolidated rock called tephra. Fine-grained tephra is ash which forms the extrusive igneous rock called tuff. Obsidian is a glassy felsic extrusive igneous rock.

Sedimentay rocks form under earth-surface processes and are usually bedded in layers. Detrital (clastic) sedimentary form from the weathered fragments (sediment) of preexisting rocks (breccia, conglomerate, sandstone, siltstones, shale, mudstone) or the broken fragments of calcium carbonate fossils (limestone) and microsiliceous fossils (chert). Note that the weathering process and transportation of sediment tends to destroy mafic minerals relative to felsic minerals. So felsic minerals are common in sedimentary rocks. Detrital rocks become lithified upon deposition, burial, and cementation of the grains together.

Chemical sedimentary rocks usually form as precipitates from water. Evaporites are generally composed of gypsum and halite and form as precipitates from evaporating sea water. Limestones are composed of calcium carbonate minerals (calcite and aragonite) which can be precipitated from sea water or fresh water when the water degasses carbon dioxide to the atmosphere. Limestones also form as cemented shell fragments of calcium carbonate minerals and hence can also be considered a detrital sedimentary rocks. Chert is composed of microcrystalline quartz and forms from lithication of hydrated silica (opal) either as a direct precipitate or as cemented opaline shell fragments. As with limestone, chert composed of shell fragments can be considered a detrital sedimentary rock.

Early in the earth's history, banded iron deposits formed as a direct precipitate of iron minerals. The calcium carbonate in limestones often recrystallizes through the addition of magnesium to form calcium magnesium carbonate rocks that are called dolomite. Petrified wood is a chemical sedimentary rock formed by the replacement of woody tissue with silica. Organic rocks such as lignite and oil shale are also chemical sedimentary rocks, formed from the preservation and modification of organic plant tissue.

Sedimentary rocks typically have structures which can be used to identify the environment of formation. These include fossils, absence of bedding, and special types of bedding such as crossbedding and graded bedding, ripple marks that are symmetrical or nonsymmetrical, and the presence of mudcracks or varves. The sorting of the grains, the composition of the minerals, and the geometry of the sedimentary rock deposit can also indicate the environment of formation.

Metamorphic rocks form by recrystallization of preexisting rocks as the result of temperature and pressure. The rocks recrystallize because they are exposed to a different environment from that in which they formed. The recrystallization requires a fluid and components in the fluid or associated gas phase may change the bulk composiiton of the rock. Directed pressure on the rock will cause a preferential growth of crystals during recrystallization called foliation, giving the rock a layered appearance (like a sedimentary rock). The maximum temperature of metamorphism is the melting temperature of the rock.

Typically, around a cooling pluton the rocks are baked (contact metamorphism) causing rocks without foliation (marbles, quartzites, skarns, hornfels). Cooling plutions are common in subduction zones and associated with hot spots in continental areas. Directed pressure metamorphism (with temperature) is also characteristic of converging plate boundaries such as subduction zones. These broad areas of convergence produce regional metamorphism, causing foliated metamorphic rocks to form (slates, phyllites, schists, and gneisses). For a rock of a particular bulk composition, the minerals that form can often be correlated with the temperature and pressure of metamorphism. This is known as the metamorphic grade and is used to tell the temperature and pressure that the rock was metamorphosed at. The mineral assemblage of a particular metamorphic grade is called the metamorphic facies.

Other types of metamorphism include burial metamorphism of sedimentary rocks in which they are lithified and recrystallized as they are gradually buried. This is a low-grade metamorphism that occurs with deltaic sediments and they are usually nonfoliated. For example, this produces the transformation of lignite to coal.

Fossils are preserved remains or tangible signs of ancient life. They are found in sedimentary rocks.

Indirect evidence of hard parts can also form from permineralization, molds and casts.

Soft parts can be preserved in amber and in anoxic environment as fossil fuel, coal, and peat.

Indirect evidence of soft parts can also form from impressions and carbonization

Life has the capacity for self-replication (reproduction) and self-regulation (use raw materials to sustain chemical reactions).

Taxonomy is the classification of living organisms. Organisms are divided into kingdoms. Monera contains primitive bacteria (Archaeobacteria) and cyanobacteria (Eubacteria). Bacteria cells are called prokaryotes and lack a nucleus and other internal cell structures such as mitochondria and chloroplasts. Nonbacteria cells are called eukaryotes which contain internal cell structures. Protoctista (also called Protista) contains algae which are simple single and multi-celled plant-like organisms and simple single-celled animal-like organisms called protozoans. Phytoplankton (diatoms and nannobacteria) and zooplankton (foraminifera and radiolarian) are in this kingdom. The more complex kingdoms include the animal-like Fungi, the animals in Animalia, and the plants in Plantae.

Phylogeny is the evolutionary history of a species. Species in Fungi, Animalia, and Plantae represent an evolutionary process that begin with Monera, passed through Protista before evolving into more complex species.

The kingdoms are subdivided into phylum, class, order, family, genus, species

kingdom, Animalia; phylum, Chordata; class, Mammalia; order, Primates; family, Hominidae; genus, Homo; species, Homo sapiens

A cluster of species that share an evolutionary ancestry is called a CLADE. Within a group of related species, some traits are primitive and some are derived and hence are not shared by all species. The common traits imply a common evolutionary origin. A cladogram shows the progression of clades sharing fewer common traits as evolution proceeds.

This is an overview covering important families of organisms, from page 70 to 89 of the text. The groups are listed below and you should learn how they are related by evolution.

Archaeobacteria - includes anaerobic bacteria which formerly lived on the earth's surface when there was little or no oxygen in the atmosphere.

Eubacteria - plant-like cyanobacteria (blue green algae) and animal-like bacteria responsible for decay of dead organisms. Cyanobacteria are photosynthetic and began the buildup of the earth's oxygen content in the atmosphere. They produce stromatolites which are boulder-shaped rocks composed of layers of algae mats covered with sediment.

Protists evolved in the Proterozoic Eon. The earliest non-bacterial fossils are of acritarchs, thought to be single-celled phytoplankton, possibly a type of dinoflagellate.

Protozoans - single-celled animal-like. Protozoans are thought to have evolved from an animal-like bacteria engulfing another (smaller) animal-like bacteria which survived to become a mitochondria in the cell. Need to know important zooplankton: radiolaria (have silica shells which produce siliceous oozes) and foraminifera (have calcium carbonate shells which produce calcareous oozes).

Algae - Thought to have evolved from a protozoan engulfing a cyanoacteria which survived to become a chloroplast within the cell. They are single-celled and simple multi-cellular plant-like organisms that have fertilization external to the plants in water. Fertilization in higher plants occurs within the parent plant. Learn important phytoplankton: diatoms (have silica shells) and nannoplankton (have calcium carbonate shells) and dinoflagellates (have chitin shells).

Spore-bearing plants> require moisture to reproduce. These include moses (primitive nonvascular plants) and spore-bearing ferns (vascular plants).They evolved in the Ordovician Period

Gymnosperms - naked seed-bearing plants, e.g., conifers, which evolved from spore-bearing plants in the Devonian Period.

Angiosperms - flowering plants, e.g., oaks, which evolved from gymnosperms in the Cretaceous Period. These are the dominant plants today.

Multicelled animals are composed of organs of tissue and evolved from Protozoans

Unless otherwise noted, the major invertebrate groups first appeared in primitive forms in the Cambrian Period. Skeletons first appeared at the beginning of this Period.

They are suspension feeders with radial symmetry. Examples include floating jellyfish and sedentary corals (tabulates, horn corals, hexacorals) which form calcium carbonate reefs in warm, shallow waters.

Sponges are suspension feeders which often contribute calcium carbonate skeltons to coral reefs, e.g., stromatoporoids.

These evolved in the Proterozoic Eon and are burrowing animals (worms) with a well-developed digestive tract

Arthropods evolved in Proterozoic Eon and are segmented, joint-legged animals with a hard exoskelton. (Although the exoskelton was lacking in the Proterozoic Eon.) Arthropods include the trilobites, insects, crustaceans, spiders and scorpions.

These are bivalves that lack mirror symmetry and include articulate and inarticulate brachiopods.

Evolved in the Ordovician Period: moss-looking corals which form colonies which help form reefs of calcium carbonate, e.g.,lacy bryozoan.

This group includes bivalve molluscs (clams), gastropods (snails), cephalopods (nautiloids, ammonoids, belemnoids, squids, octupus)

They have radial symmetry. Examples include starfish, crinoids, (sea lilies), sand dollars, and sea urchins.

Have a vertebrate column with a spinal cord. Cambrian conodonts were the earliest vertebrates. The following classes appeared in order of evolution - jawless fish, jawed fish, sharks and bony fish, amphibians, reptiles (including therapsids and thecodonts), dinosaurs, pterosaurs, mammals and birds. Jawless fish evolved in the Cambrian Period, and sharks and bony fish evolved from jawed fish in the Devonian Period. Amphibians evolved from bony fish in the late Devonian Period, and reptiles evolved from amphibians in the Pennsylvanian Period. Therapsids and thecodonts were reptile groups that evolved in the Permian and Triassic Periods, respectively. Dinosaurs and pterosaurs evolved from thecodonts in the Triassic Period, and birds evolved from dinosaurs in the Jurassic Period. Mammals evolved from the therapsids in the Triassic Period.

Ecology is the study of factors that govern the distribution and abundance of organisms in natural environments.

Habitats are the different environments close to the surface, and an ecologic community - is the populations of groups of species inhabitating a single habitat

At present land covers about 30% of the earth's surface. This is a larger percentage than has normally occurred during the earth' history, so sea level is lower than normal.

ecological niche - defined by the way that a species relates to its environment in terms of nutrients and the physical and chemical conditions that can be tolerated.

physical and chemical conditions - e.g., amount of moisture tolerated, or salinity of water tolerated, or range of temperature.

producers or autotrophs - organisms undergoing photosynthesis, e.g., plants, some bacteria, and some Protoctista

compounds produced by other organisms, e.g., animals, some bacteria, some Protoctista and some Fungi. Animals include carnivores, herbivores, omnivores.

Bacteria are at the bottom of a food chain and food web; however, they also feed on all levels of the chain and web including the top.

ineffective predation - increases diversity by making room for opportunistic species which otherwise couldn't compete.

Biogeography is the distribution and abundance of organisms on a broad geographic scale.

chemical composition: nitrogen (78%); oxygen (21%); carbon dioxide (0.03% and rising with industrial emissions); water vapor (variable but highest in warm air and lowest in cold air)

respiration by organisms, decay, combustion, oxidation (reverse of photosynthesis)

Coriolis effect - controls prevailing wind directions, produced by the earth's rotation which has a decreasing rotational velocity approaching the poles.

rain shadow - west side of mountains between 0^o to 30^o and east side of mountains between 30^o and 60^o.

tropical - near the equator, producing rain forests (jungles) due to high moisture content caused by precipitation from rising air.

deserts - near 30^o latitude where sinking air occurs, in rain shadows, on coasts near cold water currents, and far from oceans.

temperate forests - (primarily deciduous, i.e., losing leaves in winter), woodlands

evergreen coniferous forests - high latitude woodlands. Evergreen trees can undergo photosynthesis year-round so they can survive more easily than deciduous trees.

waxy leaves - warm climates, helps retain moisture in deserts and along coast.

reptiles - prefer warm climates because they can't regulate their body temperature.

clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

equatorial countercurrents - return arms of equatorial currents, pushed by westerlies to the east.

west-wind drifts - currents near 60^o in southern hemisphere which are pushed to the east by the westerlies,

Deepwater Circulation - water density increases, causing sinking, with decreasing temperature and/or increasing salinity. Cold North Atlantic Deep Water (NADW) forms by down welling between Iceland and Europe where the Atlantic Ocean is fairly shallow. The depth of the ocean bottom increases from the North Atlantic to the South Atlantic to the Indian Ocean to the Pacific Ocean. The NADW water flows south by gravity to Antarctica and then east through the Indian Ocean and into the Pacific Ocean. Upwelling occurs along the way along coasts where the surface currents are pushing water away from the coast. Mediterranean Deep Water, concentrated by evaporation, flows out into the Atlantic but is not as dense as NADW so it doesn't sink as deep. Antarctic Deep Water (ADW) down wells near Antarctic below the NADW and spreads away from Antarctica.

circular particle movement - except near coast where shallow bottom interferes with movement, causing breakers as movement becomes elliptical and top of wave moves faster than bottom of wave.

wave refraction along coasts - due to friction with wave bottom, causing the deep water wavefront to move faster than the shallow water wavefront. Wave refraction produces near parallel alignment of the wavefront with the beach.

longshore drift - sediment movement caused by longshore currents due to the wave fronts not being parallel to the shore. The longshore drift in Louisiana is to the west.

effect of moon - gravitational pull creates high tides on earth's surface nearest and furthest away from moon.

Note an epicontinental sea is a shallow sea formed by flooding a continent, e.g., Hudson Bay, Chesapeake Bay.

Pelagic life (plankton and nekton which live above the sea floor) and benthic life (live on the sea floor)

Benthic life (on the sea floor) consists of organisms that live on the sea floor, bore into rock, or burrow through soft sediment.

bacteria - heterotrophs that release nutrients which are returned to the photic zone by upwelling, etc, and

autotrophs such as cyanobacteria (stromatolites and algal mats. Note that not all bacteria are heterotrophs.

algae, e.g., within corals (dinoflagellate) and growing on surface of sea floor in photic zone.

suspension feeders, e.g., some mollusks, sponges, sea lilies, benthic foraminifera, corals

stable oxygen isotopes in shells in which the ratio of ¹⁸O/¹⁶O decreases with increasing temperature.

sea water is normally 35 ppt but ranges from 30 to 40 ppt - high species diversity because osmosis works best in salt water in removing wastes from organisms.

hypersaline waters > 40 ppt, brackish water is between 5 and 30 ppt, fresh water is < 5 ppt, brines > 100 ppt - all with low species diversity.

Name an animal fossil that indicates a warm paleoclimate on land. How are plant leaf fossils used to indicate if the paleoclimate temperature was warm?

Which way does the wind veer (due to the Coriolis effect) in the norther hemisphere? Why do deserts occur on the east side of the Cascades in Washington and Oregon?

Paleogeography - Determining the geography of the past from the rock record. The sedimentary rock record provides clues as to the original sedimentary environment of deposition.

caliche and calcrete - layer of CaCO₃ in B horizon occurring in arid soils.

laterite - soil enriched in iron and aluminum oxides in A horizon of a tropical soil.

bauxite - soil enriched in residual aluminum oxides in heavily-weathered soils

Ancient soils are located on unconformities (erosion surfaces). They are recognized by the weathering alteration of rocks (discolored and reddish), presence of caliche, sediment-filled root tubes, preserved animal burrows, etc. Weathering requires oxygen which became available in the Precambrian time in the atmosphere. Forests developed in the Devonian, allowing the accumulation of humus.

Lake sediments are often preserved because of initial deposition in low-lying areas where erosion is unlikely. Sediment grades coarse to fine towards the center of the lake. Sediment contain fresh-water fossils and fewer burrows than found in marine environments. Lake deposits are often varved, with two layers representing an annual deposit (thick light layer in the summer and a thin dark layer in the winter). The thin layer of winter deposits can result from freezing of the lake surface so that only the fine sediment in suspension is deposited. Large lakes can have delta type deposits where streams enter the lake. Lake sediments are less common than marine sediments.

Glaciers leave temporary features in valleys because subsequent erosion by streams will transform the U-shaped valleys into V-shaped valleys, removing the hanging valleys and rounding the cirques and aretes. Continental glaciers (e.g., in Greenland and Antarctica today) leave more lasting features. Continental glaciers tend to round and smooth surfaces by completely over-riding mountains.

Both continental and valley glaciers move by gravity toward the oceans. Global ice ages have occurred in the Pleistocene and Pliocene (3 my to 10,000 years ago), Permian, Ordovician, Devonian and Precambrian. The last Ice Age ended only 10,000 years ago, and we are presently in an interglacial time. Ice Ages are unusual because the earth's surface temperature has usually been warmer than it is today. Sea level has usually been higher.

Deserts occur where air masses are descending, in rain shadows of mountains, far from the oceans, and near cold ocean coasts. Deserts usually have interior drainage with the associated playa lakes. Flash floods produce alluvial fans at the mouth of canyons and result in braided streams. Salt deposits are common as the playa lakes dry up. Sand dunes occur with large crossbeds, and the sand particles have frosted surfaces. Wind erosion by sand storms produce ventifacts (faceted boulders). Removal of fine particles on desert surface will result in larger grains of gravel and cobbles (desert pavement) forming the surface. These are frequently stained black with manganese oxides (desert varnish).

River environments include the floodplain of a river with its associated delta or alluvial fan. Floodplain deposits include the stream bars within or on the edge of the channel, the natural levee deposits along the channel and the backswamp deposits away from the channel. Because the channel moves by lateral erosion, the river is constantly removing some of the floodplain deposit and depositing new sediment. The river itself can be a meandering or braided stream. Tributaries usually feed a river until it approaches a delta or alluvial fan, where the main channel splits up into distributaries. Between each distributary, in a delta, are natural levee and backswamp deposits, making the delta sequence very complex. Delta environments are usually considered to be shallow marine or lacustrine.

Meandering streams are much more common than braided streams. A meandering stream channel migrates by eroding natural levee and backswamp deposits and replacing these deposits with point bars. These point bar deposits of sand are then covered with natural levee and backswamp deposits as the stream channel continues to shift position. The result is a vertical coarse to fine sequence of sandbar deposits at the bottom grading up to natural levee, grading up to backswamp deposits at the top. This sequence is shown in cores of sediment taken in floodplains.

The surface of the floodplain will be elevated by deposition if sea level rises, e.g., by melting of ice at the end of an Ice Age, or the river valley is subsiding. What happens is that either a rise in sea level or a drop in the surface level of the valley will reduce the stream gradient by lowering the vertical drop to sea level (base level of erosion). A lower stream gradient reduces the ability of the stream to erode downward. The stream will deposit a new vertical sequence of coarse sediment as it migrates by lateral erosion over the floodplain, raising the surface of the floodplain. The result can be several vertical sequences of a coarse to fine cycle in cores taken in a large floodplain such as the lower Mississippi River valley which is undergoing continued subsidence.

A drop in sea level or vertical uplift of the river valley, increases the ability of a stream to erode its channel, resulting in lowering the surface of the floodplain. The stream erodes deep into its old floodplain deposits, removing some of the earlier coarse to fine vertical sequences and establishing a lower floodplain surface. Parts of the old floodplain surface which were not eroded (on the edge of the river valley) stand out as terraces, above the new floodplain surface, e.g., the LSU campus at Baton Rouge sits on a Pleistocene terrace.

Important Concept - Walther's Law - When depositional environments migrate laterally, sediments of one environment come to lie on top of sediments of an adjacent environment.

The typical delta grades from sandy (crossbedded) deposits nearshore (the horizontal topset beds making up the delta plain) to silts and clays deposited just offshore (the inclined foreset beds, containing both wood debris and marine fossils, forming the delta front) to clay deposits located further offshore (the gently dipping bottomset beds making the prodelta). The general horizontal gradation is coarse to fine. If the delta builds seaward (called prograding), the coarse sediment is deposited over finer sediment, resulting in a vertical upward sequence of fine to coarse. If the delta moves landward, the vertical upward sequence will be coarse to fine.

During times of static sea level, the active delta will eventually sink due to isostatic movement of the crust. When a new delta builds out over the old delta, a new packet of sediment, grading from fine to coarse, is deposited over the old delta. These packets of fine to coarse are the opposite of the coarse to fine vertical gradation of meandering rivers, developed over sinking flood valleys.

A delta will build seaward if sealevel drops (regressive sea) or landward if sealevel rises (transgressive sea). Thus, the vertical gradation of sediment size in the rock record can be an indication of changes in sea level. This assumes that both the land surface elevation and the sediment supply to the delta were not varying sufficiently to cause the delta to move.

The Mississippi delta tends to build outward until it is so far from the coast that it will shift its channel, to raise the gradient (vertical drop/horizontal distance) to the sea. It then repeats the process by building outward a new delta at the mouth of the new channel. The different deltas form the lobes of the entire delta system. The Mississippi used to switch channels every 600 years, and it last switched channels 600 years ago. The Core of Engineers is trying to prevent it from switching to the Atchafalaya channel.

Deltas in seas of low wave activity are river dominated; whereas, in high wave activity, they are wave dominated. Rivers typically split up into distributaries within the active delta lobe. The Mississippi delta is river dominated. The Nile and Niger deltas arewave dominated in which high wave energy prevents a lobe from sticking out into the sea.

Padre Island and Galveston Island are examples that occur off the Texas Coast in which the lagoon is the waterway between the barrier island and the mainland.

Barrier islands are sandbars of well-sorted sands, oriented parallel to the coast, e.g., Galveston Island. Barrier islands, like beaches, typically have sand dunes with large crossbeds. The islands origins are related to longshore currents and perhaps to a drop in sealevel, to initiate the building of the sandbar. If they lie close to the coast, they are separated from the coast by a low-energy environment called a lagoon which collects fine-grained sediments and may sometimes have preserved mud-cracks. Sometimes, the waters of the lagoon become hypersaline due to evaporations and limited access to the sea, e.g., Laguna Madre behind Padre Island off southwest Texas.

The inlets between barrier islands are called tidal inlets, sediment dropped by tidal currents as they move into the lagoon are called tidal deltas. The tidal inlets and their tidal channels in the tidal delta tend to migrate, just as river channels do, producing a coarse to fine vertical upwards sequence in the tidal delta (due to erosion of previously-deposited sediments and their replacement with coarse sediments in the new channel while depositing finer-grained sediments over the coarse grain sediments marking the previous channel).

The crossbeds in tidal deltas indicate currents moving in two opposite directions, as water flows in and out of the tidal inlets. The tidal flats are deposits of sand and silty sand, typically forming on the landward shores of lagoons. Marshes build up on the landward side of beaches bordering the tidal flats. Their deposits include peat and coal. The marshes are in the upper part of the intertidal zone and the tidal flats are in the lower part of the intertidal zone.

If the barrier island-lagoon complex moves seaward (progrades), than the sands will be deposited over the fine-grained marine deposits on the shelf, resulting in a vertical upward sequence of fine-to-coarse from marine clays to sands. Fine-grained lagoonal muds will cover the barrier-island sands. These lagoonal muds will then be covered by coarser tidal-flat and beach deposits. The complete upward vertical sequence will be fine-grained marine deposits to barrier-island sands to lagoonal muds to tidal-flat sand and silts, i.e., two fine-to-coarse sequences.

The diversity of marine species is far greater than that of the barrier island-lagoon complex. Thus, the vertical upward fossil sequence for a prograding complex will be from abundant to less abundant. In addition, the lower "marine" unit will show evidence of burrowing which will be largely absent in the wave-ridden cross-bedded beach deposits. Plant roots are typically preserved in the beach deposits which may show cross-bedding if deposited as dunes.

Modern coral reefs are porous limestone (calcium carbonate rocks composed of calcite and aragonite, CaCO₃ formed from the animal organisms, "hexacorals." The coral organism lives in warm, shallow waters and contains symbiotic dinoflagellates (plant organisms) in their tissues which remove CO₂ during photosynthesis. The removal of CO₂ helps the coral polyps precipitate calcite and aragonite. The surface of the coral is often covered by coralline algae which helps cement the reef together. The voids in the reef are filled in with carbonate sediment. The reefs are largely unbedded, when preserved as rocks.

Parts of a reef moving form the shore to offshore - Note the lateral zonation in types of corals.

barrier reefs - separate from coastlines as land sinks or by seaward growth of fringing reef.

Carbonate platforms are shallow-water limestones composed of ancient reefs and calcium-carbonate sediment. The sediment includes shells and skeletons of organisms (such as calcareous algae), algae mats, fecal pellets of deposit-feeding organisms, and the direct precipitate from sea water of aragonite and calcite as ooids in areas where cold water upwells to the surface. The ooids often form submarine dunes with crossbeds in areas where wave activity is strong. Ooids are spherical pellets with concentric layering. The algae mats trap sediment, forming layered deposits, as a new algae mat grows on top of an earlier mat. Algae mats grow in supratidal settings where burrowing and grazing marine animals do not totally destroy them. These supratidal zones often have mud cracks. Stromatolites grow today in subtidal zones where high water flow prevents grazers from destroying them.

The intertidal and supratidal areas along the leeward coast of the carbonate platform often occur together in one zone. A high "supratidal" beach ridge marks the seaward end of this zone which grades offshore into the subtidal zone, containing subtidal sediments. Cutting the beach ridge are tidal channels which are bordered by "supratidal" levees and separated by "intertidal" flats. Landward of this zone are "supratidal" marshes.

stromatolites (cyanobacteria or blue-green algae), concentric layered deposit forming in tidal channels

bird's eye limestone (algae mats with worm holes) horizontal layered deposit forming on the backslope of supratidal ridges, usually form in supratidal or intertidal zones, not in subtidal zone.

A sabka is a supratidal area with hypersaline waters due to evaporation and limited access to the sea. Typically, gypsum, dolomite, and calcite are deposited with halite (under conditions of extreme evaporation). The sabka deposits may overlie algae mat (intertidal) deposits which overlie more seaward (subtidal) deposits if the supratidal zone is prograding seaward.

Examples are the submarine movements of sediment off the New Jersey coast that cut telephone cables.

Turbidity currents moving down the continental slope deposit turbidites on the continental rise and on the sea floor as abyssal fans. These currents have eroded the submarine canyons that cut the continental slope and shelf. Turbidites have graded bedding of coarse to fine upwards in a thin vertical sequence of a few centimeters. Similarly-graded beds, deposited by meandering rivers in their flood plains are much thicker. Sole marks are "molds" along the underside of a turbidite sequence. The molds fill casts of depressions scoured by the turbidity current in the surface of the underlying bed.

Sediment accumulation on the abyssal ocean floor is slow, about 1 mm/1000 yrs. The sediment source of pelagic clays are weathering of oceanic volcanics and wind deposits of terrigenous (continental) dust. Calcium carbonate (CaCO₃) and opaline silica (SiO₂.nH₂O) deposits occur as the shells of pelagic planktonic organisms which are abundant in areas of upwelling water. The calcareous (calcium carbonate) shells are from planktonic foraminifera and calcareous nannoplankton (warm surface waters). The term globigerina ooze refers to calcareous shells of a planktonic foraminifera species. The siliceous (opaline silica) sediments are from diatoms (cold water) and radiolaria. Siliceous sediments (called diatomaceous earth) recrystallize to chert (microcrystalline quartz) layers and calcareous sediments recrystallize to limestone. Note that diatoms did not evolve until Cretaceous (late Mesozoic time).

Calcareous shells tend to dissolve in the water column if they fall below depths of about 4000 meters. This is because the surface ocean is supersaturated to calcium carbonate but becomes undersaturated below the carbonate compensation depth. The undersaturation results from lowering temperature and increasing pressure which occurs with increasing depth. If the shells reach the bottom without being dissolved, they will be preserved in the sediment even if that section of the ocean floor later moves to depths far below the carbonate compensation depth as a result of plate tectonics.

Stratigraphic Section - exposed vertical sequence of rock in outcrop or through cores, etc.

lithostratigraphic units (based on lithology) not a constant time unit in which a type section is used to describe a particular unit, e.g., supergroups: groups: formations: members

The time-stratigraphic or chronostratigraphic units (erathem, system, series, and stage) correspond to the geologic time units of era, period, epoch, and age. These time-stratigraphic or chronostratigraphic units correspond closely to the biostratigraphic units. Remember that the time-stratigraphic units were originally defined on the basis of global unconformities, global mass extinctions, etc, which took place over short time intervals. Each system, corresponding to a period, was originally represented by a single type section. At present, a boundary stratotype is being defined for each system to correspond to the "type stratigraphic" boundary between two systems.

Biostratigraphic units are based on fossilsand are approximately constant time units called zones

Index Fossils - easily distinguishable; widespread geographically so that they can be used to correlate rocks over a wide area; occur in a wide range of sedimentary rocks; restricted time interval of occurrence.

zone - segment of stratigraphic record, characterized by one or more species of index fossils. The time segment covered by a zone is approximately constant worldwide. Named for a species or if more than one species is present, for a genus. A zone can be subdivided into subzones if more than one species is present.

An example is the use of graptolite species (colonial hemichordates) to establish zones in the Ordovician and Silurian Periods.

Zones are commonly used to correlate the age of rocks between different geographic locations.

radioactive decay - release of energy through the loss of mass occurring with the disintegration of an atom.

isotopes - atomic nuclei of the same element having the same atomic number but different numbers of neutrons.

daughter isotopes - stable or unstable isotopes produced by radioactive decay.

alpha emission: 2 protons and 2 neutrons; produces -4 change in atomic mass and -2 change in atomic number.

beta emission: an electron emitted from nucleus due to the breakup of a neutron into a proton and an electron; produces 0 change in atomic mass and +1 change in atomic number.

beta capture: an electron captured from the electron cloud surrounding the nucleus in which a proton combines with the electron to form a neutron; produces 0 change in atomic mass and -1 change in atomic number.

of atoms present. ln(C/C^o) = -t_1/2y where C/C^o = 0.5, y is the decay constant and t_1/2 is the half life.

Fossils ages are determined through radioactive dating of the rocks that the fossils are found in. Fossils can be used to date rocks, once their time interval of existence has been established through radioactive dating. Because it is easier to date rocks with fossils than by radioactive dating, most rock ages are determined through the presence of fossils. But note that fossil occurrences cannot provide more accurate age dates than radioactive dating.

Beds laid down rapidly, such as volcanic ash deposits or evaporites, are of the same age over a wide geographic extent. A marker or key bed is one that can be traced over a wide geographic extent. Note that most marker beds are not truly the same age everywhere, e.g., a sandstone marking a beach deposit or till from a glacial time period. However, the range in time may be insignificant in terms of the geologic age.

sedimentary facies - set of characteristics of a rock body representing a local environment of deposition.

facies change - lateral change in characteristics of a rock body produced by a change in the local environment of deposition.

regressive sea or prograding shoreline will shift nonmarine facies away from the shoreline.

transgressive sea or retreating shoreline will shift marine facies towards the shoreline.

Global changes in sea level (eustatic changes) are recognized by global unconformities (disconformities) on continental shelves. The position of the ancient shoreline is revealed by the sedimentary facies change between nonmarine and marine deposits. A rise in sea level can be indicated by a lateral shift in this facies change towards the land. A drop in sea level can be shown by a lateral shift in this facies position away from the land. A rapid change in sea level, will shift the shoreline without leaving any residual shoreline deposits between the "old" and "new" locations. If the rapid change is due to a drop in sea level, there will be an unconformity between the "old" and "new" shoreline positions. A local change in sea level may also result from an area rising or subsiding due to "tectonic" changes. An increase or decrease in sediment supply can also cause a prograding or retreating shoreline, respectively.

Seismic data can be used to determine stratigraphic sections without drilling. Hence the data can follow facies changes in a two-dimensional (horizontal and vertical) cross-section of sedimentary rocks. Eustatic changes in sea level have often been determined from seismic profiles.

normal changes in sea level 35 ft/m.y.; whereas changes in sea level due to ice ages 330 ft/ several t.y.

The earth's magnetic field shows two changes with geologic time. First, the declination (difference between the magnetic poles and the geographic poles) shifts slowly (on the order of a few thousand years to make a circuit) as the magnetic poles rotate around the geographic poles. Second, on the order of every half million years, the polarity of the magnetic poles appears to rapidly switch so that the north and south magnetic poles reverse. The reason for this switch is the subject of debate.

The polarity reversal of the magnetic poles can be determined from studying the residual magnetism of rocks, primarily igneous rocks containing iron. The igneous rocks have the earth's magnetic field orientation frozen in as residual magnetism when they cool below about 500^oC (their Curie points). This residual magnetism can be determine from an igneous rock and the rock's age can be determined by radioactive (radiometric) dating. By doing this for rocks from stacked lava flows, the polarity of the earth's magnetic field as a function of geologic time has been determined.

The polarity of the residual magnetism is said to be "normal" if the orientation is in the same general direction as today earth's magnetic field and "reversed" if the orientation is in the opposite direction. Note that the movements of plates (through plate tectonics) around the earth's surface may have changed the apparent orientation of the earth's magnetic field from the time the rock was formed. Plate tectonics makes determining the polarity more difficult in rocks older than Mesozoic. We generally know the movement of the continents in the Mesozoic and Cenozoic, so the change in orientation from plate movement can be subtracted out.

The magnetic reversals are used to date the sea floor. The oldest sea floor is Mesozoic in age. If an area of the sea floor has a residual magnetism parallel to the present earth's magnetic field, a positive (stronger) magnetic field is measured for the earth directly over that sea floor and vice versa. A stronger magnetic field is called a positive anomaly; whereas, a weaker magnetic field is called a negative anomaly.

The symmetrical magnetic anomaly pattern of positive and negative anomalies, relative to the mid-oceanic ridges, can be related to the orientation of the earth's magnetic field at the time the sea floor was created. The basaltic sea floor originally formed at the ridge, picking up a residual magnetism corresponding to the orientation of the earth's magnetic field. The sea floor then moved away from the ridge as new sea floor was created. This movement away from the ridge has created the present symmetrical pattern of magnetic anomalies. Since the ages of changes in magnetic polarity are known from radiometric dating on land, the magnetic anomaly pattern can be used to date the sea floor.

We can determine, from the residual magnetism of igneous rocks, the location of the two poles of the earth's magnetic field at the time of formation of the rock. The movements of the plates have produced an apparent movement in the positions of the magnetic poles. We know the poles haven't actually moved because the apparent positions of the magnetic poles are different for rocks of the same age that are located on different plates. The only explanation is that the plates are moving independent of each other.

Strontium has two stable isotopes which occur in calcium carbonate shells (CaCO₃) through the substitution of strontium for calcium in the mineral lattice. The ratio of these two isotopes in marine shells (⁸⁷Sr/⁸⁶Sr) ratio) is the same as the ratio in the water in which the shells were grown. The ratio in marine shells has varied systematically throughout geologic time, with a general increase with younger rocks. One of the reasons that the ratio has varied in sea water is due to the gradual increase in strontium 87 formed by the radioactive disintegration of rubidium 87 in rocks. Please note that in the textbook, the author has written (⁸⁶Sr/⁸⁷Sr instead of ⁸⁷Sr/⁸⁶Sr on his figures which is an error.

The variation of the strontium ratio with time in sea water has been determined by measuring the ratio in shells and determining the age of the rocks (radiometric dating) containing the shells. Now that the variation with age is known, the age of marine rocks can often be determined by measuring the strontium ratio in shells within the rocks.

If a seashell has lost 7/8ths of its ¹⁴C and the half life of ¹⁴C is 5700 years, how old is the seashell? If the shell is buried to a depth of 10 cm in sediment, what is the depositional rate?

How can we explain rocks of the same age on different continents having residual magentism that predicts a different location of the magnetic poles?

Organic evolution refers to changes in populations of the same species. A basic restraint on evolution is the body structure of what is already present in a population, i.e., because evolution remodels rather than starts with a new design.

During evolution, a species as a whole can evolve in the course of time or can rapidly give rise to additional species. Natural selection is used to describe the gradual transformation of a species through transferral of selected traits through breeding from survival of the fittest. Speciation is used to describe the rapid development of new species from another species. Often, the new species is geographically isolated from the remainder of the parent species, thereby lacking the full genetic variability in the general population. Also, often no competitors exist in the environment.

Adaptations refer to specialized features of plants and animals that allow them to perform functions, e.g., the development of seeds allowed gymnosperms to colonize dry land.

The process of evolution involves changes in the gene pairs which are responsible for hereditary factors. These changes can involve

of sex cells when dividing or or exposed to chemicals and/or radiation which are subsequently preserved through breeding or by

through breeding. Each gene is a chemical segment in a DNA (deoxyribonucleic acid) strand within a chromosome in the nucleus of a cell. The chromosomes exist in pairs, half supplied by each parent during breeding.

DNA has the structure of a twisted rope ladder (double helix) in which the sides of the ladder are composed of alternating sugars (S) and phosphates (P). The steps are composed of pairs of nucleotide bases: either adenine (A) and thymine (T) or guanine (G) and cytosine (C). Genetic point mutations of DNA can occur during cell division or by an external source such as radiation or chemical exposure. These mutations are passed on to offspring through breeding in which each individual of a pair contributes half the chromosomes by way of a gamete (e.g., egg and sperm). If the mutation is favorable, the individual is more likely to survive and continue to pass on his or her genes. In this way species evolve or change with time. Genetic point mutations probably result in speciation, a quicker process than natural selection.

Species may also evolve, in the absence of genetic mutation, through a new gene combination in breeding. This is a long-term process which is called natural selection. If the combination is favorable, the individual is more likely to survive and continue to pass on his or her genes, making it more likely for the gene combination to reoccur in related offspring.

homology - The development of different functions for organs in different species which shared this organ in a common ancestor, e.g., bat wings, human hands, and whale flippers all have the same basic five-finger structure. This was part of the basic evidence for evolution cited by Darwin.

vestigial organs - organs developed in ancestral species and retained through evolution but not serving a purpose, e.g., the pelvic bones of whales. Again, this is part of Darwin's evidence for evolution.

mass (rapid) extinction - large numbers of species become extinct every 26 m.y. and this appear to be associated with asteroid impacts on the earth. The asteroid events deposits trace metals (e.g., iridium) in sediments which have been identified in the geologic record. These trace metals were within the asteroid which was vaporized upon impact. The vaporization process ejected trace metals into the atmosphere to spread arourd the earth and then slowly settle out onto the earth's surface.

pseudoextinction - evolution into a new species, suggesting that the ancestors have become extinct.

adaptive radiation - rapid expansion of a group into many new species as the result of a friendly environment, e.g., other competing groups have become extinct or introduction into a new environment lacking competitors, e.g., the adaptive radiation of mammals in the Cenozoic following the extinction of the dinosaurs at the end of the Mesozoic. This is part of speciation.

adaptive breakthroughs - development of key features that along with ecologic opportunities allow adaptive radiation to take place. In corals, the development of a porous skeleton allowed more rapid growth and the development of the symbiotic relationship with dinoflagellates allowed for removal of their produced carbon dioxide as well as a convenient food supply.

extinction rates vary between different groups. Mammals have a survival rate of 1 to 2 m.y. The average extinction rate for all groups is about 3 m.y. Human species have been here for 200,000 years.

evolutionary convergence - adaptive radiation evolution of two different taxonomic groups, e.g., marsupials and placentals to produce similar-looking species to occupy similar environments, e.g., Tasmanian wolf and the wolf.

iterative convergence - evolution of a taxonomic group producing similar-looking species at different time periods; however, they are not identical because evolution does not repeat itself.

The history of evolution of species is called phylogeny. The sequence of development of an individual from its origin by fertilization of an egg to its death is called ontogeny. Frequently, phylogeny is preserved in ontogeny in the different stages going from embryonic to adult, leading to the saying "Ontogeny recapitulates phylogeny." The preservation shows up in common embryonic stages between species having a common ancestor group. This evidence for evolution was cited by Darwin.

Cope's rule - Organisms increase in size through evolution (now somewhat discredited)

One common evolutionary trend is to eliminate the adult form or the latest stages of development or by retaining some of the juvenile or embryonic features. This involves the transfer of sexual maturity to an earlier juvenile stage and the arrestment of development at that stage. Amphibians that have an aquatic juvenile stage may remain in that stage, never progressing to four-legged terrestrial adults. The axolotl evolved from the juvenile stage of a salamander, through the loss of thyroxine from the thyroid gland, which is needed for transformation to an adult salamander. The domestic dog has lost the adult form found in wolves. Dogs like wolf pups bark, but adult wolves do not. Humans have lost the adult ape form through evolution. We lack the body hair and pointed faces of adult apes, retaining the juvenile, lesser hair, and flat face of young apes.

Most evolution probably involves speciation, evolving rapidly from existing species, rather than by natural selection, the slow transformation of an existing species through breeding.

The evolutionary trend may be controlled by species selection in which the significance of a species in evolution is related to its length of duration and its rate of producing descendant species. If a species has a long length of duration and produces a lot of descendant species than more of its traits are likely to be passed on.

Dollo's Law - Evolution doesn't reverse itself to perfectly duplicate an earlier extinct species.

The Frenchman Antonio-Snider-Pellegrini published a map in 1858 suggesting that a supercontinent had broken apart to form North America, South America, Europe, and Africa, using the puzzle-like fit of the continents. Even earlier, the Englishmen Bacon had proposed this possibility.

Fossil evidence pointed for a connection between the continents. The name Gondwanaland, proposed by Edward Suess, originally referred to a hypothetical southern hemisphere super continent containing land bridges connecting India, Antarctica, South Africa, South America, and Australia. All of these areas contained flora of fossil plants called the Glossopteris flora which are best exposed in Gondwana, India. The most conspicuous Glossopteris genus is the Glossopteris seed fern. The early geologists speculated that land bridges of felsic rock had connected the continents and then subsequently sank. However, lower-density felsic rock cannot sink in the higher-density mafic rock.

In 1908, the American Frank Taylor suggested that the supercontinent had split along the mid-Atlantic ridge. This midocean ridge had been discovered by soundings made by the British Challenger expedition between 1872 and 1876.

Alfred Wegener, a German meteorologist, published "On the Origin of Continents and Oceans" in 1915, a book proposing continental drift in which a supercontinent called Pangea split apart in the late Mesozoic. Wegener used different lines of evidence besides the puzzle-like fit of the continents. He pointed out the African rift valleys were early stages of a continental rifting process. He used geologic structures and fossil evidence to support the occurrence of Pangea as well as paleoclimate data which different from present-day climates.

The South African geologist Alexander du Toit showed that striations from Permian glaciers indicated the impossible movement from oceans onto present landmasses. Du Toit supported continental drift with the present-day common occurrences of earthworm genera, the common presence of the Early Permian fresh-water fossils of Mesosaurus genera, and the Early Triassic mammal-like herbivores Lystrosaurus genera, and the common Gondwana sequence which includes Permian tillites, Triassic dune deposits, and Jurassic lava flows. Du Toit also pointed out that a reconstruction of Gondwanaland allowed the mountain ranges along their margins to form a continuous chain between the continents. Du Toit proposed that Pangea did not form until the late Paleozoic. The northern supercontinent Laurasia and the souther supercontinent Gondwanaland existed prior to the formation of Pangea. Unfortunately du Toit and Wegener could not come up with an adequate force to allow the continents to push through the oceanic crust.

Wegener confused the issue by pushing for an early Cenozoic split of Pangea, rather than the early Mesozoic split. This dating error confused palaeontologists who realized that not all of the evolution occurring on separate continents could have occurred in the Cenozoic time frame

In 1931, the Englishman Arthur Holmes proposed that continents split along midoceanic ridges, that oceanic crust carried continents along with it (as on a conveyor belt), and that stretched oceanic crust was destroyed in trenches. However, he thought the midoceanic ridges were remnants of continents. He did not think that new oceanic crust was being formed at the ridges.

In the 1950s', apparent magnetic polar wandering was used to support the movement of plates. Remnant magnetism can give magnetic latitude and the location of the poles but no longitude. The remnant magnetism from dated (by radioactive means) igneous rocks on different continents pointed to different locations of the poles as a function of geologic time, indicating separate movements of the plates rather than true polar wandering.

In 1962, the American Harry Hess published the "History of Oceanic Basins," proposing the creation of sea floor at the midoceanic ridges and its destruction at trenches or subduction zones. He was able to explain the young age of the sea floor. He also could explain the movement of the continents as being carried along with the sea floor, rather than being pushed through the oceanic crust. He supported his evidence with the small numbers of volcanic seamounts, the presence of guyots. He proposed mantle convection as the force moving the sea floor and continents. In 1963, Vine and Matthew explained the magnetic sea-floor anomalies symmetrical to the midoceanic ridges using Hess's hypothesis.

Sea floor spreading explains the rift valleys (grabens and normal faulting) associated with the midoceanic ridges, the presence of strike-slip faults (transform faults connecting other plate boundaries), and the occurrence of pillow basalts in oceanic regions. The spreading is of the order of 5 cm/year for the Pacific sea floor and less for the Atlantic sea floor. Presently there are 8 large plates and several smaller plates.

The occurrence of shallow and deep earthquakes in the Benioff zones was explained by subduction in plate tectonics. The associated andesite volcanism in island arcs and on continental margins was explained as due to partial melting of the sea floor in the subduction zones. The arcuate shape of the volcanos is related to the arcuate shape of the trenches which is due to the spherical shape of the earth, e.g., try pressing down on a ping pong ball and see the arcuate outline of the depression). The great age of the continents and the general age trend to increasing age in the center of the continents was due to the inability to subduct the lighter continental crust and the gradual accretion of additional material to the continents along their margins. Hot spots or mantle plumes or thermal plumes are used to explain nearly immobile points in the mantle which serve as the source of magma for volcanoes erupting on the surface of the earth and then carried away from the magma source These hot spots create aseismic ridges or chains of volcanoes in which the only active volcano is over the hot spot, e.g., the big island of Hawaii in the Hawaiian Island chain.

Rifting appears to begin with a mantle plume, producing a three armed rifting center, a triple junction. The three arms may consist of a spreading zone, a subduction zone or a transform fault. In general, one of the arms become inactive and the other two arms connect up with active arms from other mantle plumes. The result is a linear zone of rifting which can begin under a continent or under an ocean. The failed rift arms are frequently the sites for river valleys, e.g., the Mississippi River and the Amazon River.

The rifting of a continent probably involves a nearly stationary continent, rather than an actively moving one over the asthenosphere. Doming occurs along with rifting as a continent splits. East Africa is nearly stationary and appears to be about to begin splitting along the East African Rift connecting to the Red Sea rift and the Gulf of Aden rift (triple junction). In this case all three arms of the triple junction are remaining active.

Continental rifting in the early stages results in basaltic magma with normal faulting (plateau or flood basalts and shield volcanoes and a rift valley). The early seaway is shallow, producing nonmarine clastics (red beds), followed by evaporites as the seaway enters the rifting region, followed by the development of passive continental margins as the continent separates. The passive continental margins accumulate large deposits of marine sediments.

Evidence of ancient subduction zones occurs in the formation of melanges in the accretionary wedge which separates the trench from the forearc basin lying in front of the active volcanoes. On continents, behind the zone of volcanoes, deep-water marine flysch is deposited in the foredeep basin, followed by shallow, non-marine mollase deposits. Ophiolites, representing ancient sea floor, are frequently exposed in regions where two continents have sutured together following the destruction of an intervening ancient ocean (in a subduction zone). In the process of suturing, the crust is thickened by thrusting and uplifted.

Evidence for ancient transform faults lie in the coexistence of terranes, formed in different regions by different processes.

What is the significance of the Louann Salt and the Eagle Mills Red Beds in Louisiana?

Where is the mantle plume that is responsible for the East African Rift Valley, the Red Sea, and the Gulf of Aden?

The process of mountain building is called orogenesis and a particular episode of mountain building is called orogeny. Mountain building occurs near subduction zones and where two continents are sutured together, following the destruction of an intervening ocean by subduction. The suturing is the result of the continental crust being too light to be subducted. Mountain building results in a thickening of the continental crust. Because of crustal underthrusting, the crust is doubled in thickness when two continents are sutured together. Remnants of oceanic crust are often preserved as ophiolites.

Along a continental margin behind a subduction zone, there will be an igneous arc of volcanic activity associated with partial melting of the oceanic crust. The crust is thickened as the result of emplacement of plutons in the igneous arc and the extrusion of volcanics. On either side of the igneous arc will be a zone of metamorphism due to heat and compression. The mountain building can be due to compression from the subducting plate combined with gravity spreading once the crust has been thickened to the point that it is unstable vertically.

Towards the continental interior will be a fold and thrust belt bordering the foredeep or foreland or backarc basin. The foredeep or foreland basin will first fill with deep-water marine flysch (black shale and turbidite) deposits and then with fresh-water (alluvial fans, flood plains, etc,) molasse deposits. Thick molasse deposits are sometimes referred to as a clastic wedge. The thrusting in the fold and thrust belt is along an inclined basal thrust surface called a decollement. The decollement slopes upward towards the interior of the continent. Much of the movement along the decollement may be due to gravity spreading in the igneous arc and metamorphic zone.

Towards the subduction zone, is the forearc basin, filled with debris from erosion of the igneous arc and metamorphic zone. The accretionary wedge separates the forearc basin from the subduction zone. The accretionary wedge contains a melange of material derived from the subduction zone. Ophiolites are preserved in the accretionary wedge.

Mountain building occurred along the western coast of South America in Devonian time. A subduction zone occurred along the western coast of South America in early Mesozoic time. Within the Cenozoic Era, the belt of igneous activity has steadily shifted from the west to the east with the passage of time, due to a decrease in the dip of the subduction zone.

The Alps are part of a chain of Cenozoic mountains stretching from the Pyrenees in Spain, through the Himalayas to Indochina. These mountain ranges formed following the break-up of Laurasia and Gondwanaland in the region of the shallow Tethy sea which existed between Africa and Europe. A deep seaway formed in this area called the Penninic Ocean. Various microcontinents existed in this area, e.g., the Iberian and Adriatic plates. The subsequent collision of these microplates (pushed by Africa) and India and Australia against the Eurasian landmass formed this long mountain chain of Cenozoic mountains.

The Alps formed as the result of the collision of the Eurasian land mass with the Adriatic (Italian) subcontinent, following the destruction of the Penninic Ocean. The subduction zone dipped southward against Africa and the microplates. The subcontinent first sutured against the coasts of Yugoslavia and Greece, forming the Dinarides and the Hellenides. The subcontinent then rifted away from Eurasia, and moved northeastward, suturing onto Switzerland and Austria, forming the Alps in Eocene and Oligocene time. The igneous core of the Alps are the Southern Alps or Dolomites in northern Italy. North of the Dolomites, the Pennides and overlying Austrides form the metamorphic zone and are overlain by the fold and thrust zone of the Helvetides. The Pennides, Austrides, and Helvetides form nappes or sheets that have slid by gravity towards the north. The decollement surface is that of underlying Triassic evaporites. The flysch beds are part of the Helvetides and the molasse beds lie further north in Bavaria and France. The Alpine orogeny died out in Miocene time.

The Himalayas began forming in the Miocene with the collision of the Indian subcontinent against Asia. The Indian subcontinent had been moving with the Australian continent as part of the Australian plate moving away from Antarctica. The rift from Antarctica began at the end of the Cretaceous. The convergence with Asia was complete about 20 m.y. ago in the Miocene; however, India continued to move northeastward as the result of continued movement of the Australian plate. The movement has destroyed the accretionary wedge and forearc basin lying along the Asian margin and produced a foredeep basin in India, south of the suturing line. The Indian subcontinent has slid beneath the Asian margin for at least 100 km along the Main Central Thrust. Movement stopped 10 m.y. ago on this decollement surface and switched to a new and deeper decollement surface called the Main Boundary Fault. Movement continues to the present day.

The Appalachians formed as the result of three different orogenies: the Ordovician Taconian orogeny, the Devonian Acadian orogeny, and the Alleghanian Pennsylvanian orogeny. The Taconian orogeny corresponded to the collision of an island arc with the eastern margin of North America. The subduction zone dipped eastward under the island arc which was thrust westward over a carbonate platform lying off the N. Am. coast. This island arc was associated with Baltica (northern Europe). North America was part of Laurentia which included Greenland. A foreland basin developed west of the sutured island arc and is preserved as a vertical sequence of foreland deposits: shallow water carbonates overlain by flysch and greywackes overlain by molasse deposits.

The Acadian orogeny resulted from suturing of Baltica (northern Europe through Norway and England) to Laurentia (Canada and U.S. through Greenland) by the destruction of the Iapetus Ocean, producing the Old Red Sandstone continent. This orogeny is known as the Caledonian orogeny in Europe. Additionally, an island arc south of Baltica was sutured onto the eastern coast of North America, forming the Avalon terrane. The Acadian orogeny is preserved in the North American rock record as a vertical sequence of foreland deposits: shallow water carbonates overlain by flysch and greywackes overlain by molasse deposits, overlying a similar cycle of deposits from the Taconic orogeny.

The Alleghanian orogeny resulted from suturing Gondwanaland to the Old Red Sandstone continent through the collision of North America with Africa. The Ouachita mountains in southern Missouri and northern Arkansas formed as part of the Alleghanian orogeny. The Alleghanian orogeny is known as the Variscan orogeny or Hercynian orogeny in Europe. The molasse deposits from the Acadian orogeny are overlain by the molasse deposits from the Alleghanian orogeny, indicating a foredeep basin did not completely develop.

After the Alleghanian orogeny, Pangea was completed by the suturing of Siberia to Europe along the Ural Mountains in the Permian Period.

The present-day Appalachians consist of the Piedmont region (ancestral Appalachians). These are highly metamorphosed rocks lying east of the Blue Ridge which have been eroded down. The Blue Ridge lies west of the Piedmont and consist of a crystalline Precambrian core that has moved upward by faulting. The Valley and Ridge, west of the Blue Ridge, is the fold and thrust belt.

The chemical cycles involve the movements (fluxes) of chemical components between reservoirs. Frequently, the movement sets up a feedback which can accelerate the fluxes (positive feedback) or decrease the fluxes (negative feedback). This chapter examines the related fluxes of carbon dioxide, oxygen, and water between the atmospheric reservoir, the oceans, and rocks.

During photosynthesis, land plants take carbon dioxide from the atmosphere and ground water and aquatic plants take it from water. They combine carbon dioxide with water to form sugars while releasing oxygen to the atmosphere or to water in aquatic systems. Respiration is the reverse of this process in which sugars and tissue are oxidized with oxygen to release carbon dioxide and water. Plants and plant-like organism (algae) and photosynthetic bacteria (cyanobacteria and green and purple bacteria) undergo photosynthesis using solar energy to drive the process. However, both plants and animals and animal-like organisms (fungi, protozoans) and animal-like bacteria (decomposers)undergo respiration. The result is a cycle in which oxygen is created by photosynthesis and destroyed by respiration. Carbon dioxide is destroyed by photosynthesis and created by respiration.

The accumulation of organic matter in buried sediment is an important reservoir for carbon dioxide. When dissolved oxygen is low in the oceans (anoxic conditions) or rapid burial is occurring in swamps, there is less bacterial decomposition of organic matter. Hence, less carbon dioxide is produced and returned to the atmosphere which lowers the Greenhouse Effect, making the earth's surface cooler. The oxygen content increases in the atmosphere because it isn't being removed by conversion into carbon dioxide. Because organic carbon is enriched in ¹²C isotope, relative to the ¹³C isotope, the atmospheric carbon and ocean carbon becomes heavier as more of the ¹²C is buried in sediments and loss to the atmosphere. The carbon in calcium carbonate (from plankton shells) in limestone will be heavier (higher ¹³C/¹²C ratio), reflecting the heavier carbon in the ocean.

Interestingly, anoxic conditions in the ocean usually occur when the earth's surface is too warm to produce heavy cold surface water that brings dissolved oxygen to the bottom of the ocean when it sinks. The resulting burial of organic carbon would make the earth's surface colder (as described above), so this would produce a negative feedback for a warm earth's surface temperature, reversing a warm trend.

Weathering is another process (like photosynthesis) that can remove carbon dioxide from the atmosphere. Dissolved carbon dioxide forms a weak acid which causes chemical weathering. The process takes carbon dioxide from the atmosphere, dissolves it in ground water and moves it to the ocean. Eventually, it may be returned to the atmosphere by equilibration between the ocean surface and the atmosphere. Weathering is accelerated by a rise in the earth's surface temperature so this produces a negative feedback to a warming trend. Weathering is also accelerated when mountain building is occurring due to the exposure of rock to earth-surface processes.

Oxygen isotopes in shells are affected by temperature. During times of cold surface temperatures, the accumulation of ice on land stores water that had evaporated from the oceans. The evaporation process preferentially removes the lighter isotope leaving the ocean water enriched in ¹⁸O. Hence, organisms making their shells in seawater will have a higher ratio of ¹⁸O/¹⁶O in their shells.

Without evaporation, the lowering temperature also raises the isotopic ratio of ¹⁸O/¹⁶O. This is because the heavier isotope is preferentially incorporated in the shell due to an energy difference between the two isotopes. The energy difference between the two isotopes increases as the temperature is lowered. (This is due to thermal energy effects.) Hence, the organism will have a higher ¹⁸O/¹⁶O ratio because its selectivity for ¹⁸O increases with a decrease in temperature.

So the temperature and evaporation effect on the ¹⁸O/¹⁶O ratio in shells is the same.

Effect of Ocean Chemistry on Calcium Carbonate Composition of Shells and Skeletons

During times of low Mg/Ca ratios, calcareous nannoplanton precipitate low-magnesium calcite shells and are very abundant in the fossil record. During times of high Mg/Ca ratios, coral organisms precipitate aragonite and high magnesium calcite precipitates (ooids) and are very abundant. The Mg/Ca ratio in sea water appears to be related to the rate of sea floor spreading that is occurring in the oceans. When this rate is high, Mg is rapidly removed from sea water in thermally driven sea water-basalt reactions and replaced by Ca, producing a low Mg/Ca ratio. At present, the ratio is high, indicating lower rates of sea floor spreading. The fact that sea floor spreading is low also helps lower sea level because the ocean ridgers are smaller and occupy less of the ocean basins.

Why does sea water evaporation raise the ¹⁸O/¹⁶O ratio in the shells of plankton?

If organic tissue is buried in sediments and not oxidized to CO₂, how does that change the ¹³C/¹²C in atmospheric CO₂?

Archean Eon - from the beginning of the earth, about 4.6 b.y. ago to 2.5 b.y. ago. The end of the Archean is thought to represent the time of the beginning of modern plate tectonics on the earth's surface.

Archean time is part of the Precambrian time which includes both the Archean and Proterozoic Eras from the beginning of the earth to 570 m.y. ago. The Archean rocks are exposed in parts of the Precambrian shield areas in the centers of each continent. The Precambrian shield consists of exposed Precambrian rocks within the craton of each continent. The craton includes both the Precambrian shield and those areas covering the Precambrian rocks that have been undeformed since the beginning of the Paleozoic era.

Most of the Archean rocks are crystalline rocks, either igneous or metamorphic. They are generally divided into two

groups: even-grained, high-grade metamorphic rocks known as granulites and volcanic and sedimentary rock (derived from volcanic sediments near subduction zones) known as greenstone belts. Because of their great age, most sedimentary Archean rocks will have been metamorphosed or otherwise destroyed.

Fossils are generally limited to stromatolites, made by cyanobacteria, also called blue green algae. These photosynthesis bacteria began producing the oxygen for the atmosphere.

Our solar system is about 4.6 b.y. old as shown by the ages of most meteorites which date back to that time.

carbonaceous chondrites (carbon-rich stony meteorites in which the carbon molecules contain amino acids that are different in structure from similar amino acids produced by Earth's organisms. They also have different ¹³C/¹²C and ¹⁸O/¹⁶O ratios.

Another indication of the age of the solar system is that Lunar rocks have been dated as old as 4.6 b.y.

The oldest rocks dated on earth are about 3.8 b.y in Greenland. These are younger than lunar rocks and meteorites because the earth's crust is continually being destroyed by erosion and weathering, and the radiometric clocks are reset by metamorphism and igneous activity. Note that zircon grains in Archean sedimentary rocks, derived by erosion of older rocks, have been dated as old as 4.2 b.y. in Western Australia.

The universe must be older than 4.6 b.y. as shown by the Doppler effect of light waves. This is an increase in wavelength of waves reaching an observer if the object sending the waves is moving away from the observer. The effect is proportional to the speed of the object and can be used to calculate a time of 15 to 20 b.y. ago when the matter in the universe was at the same location. A subsequent explosion, the big bang, may have begun the expansion of the universe to the present day.

With the exception of Pluto, the planes revolve around the sun in nearly the same plane and rotate in the same direction, suggesting that they formed at the same time from the same rotating dust cloud.

solar nebula theory - The origin begin with a collapsing sun which formed heavy elements, followed by the formation of an exploding star, a supernova, that casts off matter of low density. The resulting explosion left a dense cloud of cosmic dust, a nebula, that was rotating. The cloud condensed, cooled and begin to rotate more rapidly as it contracted in order to preserve angular momentum. Less dense materials, e.g., gaseous elements, were expelled towards the edge of the rotating cloud and beyond into space. The cloud flattened into a disc which segregated into rings. The material in each ring coalesced into a planet with the sun forming in the center. An alternative theory would have the sun and planets forming from separate dust clouds in which the planetary dust cloud was captured by the sun's gravitational pull. The origin of the earth's moon is thought to be related to the earth as the result of a planetary collision of the earth with another object. Moon rocks show a close relationship with earth rocks, having similar ¹⁸O/¹⁶O ratios, which are not generally similar with or between meteorites. Ancient moon rocks have a weak residual magnetic field, indicating a molten core was present. Presently, the moon lacks a magnetic field.

Excesses (relative to meteorites from outside the solar system) in amounts of Xenon 129 and Plutonium 244 in some meteorites indicate the formation of these meteorites occurred within a 100 m.y. of the formation of the sun. Excess xenon 129 formed from iodine 129 which was created during the formation of heavy elements by the collapsing sun and preferentially incorporated into meteorites. The half life of iodine 129 is only 17 m.y., so the meteorites formed prior to the decay of all the iodine 129. Thus, the entire solar system may have formed in this time interval of about 100 m.y.

Origin of the Earth's Structure, Atmosphere, and Oceans and of the Moon's Surface

Concentric Layering of the Earth and the Development of the Oceans, Atmosphere, and Crust

Homogeneous - condensed material of both heavy and light elements which were segregated gravitationally once the heat released by condensing, melted the material. Radioactive elements followed the light elements because they formed compounds with these elements.

Heterogeneous - a dense core condensed first followed by condensations of less dense silicates.

atmosphere - degassing of the earth, primarily from molten material during early formation of earth. Degassing continues with volcano emission. Present atmosphere is 78% N₂ and 21% O₂ with minor amounts of H₂O and CO₂. The O₂ is the result of photosynthesis of plants and the early atmosphere was deficient in O₂.

oceans - degassing of H₂O from molten material during early stage of earth formation. Upon cooling of the planet's surface, the water condensed to form oceans. Dissolved salts are in steady state with inputs balancing outputs.

oceanic crust - first crust to form. Basaltic rock resulting from partial melting of ultramafic rock.

continental crust - second crust to form. Felsic rock from partial melting of oceanic crust at the base of thick piles of basalt (Greenstone Belts) and in primitive subduction zones.

mantle - ultramafic silicates, forming during gravitational segregation of lighter elements from heavy elements - has subsequently solidified by cooling from convection currents.

outer core - liquid iron with minor sulfur and nickel- formed during gravitational segregation of heavy elements from lighter elements - is still liquid - perhaps it is solidifying by increasing the size of the inner core.

inner core - solid iron with minor sulfur and nickel - formed during gravitational segregation of heavy elements from lighter elements - has subsequently solidified by cooling from convection currents.

The lunar rocks from the maria craters date at between 3.9 and 4.6 b.y., indicating great meteorite showers occurred during this time. Note that an asteroid impact melts the surface rock and resets the radiometric clock, causing the age dates on the rock to give the age of the impact. The composition of the earth's crust may have been altered by these meteorite showers. The actual impacts have been removed by erosion and other earth-surface processes.

Greenstone belts provide more information than Archean granulites which are rocks subjected to intense metamorphism. The greenstone belts typically occur as pods or synclines within granulites in which ultramafic volcanics grade upward into mafic volcanics (basalt) into felsic volcanics (andesite to rhyolite) into sediments. Perhaps the more felsic volcanics formed by partial melting of a thick sequence of mafic volcanics which were subsequently covered by sediments. The sediments are not commonly those of continental-shelf sequences, e.g., carbonates (limestones) or delta sequences with cross-bedded sandstones (quartz sandstones and feldspar-rich sandstones or arkoses). Instead, they are typically marine, e.g., mudstones (marine shales) and turbidites (greywackes and conglomerates), with few indications of terrestrial or fresh water deposition. These sediments are explained by the presence of small protocontinents of felsic crust and the absence of large continents and their associated shelves during Archean time.

The first large continent apparently formed in South Africa, about 3 b.y. ago. Other large continents were formed between 2.1 to 2.7 b.y. ago as indicated by dating of metamorphism related to intrusion of magma in cratons. The end of the Archean time at 2.5 b.y. marks the beginning of the widespread occurrence of continental crust together with modern plate tectonics.

Banded iron formations (iron-rich layers alternating with cherts) are common during the Precambrian, particularly during the Proterozoic. They also occur during the Archean and make up the oldest dated rocks. The lack of oxygen (no higher plants to make it by photosynthesis) in the Precambrian atmosphere may help explain these iron formations which are not forming today during Phanerozoic time. Oxygen tends to prevent accumulations of iron in aqueous solution, preventing subsequent precipitation of iron-rich minerals.

Graphite in banded-iron formations has ¹³C/¹²C ratios similar to those in organic tissue. Bacteria are the only life forms represented in Archean fossils. These include both animal-like and plant-like bacteria. Stromatolites, thought to have been formed by cyanobacteria (of which modern species undergo photosynthesis), occur in rocks dated as old as 3.5 b.y. They occur as layered structures of organic-rich calcium-carbonate sediment alternating with layers of purer calcium-carbonate sediment. Stromatolites grow in shallow seas on continental shelves and are thus rare in Archean rocks because of the scarcity of continental shelves around the small protocontinents existing during this time. They were much more abundant during the Proterozoic and become less abundant during the Phanerozoic when competitors evolved to limit their growth.

Bacteria fall in the kingdom Monera and are "prokaryotes", meaning that they lack a cell nucleus, DNA in chromosomes, and other cell organelles, that are present in the advanced cells called "eukaryotes" which did not exist in the Archean.

Amino acids are the building blocks of proteins needed by living organisms. There are 22 naturally-occurring amino acids. An amino acid contains the group:

Proteins are formed by linking amino acids between the NH₂ of one amino acid with the COOH groups of another amino acid. Each linkage is accompanied by the release of one molecule of water H₂O. Amino acids are found in carbonaceous chondrites (carbon-rich stony meteorites) as well as on earth. Electrical sparks or ultraviolet light in a gas phase of ammonia (NH₃), methane (CH₄), hydrogen (H₂), and steam (H₂O) can produce amino acids. Other gas phases can be used such as carbon monoxide (CO), nitrogen (N₂) and hydrogen (H₂). Could this type of reaction have anything to do with the beginning of life on earth? ATP (adenosine triphosphate) is used by modern cells as the source of energy to build organic compounds. Instead of manufacturing ATP, it can be produced from simple gases. Could the early cells have obtained ATP from their environment rather than manufacturing it.

fermenting bacteria - fermentation - breakdown of sugars to form alcohol and carbon dioxide and water

sulfate-reducing bacteria - use sulfate to breakdown sugars to form sulfide and carbon dioxide and water.

aerobic respiration - use oxygen to breakdown sugars to form carbon dioxide and water - however, little oxygen available in Archean time and there may not have been bacteria using aerobic respiration.

Photosynthesis - use of light energy to form chemical compounds within a cell

photosynthesis - cyanobacteria which use carbon dioxide and water with chlorophyll and light energy to form oxygen and sugars. The buildup of oxygen began with photosynthesis of cyanobacteria.

sulfide-oxidizing bacteria - purple and green bacteria which use carbon dioxide and hydrogen sulfide and light energy to form sulfur and sugars.

What was the typical sedimentary depositional environment during the Archean Eon?

During the Proterozoic Eon, the modern style of orogeny (mountain building), which is related to plate tectonics, began. The first modern-style orogeny was the Wopmay orogeny of Canada which occurred between 2.1 and 1.8 b.y. ago along the western margin of the present-day Archean age microplates forming the core of the Canadian Shield. In this area, continental crust had rifted two continental masses apart and subsequently, a subduction zone developed along the boundary. The igneous belt, metamorphic belt, and a fold and thrust belt (containing the foredeep (or foreland) basin sediments) are preserved in the rock record. Stromatolites are preserved in the preorogenic shallow marine sediments of the foredeep basin.

During the Proterozoic Eon deposition of sediment shifted from being primarily deep-water marine (in the Archean Eon) to shallow-water as the shelf areas along continents developed.

As described briefly below, mountain building occurs along plate margins where plates are converging.

Andesitic volcanoes form along the edge of an un subducted plate near a subduction zone in which oceanic crust is being destroyed, e.g., Cascades, Sierras, Rockies (difficult example), Aleutians, Japanese Islands, Philippine Islands, and Andes. Once all oceanic crust is destroyed in a subduction zone, continental crust will collide with continental crust, between the two plates. Because continental crust cannot be subducted (due too its low density), the two plates then fuse together and compress any sediment along the margins into mountains of folded sediment, e.g., Alps, Urals, Appalachians, and Himalayas. In both types of mountain building, subsequent erosion will result in the continental crust being uplifted by block faulting due to isostasy.

Described below is a general vertical upward sequence of shallow-water to deep-water to shallow-water and fresh-water sediments (all in the foredeep or foreland basin) and a lateral sequence of rocks going (away from the craton) from folded sediments (fold and thrust belt) to metamorphosed sediments, to igneous rocks. These sequences are associated with Phanerozoic mountain building such as the Wopmay orogeny of Canada.

A depositional basin exists between the volcanoes forming along the edge of a continent and the stable craton. The vertical trend in sediments in this basin grade upward from the (oldest) shallow-shelf marine shelf deposits which are preorogenic (sandstones and carbonates) to deep-water marine deposits (flysch: mudstones and turbidites) and then back to the (youngest) shallow-water marine and fresh-water deposits (molasse). The trend reflects the downwarping of a shallow basin by the stresses associated with mountain building (occurring further offshore), followed by filling the deep-water basin with sediments from the newly-formed mountains and from the craton. These sediments have been folded because of the compression associated with mountain building. In addition, the sediments closest to the mountain-building belt are metamorphosed because of the generally higher compression and the higher temperatures due to the presence of magmas associated with the volcanoes.

The lateral rock sequence going from the craton to the area of mountain building is a belt of folded and thrusted sediments, a belt of metamorphosed and folded sediments, to a belt of igneous rock. The first two belts are formed in the depositional basin described above and the third belt consists of rock making-up the volcanoes.

Early Proterozoic glaciation occurred about 2.3 b.y. ago, as shown by glacially-derived features (tillites, dropstones, laminated mudstones with varves due to deposition in glacial lakes in the Gowganda Formation in southern Canada. Similar age tillites are also found in Wyoming, Finland, southern Africa, and India. Thus the glaciation was world-wide.

Late Proterozoic glaciation occurred in several pulses between 850 and 600 m.y. ago, as shown by tillites on all major continents except Antarctica. A mass extinction occurred with the glaciation at 600 m.y. which affected the acritarchs and other species and may have been due to a cooling of the surface waters of the oceans.

Between 2 and 3 b.y. ago atmospheric oxygen was thought to have been only about 2% of present-day concentrations, reaching moderate (15%) concentrations at around 2 b.y. ago. Cyanobacteria and other organisms undergoing photosynthesis and producing oxygen, did not become abundant until about 2.3 b.y. ago, although cyanobacteria actually evolved about 3.5 b.y. ago. Sulfate-reducing bacteria, sulfide-oxidizing or purple-green bacteria, methane-producing bacteria, and fermenting bacteria cannot survive in oxygenated environments. These bacteria have become less common since Archean time, being restricted to anaerobic (without oxygen) environments, out-of-contact with the atmosphere.

The generally low levels of atmospheric oxygen in the Precambrian time are needed to explain the banded iron formations which commonly formed between 2.5 and 1.8 b.y. ago, as well as the occurrence of uranium minerals with uranium in a reduced form U⁴⁺. The common occurrence of pyrite (having both reduced iron and reduced sulfur) in Precambrian sediments, is another indicator of low levels of atmospheric oxygen. Pyrite is not stable in an oxygenated environment and decomposes. The banded iron formations consist of alternating bands of chert with layers that are usually composed of iron-rich minerals, with iron in a reduced state (Fe²⁺). Iron and uranium are sinks for oxygen (combine with it) and would not normally be deposited in reduced forms under earth-surface conditions, unless the oxygen in the atmosphere was at low concentrations. In addition, uranium is actually very soluble in its oxidized form and would have been dissolved quickly if deposited in an oxidizing environment.

Red beds, containing oxidized iron as Fe³⁺ in hematite, Fe₂O₃, appear in rocks younger than 2.3 b.y. ago, as the result of a gradual increase in oxygen content of the atmosphere. In addition, not all of the iron in banded iron formations is in the reduced state. Some of the minerals contain iron in both reduced and oxidized forms (magnetite, Fe₃O₄)) indicating some oxygen in the atmosphere.

Before the concentration of oxygen in the atmosphere could be built up to present-day levels by photosynthesis, various oxygen sinks had to be filled. Carbon monoxide (emitted by volcanoes) had to be oxidized to carbon dioxide in the atmosphere, sulfide minerals exposed at the earth's surface had to be oxidized to sulfates, and reduced iron, uranium, and other metals exposed to the atmosphere had to be oxidized to metal oxides, e.g., rust. A lot of the initial oxygen produced by photosynthesis was used up in these reactions. Once levels of oxygen were built-up in the atmosphere, they stopped increasing and reached a steady state concentration due to the evolution of organisms, using oxygen in respiration. In essence, photosynthesis became balanced by respiration, producing a balance between atmospheric oxygen and carbon dioxide.

At the beginning of the Proterozoic Eon, life consisted of single-celled prokaryotic producers of the kingdom Monera, such as cyanobacteria. By the middle of the Proterozoic Eon, life had evolved to produce single-celled eukaryotes in the kingdom Protoctista: first into protozoans, animal-like protoctists (more than 1.8 b.y. ago) and then into plant-like protoctists, such as acritarchs (about 1.4 b.y. ago). The acritarchs are thought to have been dinoflagellates. They underwent adaptive radiation between 900 and 700 m.y. ago before undergoing a mass extinction 600 m.y. ago at the time of the Late Proterozoic glaciation. Only a few spherical forms survived. Present-day representatives of acritarchs are dinoflagellates.

By the end of the Proterozoic Eon, multi-celled plant-like protoctists had evolved together with multi-celled plants of the kingdom Plantae and multi-celled animals of the kingdom Animalia, such as jellyfish and sea pens of the phylum Coelentera, segmented worms of phylum Annelida, and organisms with external skeletons and jointed appendages of phylum Arthropoda. The evolution of multicellular animals required high oxygen concentrations in the atmosphere and advanced nerve cells and occurred less than 1 b.y. ago (as evidenced from trace fossil evidence of burrows).

As noted earlier, stromatolites originated about 3.5 b.y. ago, became very abundant, as shallow water environments became more abundant, about 2.3 b.y. ago and exist today. They are not useful as index fossils because they tend to have different shapes in different environments. They are no longer as abundant because multi-cellular animals evolved that feed on the cyanobacteria. They only survive in shallow-water environments too harsh for these predators, e.g., tidal channels.

Eukaryotes, (cells containing nuclei, chromosomes, and other organelles) evolved from the prokaryotes in the Proterozoic about 1.8 b.y. ago, based on the first appearance of cells with thick walls (cannot identify as to being plant-like or animal-like). The first appearance of large cells in the fossil record date back to 1.4 b.y. ago in the acritarchs. In general, prokaryotes have much smaller cell sizes than acritarchs.

Eukaryotes are thought to have evolved through the union of two or more prokaryotic cells. Protozoans, animal-like cells in the kingdom Protoctista, are thought to have evolved first. The evolutionary process consisted of one prokaryote devouring a smaller bacterium, in which the smaller cell survived, although altered, and became a mitochondrion or an organelle capable of using oxygen to break down organic compounds, i.e., respiration in the first animal-like cell. This scenario is supported by the observation that bacteria are known to inhabit single-cell organisms and perform respiration for their host.

The first plant-like cell in the kingdom Protoctista is thought to be the result of a protozoan consuming a cyanobacteria which became an intracellular body known as a chloroplast (where photosynthesis occurs). This possibility is supported by structural similarities between cyanobacteria and chloroplasts. The oldest plant-like protoctist fossils date back 1.4 b.y. (algae plankton called "acritarchs" and thought to be dinoflagellates); whereas, the oldest positively-identified fossils of protozoans date back only 0.8 b.y.. However, since the plant-like protoctists evolved from protozoans, the protozoans must have originated more than 1.8 b.y. ago.

Algae include plant-like organisms, ranging from cyanobacteria to single-cell and multi-cellular photosynthesis protoctists to multi-cellular plants that (unlike advanced land plants) lack multicellular reproductive structures to protect their eggs and embryos). Multicellular algae are present in the fossil record beginning about 0.9 b.y. ago.

Multicellular animals evolved from protozoans by developing multicellular body forms and advanced nerve cells. Between 900 to 600 m.y. ago, a series of accumulations of ¹³C-rich carbonate rock deposits, provide evidence that not much oxidation of organic material was occurring. (This follows because organic matter is rich in ¹²C, which upon oxidation would also be incorporated into carbonate rock, preventing carbonate rock from being rich in ¹³C.) The lack of oxidation of organic matter allowed rapid build-up of oxygen in the earth's atmosphere, which then promoted the evolution of multicellular animals, depended upon oxidation for respiration.

Multicellular animals may have not developed rapidly until near the end of the Proterozoic for other reasons besides a low atmospheric oxygen content. Evolution took time to produce the nerve cells which coordinate muscle movement. Perhaps, these nerve cells only developed near the end of the Proterozoic.

The oldest multi-celled animal fossil records are trace fossils (burrows, trails, tracks) of soft-bodied animals in rocks less than 1 b.y. old. Imprints of jellyfish and sea pens have been found. Fossil skeletons were not preserved until near the end of the Proterozoic Eon, about 600 m.y. ago, because organisms with skeletons evolved after the non-skeleton forms. The multicellular animals present at the end of the Proterozoic Eon included coelenterates (jellyfish and sea pens in phylum Coelenterata), segmented worms (annelids in phylum Annelida), and arthropods in phylum Arthropoda. The late Proterozoic Ediacara fauna in Australia contains fossils of many soft-bodied animals.

(1) suturing two continents together, following the destruction, in a subduction zone, of oceanic crust between them.

(2) accretion by attachment of microplate (e.g., an island arc) to a large craton, following the destruction, in a subduction zone, of oceanic crust between the craton and the microplate. The microplates are called exotic terrains.

(2b) accretion by metamorphism of sediments, near a subduction zone, that are on the continental margin. These can also be oceanic sediments that were smeared onto the edge of the continental margin, as they were pushed into the subduction zone.

(2c) accretion through the introduction of magma in volcanoes overlying a subduction zone on the continental crust. The magma represents partially melted oceanic crust and melted oceanic sediments in the subduction zone.

(3) introduction of basaltic magma in failed rifts within a continent. Although, continents become smaller by rifting, if the rifting proceeds to split the continents.

During the Proterozoic Eon, a large supercontinent developed called Rodina. The continent eventually included North America, Greenland, Scotland, Ireland, eastern Russia, Baltica (Scandinavia), and possibly Siberia along its northeastern margin. Also attached to the continent were Australia (along its northern margin), Antarctica (along its western margin, and Africa (along its southwestern margin). The final stage of the supercontinent creation was the Grenville Orogeny in which South America sutured to Laurentia in Late Proterozoic. This supercontinent subsequently broke apart at the end of the Proterozoic: separating into Laurentia (North America and Greenland), Baltica, other northern hemisphere continents, the southern continents (that later joined in the early Paleozoic to become Gondwanaland), and several microcontinents. Eurasia did not exist during the Proterozoic Eon but formed later from fragments rifted from Laurentia and Gondwanaland in the Phanerozoic Eon. Note that the super-continent which existed at the end of the Proterozoic Eon was reassembled at the end of the Paleozoic Era to form the supercontinent named Pangaea. When Pangaea split into two supercontinents during the Mesozoic, these were Laurasia and Gondwanaland.

Laurentia began to form 1.95 to 1.85 b.y. ago through the assembly together of at least six microplates of Archean age to form the Canadian Shield. Note that Laurentia included Greenland which contains Archean age rocks. South of the shield area, a series of Proterozoic island arcs developed (in the region of the present United States) that were accreted onto the craton between 1.8 and 1.6 b.y. ago.

About 1.5 b.y. ago, Laurentia may have become smaller in size through the rifting of one of the microplates, Siberia, away from the western margin of the craton.

A rift began to develop in central Laurentia between 1.2 and 1.0 b.y. ago. The rift extended south from the Great Lakes region to Kansas, forming the Mid-continental rift (also called the Keweenawan Rift) which failed but can be recognized by basaltic rocks south of the Canadian Shield. The copper-rich Keweenawan basalts were extruded at this time.

Between 1.2 and 1.0 b.y. ago, the western margin of South America and Laurentia sutured together along the present eastern margin of the United States, during the Grenville orogeny to complete the formation of the supercontinent Rodina. During this time, Ireland, Scotland, Scandinavia, and eastern Russia were part of Baltica already joined to Laurentia. Siberia was already joined to Laurentia. The rest of what was to become Gondwanaland formed a crescent wrapping around Laurentia. Some of the suturing of these southern continents to Laurentia may also have occurred during the time of the Grenville Orogeny. The reassembly of the southern hemisphere continents in the vicinity of Laurentia follows the Samfrau belt of mountains, between South America and Africa, between South Africa and Antarctica and between Australia and Antarctica. The Samfrau mountains were active during the time of the Grenville Orogeny indicating suturing together of the southern continents at the same time they were possibly being sutured to Laurentia. The North America east coast suturing with South America produced a range of mountains in the Grenville Province. Remnants of those mountains are exposed in the Adirondack Mountains in New York. Rocks formed during the Grenville orogeny are exposed in the Blue Ridge Mountains in Virginia, and in the Llano Uplift in Texas.

The supercontinent Rodina rifted apart between 0.8 and 0.6 b.y. ago. Laurentia and Baltica split apart from the continents which became Gondwanaland. There were several microcontinents such as northern Ireland, Scotland, southern Ireland, and England. The Iapetus Ocean separated the other continents from Laurentia. The rifting separating Gondwanaland from the western margin of Laurentia produced a clastic wedge of eroded sediments in basins preserved in the Belt Supergroup.

The sequence of accretion of continents to form Gondwanaland at the end of the Proterozoic is unknown. Metamorphic belts in the Proterozoic follow the present-day outlines of the different continents which formed Gondwanaland. However, these metamorphic belts appeared to form in the interior of Gondwanaland and the different continents later broke apart along these belts. The origin of the metamorphic belts is unknown. They are not the result of the normal mountain-building processes of plate tectonics.

Near the end of the Proterozoic Eon, the continents had merged briefly into a supercontinent (through collisions) which included North America. Subsequently, the supercontinent split apart at the beginning of the Paleozoic Era, producing at least three continents: Laurentia (North America and Greenland), Baltica (Scandinavia), Gondwanaland (South America, Africa, Antarctica, Australia, India, Arabian Peninsula). The Iapetus Ocean formed as Laurentia (North America) and Gondwanaland separated by rifting.

Apparently, Gondwanaland remained intact throughout the Paleozoic Era, not breaking apart until the Mesozoic Era. See figure 10.25 on page 258. Eurasia did not exist as a continent at the beginning of the Paleozoic Era. It began forming during the Paleozoic Era by accretion of continental fragments onto Laurentia to form Laurasia. These fragments included Baltica, and the suturing of the Russian and Siberian platforms at the Ural Mountains at the end of the Paleozoic.

During the Cambrian Period the continents are thought to have been near the equator and not near the poles. A near-equatorial location explains the common presence of reef limestones, requiring both shallow waters and warm waters. Sea level was low at the end of the Proterozoic Eon so most of the continental landmasses were exposed. A low sea level explains the general absence of marine sediments marking the transition between the end of the Proterozoic Eon and the beginning of the Phanerozoic Eon (Paleozoic Era and Cambrian Period). However, during the Cambrian Period, sea level rose and flooded most of the continents (Sauk Sea {Cam.} and Tippecanoe Sea {Ord.} in Laurentia) and remained high throughout the Ordovician Period. Sea level gradually rose throughout the early Paleozoic, reaching a maximum level in Late Ordovician and in the Silurian, followed by a gradual drop until the end of the Paleozoic. In each period however, there were smaller rises and falls of sea level, e.g., the sea level fall at the end of the Ordovician and Devonian Periods that corresponded to major glaciation.

Note that the center of the craton of Laurentia was generally unflooded during the time of high sea levels in the early Paleozoic. This region called the Transcontinental Arch served as a source of sediment for deposition in the interior epicontinental seas. Between Late Proterozoic and Middle Cambrian time, an active rift existed in Oklahoma, the "Southern Oklahoma Rift" which contain basalts and was covered with shallow limestones from Late Cambrian to Early Devonian time after the rift became inactive.

During the Ordovician Period, Gondwanaland begin moving south towards the south pole, eventually covering part of the pole by what is now North Africa during the end of the Ordovician Period. A major period of glaciation occurred on Gondwanaland at this time; however, Laurentia and Baltica did not experience glaciation because they were too close to the equator; however, Baltica was further south than Laurentia. Sea level fell during this time of glaciation and a major mass extinction of marine organisms occurred at the end of the Ordovician Period. The mass extinction was greatest in tropical waters which presumably underwent global cooling due to glaciation occurring in Gondwanaland.

As Gondwanaland moved south during the Ordovician Period, Baltica and two small micro-continents (England and the south half of Ireland) were moving north and approaching Laurentia, closing the Iapetus Ocean. During late Ordovician time, mountain building, associated with island arcs near subduction zones, occurred along the eastern margin of Laurentia and along the margin of Scotland. This first period of Appalachian mountain building is called the Taconian orogeny. Eventually, in Silurian and Devonian time, Baltica and the two small microcontinents fused onto the eastern margin of Laurentia, producing the second phase of Appalachian mountain building called the Acadian or Caledonian orogeny. The former is the American name and the latter is the European name.

Typically, during the early Paleozoic Era, around each continent were concentric belts of sediment changing laterally in a seaward direction from shallow deposits of siliciclastic debris (due to weathering of silicates on the craton) to limestones (carbonate platform deposits, reefs of coral and stromatoporoids or sponges, stromatolites) to deep water deposits of muds and breccias (turbidites or flysch deposits). Added to the deep water deposits were weathered debris from volcanics if a subduction zone existed along the continental margin. The carbonate platform bordering the eastern margin of Laurentia was destroyed by subduction associated with the closing of the Iapetus Ocean during late Ordovician time. However, the carbonate platform along the western margin remained intact into middle Paleozoic time. It was not until the end of the Devonian Period that orogeny affected the western margin. The Burgess Shale of British Colombia, which contained many soft-bodied fossils, formed at the base of the Middle Cambrian Period. The shale formed at the seaward foot of the carbonate platform.

Near the end of the Proterozoic Eon, there was a mass extinction followed by adaptive radiation which produced many soft-bodied animals such as jellyfish (phylum Coelenterata) and worms, e.g., polychaete worms (phylum Annelida). By the beginning of the Paleozoic, the seas had also become inhabited by small, shelled animals, e.g., primitive mollusks in phylum Mollusca, with external skeletons. This occurred during the Tommotain Stage at the base of the Cambrian. The evolution of skeletons may be due in part to the development of carnivores and possibly a change in sea-water chemistry. The skeletons were made of either calcium carbonate or calcium phosphate. Note that calcium phosphate is what our teeth are made of.

The Cambrian life that has been preserved in the fossil record is of marine life, without any terrestrial life or fresh-water life. The first reefs, formed during the Tommotain Stage, were composed mainly of the cone-shaped archaeocyathids (phylum Archaeocyatha), together with calcareous algae. Later Cambrian reefs were composed of corals, e.g., rugose (horn) corals (phylum Coelenterata) and stromatoporoids (sponge-like in phylum Porifera); however, reefs did not become abundant again until the Ordovician Period. Other marine organisms which evolved during the Cambrian include crinoids (sea lilies in phylum Echinodermata); brachiopods (unsymmetric bivalves in phylum Brachiopoda); conodonts (tooth structures of ell-like creatures in phylum Chordata); trilobites, ostracods (phylum Arthropoda); bivalves, snails, and nautiloids (phylum Mollusca); jawless fishes (phylum Chordata); and graptolites (phylum Hemichordata).

The dominant skeletonized fossils during the Middle and Late Cambrian Period are of trilobites which evolved in the Early Cambrian Period (following the appearance and extinction of the Tommotian fauna). The trilobites which lived in warm, shallow seas underwent several extinctions during the late Cambrian Period, followed by adaptive radiations (speciation). Trilobites living in cool, deep waters did not undergo mass extinctions, suggesting that the periodic cooling of the surface waters may have caused the extinctions. Nautiloids, which evolved in the Late Cambrian, underwent adaptive radiation in the Late Cambrian Period, followed by a mass extinction at the end of the Cambrian Period. Nautiloids have chambers in their shell which contain gas used for buoyancy control as they swim. Stromatolites (that first appeared during the Archean) remained abundant throughout the Cambrian Period. The phytoplankton Acritarchs (the earliest preserved eukaryotes that first appeared in the Proterozoic and suffer a mass extinction at the end of the Proterozoic Eon) were also abundant during the Cambrian Period.

There were about 400 known families of marine invertebrate families at the end of the Ordovician Period as compared to 150 known fauna at the end of the Cambrian Period. A great adaptive radiation occurred in the Ordovician. Since that time the number of marine invertebrate families of marine animals have remained about the same through the end of the Paleozoic Era. This may be due to (1) filling environments to the point that it is difficult for new life to evolve, (2) evolution of effective predators limited the development of new life, and (3) animals became too specialized to easily evolve into totally new different species.

The mass extinction which ended the Ordovician Period wiped out about 100 of the families of marine animals; however, these were replaced in the Silurian Period.

During the Ordovician Period, stromatolites began to decrease in abundance due to the evolution of animals that graze on algae mats. The mass extinction of trilobites at the end of the Cambrian Period began the decline in overall abundance of trilobites in the Ordovician Period. Adaptive radiation of other groups of organisms evolved many new species of phyla that first appeared during the Cambrian Period. Reefs were composed of bryozoans, rugose and tabulate corals, and stromatoporoids. Brachiopods, graptolites, and conodonts evolved rapidly and provide important index fossils for the Ordovician Period. The bryozoans or moss animals (phylum Bryozoa) which live on the sea floor, first appeared during the Ordovician Period as did sea urchins and starfishes) (phylum Echinodermata). Tabulate corals (phylum Coelenterata) first appeared in the Ordovician.

Non-seed bearing plants (spore-releasing mosses) probably first appeared on land in moist environments during the late Ordovician Period. However, land plants did not become abundant until the Silurian Period.

A large mass extinction occurred at the end of the Ordovician due to cooling from glaciation on Gondwanaland.

Chapter 13 - Review Questions - The Early Paleozoic (Cambrian and Ordovician Periods)

What were the first land plants and in which geologic period did they occur? (4)

What were the first reef building organisms and in which period did they become extinct? (4)

High sea level occurred throughout most of the Silurian and Devonian Periods. Laurentia was flooded by the Tippecanoe Sea (Sil.) and the Kaskaskia Sea (Dev). Lower sea level occurred at the transition between the Silurian and Devonian periods and at the transition between the Devonian and Mississippian Periods which was also a period of glaciation in Gondwanaland. From the end of the Devonian to the end of the Paleozoic, sea level gradually falls. Note that because the periods were distinguished on the basis of unconformities by the early geologists, it is not surprising that sea level was low at the end of each period (which accounts for the unconformities).

Abundant warm-water deposits (reefs and evaporites) during the middle Paleozoic imply that Laurentia and Baltica were close to the equator. Gondwanaland contains glacial deposits at the end of the Devonian but not at the end of the Silurian. The period of glaciation at the end of the Ordovician on Gondwanaland ended in early Silurian as Gondwanaland moved away from the South Pole. Apparently Gondwanaland moved back to the South Pole in the Devonian. The Late Devonian glacial deposits correspond to the Devonian mass extinction. Australia had a Great Barrier Reef in Late Devonian time so not all of Gondwanaland was near the South Pole.

At the end of the Ordovician, Baltica was approaching Laurentia and the Iapetus Ocean was being destroyed in a subduction zone (near Laurentia's eastern margin) between the two continents. The island arcs associated with the subduction zone, formed mountains in the Appalachian Mountain trend in the Taconic orogeny of Ordovician time. Eventually, in late Silurian time through Devonian time, Baltica was sutured onto Laurentia, forming another part of the Appalachian Mountain trend in the Acadian orogeny of North America (called the Caledonian orogeny in Great Britain and Scandinavia). This new continent (the so-called "Old Red Sandstone Continent") was close to Gondwanaland during the Devonian Period, as suggested by the common Late Devonian genera of marine invertebrates. They may have even been sutured together in late Devonian and then separated by rifting? We don't know. However, Pangaea, the supercontinent at the end of the Paleozoic formed later in the Pennsylvanian and Permian Periods by the suturing of the Old Red Sandstone Continent to Gondwanaland. This later suturing event completed forming the Appalachian Mountain trend with the Alleghanian orogeny. The Alleghanian orogeny is called the Variscan or Hercynian orogeny in Europe.

During late Devonian, an island arc lay offshore of the western margin of the Old Red Sandstone continent (North America). The intervening seaway was destroyed by a subduction zone dipping under the island arc (to the west). The remnants of this arc are recorded in ophiolite deposits in the Klamath Mountains of California. (An ophiolite is a cross-section of oceanic crust which was not destroyed in a subduction zone but smeared and later uplifted onto the overriding edge of a subduction zone.) During the late Devonian and Mississippian, this island arc was accreted onto the western edge of the Old Red Sandstone continent in the Antler orogeny, creating the Klamath Mountains. This accretion formed the Roberts Mountains Thrust in Nevada in which shelf sediments were thrust eastward onto the craton.

During late Devonian time, continental glaciation developed on Gondwanaland, near northern South America.

Trilobites failed to recover fully after the mass extinction at the end of the Ordovician, becoming less abundant. Stromatolites continued their decrease in abundance. In general, the other groups of animals recovered by undergoing adaptive radiation. Acritarchs were the dominant phytoplankton. Mollusks and brachiopods were abundant as were bryozoans and graptolites. Reefs were abundant, composed primarily of rugose and tabulate corals and stromatoporoids together with bryozoans. The dominant reef-building organisms were the tabulate corals and the stromatoporoids, resulting in the name "tabulate-strome" reefs. Scorpion-like arthropods (eurypterids) evolved in Late Silurian time and moved into fresh water environments. Ammonoids evolved from nautiloids in Devonian time. Fishes with jaws (acanthodians, jaws evolved from gill supports) developed in Late Silurian time. Modern ray-finned and our ancestors, the lobe-finned (lung fish) fish evolved in Devonian time. Sharks evolved in Devonian time. Amphibians evolved in late Devonian time from lobe-finned fish and moved onto moist land environments. Spore-bearing plants were abundant in Silurian time in which a vascular system developed by Late Silurian or Early Devonian time (the lycopods). Gymnosperms (seed-bearing plants) developed in late Devonian time. Note that gymnosperms are flowerless plants.

The ammonoids evolved from nautiloids in the Devonian. Their septa separating the internal chambers are crenulated which makes the chamber walls stronger. Hence the shells can be made thinner and lighter. The ammonoids also had a curved shape which produced better balance as the organism reached out to grab.

Eurypterid arthropods (relatives of spiders) were common and lived in fresh and brackish waters. They invaded the land in Late Silurian time.

The Devonian time was the age of the fishes because they were the only vertebrates (effective predators without competition). Fish moved from marine environments into fresh-water environments during the Silurian. The jawless ostracoderms evolved in Silurian time but became extinct by the end of the Devonian. These ostracoderms still had bony plates rather than scales and lacked paired fins. Jawed fish (acanthodians) first evolved in late Silurian. Apparently the jaw evolved from gill supports and the teeth developed from scales. Acanthodians were also the first fish which had paired fins and scales covering their bodies. They underwent adaptive radiation in the Devonian. Placoderms (armored fish, covered with bone over much of their body) appeared in Late Silurian and were abundant in the Devonian before becoming extinct in early Carboniferous. The placoderms were at the top of the food chain in the Devonian.

Cartilage fish such as sharks evolved in the Early Devonian. Bony fish such as the ray-finned fish and the lobe-finned fish also evolved in the Devonian. The ray-finned fish have radiating bones from the body supporting the fins; whereas, the lobed-finned fish have a single bone attaching the fins to their body. Ray-finned fish are the first modern-type fish. The primitive ray-finned fish lacked a symmetrical tail and their scales did not overlap. Lobe-finned fish declined after the Devonian but are still present today as a few living fossil species. The lobe-finned fish have secondary lungs, allowing them to live in shallow-water environments as they dry up. The lungs evolved from air sacks which fish used to maintain buoyancy in water. The lobe-finned fish appear to have been the ancestors of amphibians which evolved soon after. The two pairs of legs of amphibians apparently evolved from the single bone supports of ventral fins of which there are two pairs on the lobe-finned fish. Perhaps, amphibians, reptiles, dinosaurs, birds, and mammals would have had more than two pairs of appendages if lobe-finned fish had possessed more pairs of ventral fins.

The ammonoids, eurypterids, and armored fish were predators which helped in the decline of trilobites and other defenseless organisms (including the ostracoderms).

Spore-bearing (nonvascular plants) probably first appeared on land in the late Ordovician Period and became common in the Silurian Period. The development of vascular tissue, two sets of tissue (one to carry water and nutrients and another to carry manufactured food) was needed to evolve plants with roots and leaves. Vascular spore-bearing plants evolved in late Silurian, and into male and female plants in mid-Devonian (lycopods). Spore-bearing plants require a moist environment for the fertilization process. The first forests were of spore-bearing plants in late Devonian. Seed-bearing plants (gymnosperms) evolved from the male and female spore-bearing plants in the late Devonian and do not require a moist environment because fertilization is internal. Dry land was invaded successfully by gymnosperms to form forests in the Late Devonian. Note that advanced seed plants with flowers (angiosperms) did not evolve until the Cretaceous Period.

Arthropods (euryptids, relatives of spiders) invaded land in the late Silurian Period; however, vertebrate animals (Phylum Chordata), such as amphibians (frogs, toads, salamanders), did not invade land until late Devonian. The amphibians are thought to have evolved from lobe-finned fishes (have paired fleshy fins and the ability to breath air) due to adaptive radiation on land to fill an environment without competition. Note that the amphibians evolved after vascular plants. Like the spore-bearing plants, they required a moist environment, because amphibians hatch from eggs in water and have an aquatic juvenile stage.

Remember that reefs composed of archaeocyathids and calcareous algae were common in the early Cambrian. Although stromatoporoids and rugose corals evolved in the Cambrian, reefs were uncommon until the middle Ordovician when reefs of bryozoan, rugose and tabulate corals, calcareous algae, and stromatoporoids were common until the end of the Devonian. In the Devonian Period, reefs were particularly abundant along the western continental shelf of Laurasia (western Canada) and along the margin of Gondwanaland corresponding to the northwestern margin of Australia. These must have been warm-water areas. The Australian reefs are unusual in that they are built not only of corals and stromatoporoids, but also stromatolites. Predators in this region were not effective in destroying the cyanobacteria that built the stromatolites.

A mass extinction occurred during late Devonian time that affected marine organisms living in warm, shallow waters or surface waters, e.g., trilobites, gastropods, brachiopods, corals, stromatoporoids, acritarchs, ammonites, rugose corals, tabulate corals, stromatoporoids, and Devonian fish. Organisms living in colder water were unaffected. The extinction must be related to glaciation and global cooling of the surface ocean that occurred during the late Devonian. The decline in reef-building organisms meant that reefs almost ceased to be built at the end of the Devonian. Reefs were not important for the rest of the Paleozoic.

What did amphibians evolve from and why do they have only two pairs of appendages? (4)

What caused the Acadian Orogeny and in which geologic Period did this occur? (4)

Permian: 290-245 m.y. ago, characterized by drier climates and glaciation on Pangaea and ending with the greatest mass extinction in earth's history.

Pennsylvanian: 323-290 m.y. ago, called Late Carboniferous, characterized by coal-bearing deposits and limestones of fusalind foraminifera.

Mississippian: 363-323 m.y. ago, called Early Carboniferous, characterized by limestone composed of crinoids.

Late Paleozoic glaciation was initiated at the end of the Devonian Period and persisted in non-tropical regions throughout the Late Paleozoic. Sea level was low at the beginning of the Mississippian Period (due to Late Devonian glaciation), at the beginning of the Pennsylvanian Period (due to expansion of Gondwanaland glaciation to near the equatorial regions) and at the end of the Permian (due to glaciation developing on parts of Pangaea existing near both the North and South Poles.

The epicontinental seas that flooded the continental interior of North America were the Kaskaskia Sea during the Devonian and Mississippian Period and the Absaroka Sea during the Pennsylvanian and Permian Periods.

During the Mississippian Period, Gondwanaland stretched from the South Pole to near the equator. The Old Red Sandstone continent reached from the equator to the middle latitudes in the northern hemisphere, separated from Gondwanaland by the Tethys Sea. Siberia lay further north, near the North Pole. Kazakhstania and China lay east of the Old Red Sandstone continent and north of the equator. A continental ice sheet covered the area near the South Pole. During the Pennsylvanian Period, glaciers reached within 30^o of the equator, while tropical coal swamps existed near the equator in the Old Red Sandstone continent. The tropical regions can be distinguished from the nontropical regions by the presence or absence of tree rings in the fossil tree record. Colder climates produce seasonal tree rings (due to different growth rates in different seasons) that are lacking or poorly developed in tropical climates. During the Permian Period, the climate was much drier in much of North America and Europe. The formation of Pangaea created a supercontinent, stretching from the South Pole to the North Pole, that included land areas far from a source of moisture, e.g., the oceans. Most of the mountain ranges lay along the eastern coasts in the equatorial regions, producing large rain-shadow areas to the west (because of the prevailing easterly winds). Evaporite and dune deposits are characteristic of the Permian Period. The transition of the Permian to the Triassic is marked with disconformities due to low sea level, resulting from glaciation near both the north and south poles.

Gondwanaland and the Old Red Sandstone continent were eventually sutured together to form Pangaea during the Pennsylvanian Period in the Alleghanian orogeny (American term), also called the Variscan or Hercynian orogeny (European terms). This orogeny extended the Appalachians to the southwest, through the Ouachita mountains, and also formed the Hercynian mountains in Europe that extended into northwest Africa. Kazakhstania sutured onto Siberia during the Pennsylvanian Period. In North America, weathering and erosion of the young Appalachian Mountains produced molasse deposits to the west in central North America. The ancestral Mississippi River was one of the rivers (the Michigan river) draining sediment from the Appalachian mountains.

The Ancestral Rocky mountains developed during the Pennsylvanian Period, as the Uncompahgre Uplift in central Utah and the Ancestral Front Range trending across southwest Colorado, both associated (somehow?) with the Ouachita-Appalachian mountains created during the suturing of Gondwanaland to the Old Red Sandstone continent to form Pangaea. The ancestral Rockies lay to the west of the modern Rockies which initially formed in the Cretaceous Period. The Pennsylvanian Fountain Arkose, containing hogbacks and cuestas (and forming the Garden of the Gods in Colorado Springs), was formed from sediment eroded from the Ancestral front Range.

Some of the coal swamps of North America (and Europe) in the Pennsylvanian Period formed in the floodplains of rivers draining the Appalachians and Hercynians. These were floodplain forests growing in the backswamps behind the natural levees of meandering rivers. Other widespread, thin coal beds in central North America formed in swamps associated with shallow epicontinental seas. Such a coastal swamp would exist during times when sea level fell (due to an increase in glaciation near the polar regions) and would be covered with marine deposits as sea level rose (due to melting ice). Both types of coal deposits produce cyclic coal deposits as described below.

The backswamp deposits produce a cyclic deposit of coal as the result of the river meandering, depositing channel deposits over old backswamps, as the river raises the surface of its floodplain. The swamp associated with an epicontinental sea will produce a coal deposit at low sea level, which is then covered with marine deposits from a transgressing sea as sea level rises, followed by deposits from a regressing sea as sea level falls, before another coal seam is formed. The complete cycle is called a cyclothem. The Everglades is an example of a coastal swamp which would be affected by small changes in sea level, resulting in cyclic deposits of coal over time spans of tens or hundreds of thousands of years. Cyclothems did not form during the recent glacial times in the Pleistocene because of the general absence of these shallow epicontinental seas that would expand and contract over large areas of land surface with minor changes in sea level.

Because, the central western area of North America was near the equator in Pennsylvanian time, rain shadows lay west of mountain ranges. A rain shadow was created to the west of the Ancestral Rocky Mountains in the Paradox Basin of southern Utah, northern Arizona, and western New Mexico, where Pennsylvanian evaporites accumulated. Similarly, in western Texas in Permian time, the rain-shadow effect of the Ouachita mountains to the east produced evaporite deposits in the Delaware Basin. This same basin has the beautifully preserved El Capitan carbonate reef, composed of lacy bryozoan, calcareous algae, and calcareous sponges, along its margins. The development of evaporites took place in subsiding basins flooded by shallow epicontinental seas.

Further west, off the margin of the North American craton, a volcanic arc, the Golconda Arc, attached to a microcontinent (Sonomia) was shedding sediment during the late Paleozoic. The subduction zone dipped westward under the Golconda arc, producing an accretionary wedge. In late Permian time, the accretionary wedge, the Golconda terrain, was thrust eastward onto the continental margin in Nevada (similar to what was done in the Antler orogeny in late Devonian and early Mississippian time). This is called the Sonoma orogeny and the additional sediment resulted in the westward growth of the North American continental margin. The orogeny continued into the Triassic in which the Sonomia microcontinent was fused to North America, producing present-day southwestern Oregon and northwestern California.

Pangaea continued to enlarge during the Permian Period. Siberia (including Kazakhstania) was sutured to eastern Europe forming the Ural Mountains. Siberia remained near the North Pole. Pangaea stretched from the South Pole to the North Pole with the zone of attachment between Gondwanaland and Laurasia near the equator (Fig. 14-35). Only China was unattached to Pangaea at the end of the Paleozoic Era and it was attached during the Mesozoic Era. The water body adjacent and east of Pangaea along the Equator was called the Tethys Sea, the same name previously given to the ocean destroyed by the collision of the Old Red Sandstone with Gondwanaland in the Alleghanian orogeny and the same name subsequently used to describe the Mesozoic Sea formed by the rifting between Laurasia and Gondwanaland.

In eastern Europe, lower Permian evaporite deposits accumulated along the western margin of the Ural mountains. The deposits formed in a subsiding basin (foredeep) which was collecting sediment from the Ural mountains. Evaporites precipitated as the result of evaporation and restricted circulation of sea water in the foredeep. During the late Permian, evaporites were deposited across northern Europe which was also acting as a subsiding basin. The region was flooded with sea water from the north during four successive periods. Each time evaporites precipitated because of evaporation and the restricted circulation of sea water back with the open ocean. These four periods of salt deposition produced the Zechstein salt deposits of northern Germany and the North Sea area. These salt deposits underlie the oil and gas production in the North Sea just as the Louann Salt underlies the oil and gas production in the Gulf Coast region.

The mass extinction at the end of the Devonian had eliminated the colonial rugose and tabulate corals that were important to reef building. Stromatoporoids never recovered to become important reef builders. Hence, reefs were not very important during the Late Paleozoic. Limestones were commonly composed of crinoids during the Mississippian Period and of foraminifera fusulinds during the Pennsylvanian and Permian. The acritarchs were hard hit in the Late Devonian extinction and never recovered after the end of the Devonian to become as abundant.

At the separation between the Mississippian and Pennsylvanian Periods (early and late Carboniferous), sea level fell and a mass extinction occurred. Presumably, an expansion in glaciation lowered sea level and cooled the surface water of the oceans, causing the extinction. Crinoid and ammonoid species were particularly reduced in this mass extinction. There does not seem to have been a major mass extinction between the Pennsylvanian and the Permian.

During the Permian Period, the climate was much drier than during the Carboniferous. Evaporites were deposited and plants flourished that could survive under drier conditions.

The mass extinction at the end of the Permian Period was the largest mass extinction in the history of the earth. It coincided with a drop in sea level and probably global cooling of the surface sea water. Although the period of glaciation near the South Pole on Gondwanaland was on the decline, the presence of the large land mass of Siberia near the North Pole, added to the overall global glaciation. On land, most of the therapsids, the mammal-like reptiles, failed to survive into the Triassic Period. In the marine world, all species of the shallow-water foraminifera fusulinds, the remaining genera of rugose and tabulate corals, and the trilobites became extinct. Only a few species of ammonoids survived, and there were major losses of species of brachiopods, bryozoans, and stalked echinoderms such as crinoids. Of the bryozoans that were reef builders, the lacy bryozoans became extinct. Bivalve mollusks and gastropods were eliminated in lesser amounts.

Fish and other families of marine life became more mobile (faster swimmers) during the late Paleozoic, resulting in the decline of heavily-armored animals such as the armored fishes and heavy-shelled nautiloids. These heavier predators couldn't catch their faster prey and gradually became extinct. The armored placoderms became totally extinct in early Mississippian time. Similarly, the heavier prey were slower, falling victim to faster predators. There was a major extinction of ammonoids at the end of the Mississippian Period.

Trilobites finally became extinct at the end of the Paleozoic Era. The acritarchs (phytoplankton), tabulates (corals), and stromatoporoids (sponges) persisted but did not re-expand after the Devonian extinction. Both tabulate and rugose corals became extinct at the end of the Paleozoic Era. While the bryozoans were reduced at the mass extinction at the end of the Paleozoic, they persisted into the Mesozoic and Cenozoic. The general absence of reef-building structures such as the tabulates and the stromatoporoids is reflected in the general absence of reefs in the late Paleozoic. Reefs did not become abundant again until the Triassic Period when the modern Hexacorals evolved, although low reef mounds were built out of bryozoans (lacy bryozoan), calcareous sponges, and calcareous algae (red algae) in the late Paleozoic, e.g., El Capitan reef in southwest Texas.

Brachiopods, gastropods, and crinoids flourished during the late Paleozoic. In particular, the crinoids underwent adaptive radiation in Mississippian Period, before being diminished by mass extinctions at the end of Mississippian time. Benthic fusulinds (foraminifera with calcium carbonate shells), living on the shallow sea floor, expanded in Pennsylvanian (late Carboniferous) and Permian time before becoming extinct at the end of the Paleozoic Era. Limestones in the Mississippian were frequently composed of crinoids; whereas those in Pennsylvanian and Permian time were frequently composed of fusulinds.

The belemnoids probably evolved from the ammonoids in the late Paleozoic. They lack an external skeleton and had an internal weight attached to an air sac which was used to control buoyancy.

Lycopods, spore-bearing trees, were the predominant trees in the Mississippian and Pennsylvanian Periods (Carboniferous); however, cordaites (primitive seed-bearing gymnosperms) trees were abundant in non-moist habitats in the Pennsylvanian as were seed ferns, spore ferns, and other spore plants called sphenopsids. A modern sphenopsid is the horsetail. Fossils of Glossopteris, the seed fern, were abundant in Gondwanaland. Remember, these were used by Wegener to argue for continental movement). By the end of the late Paleozoic, the remaining lycopods and sphenopsids were much smaller and less abundant. The cordaites were totally extinct. Other gymnosperms such as the conifers evolved and predominated in the Permian Period. The gymnosperms subsequently prevailed throughout much of the Mesozoic: the Triassic Period, Jurassic Period, and early Cretaceous Period, until the appearance of the (flower-bearing) angiosperms.

During the late Paleozoic, insects (phylum Arthropoda) became abundant, having first appeared in early Devonian. Winged insects had evolved wings which in general were unfoldable (like dragonflies) by Early Pennsylvanian time. They had probably evolved in the Mississippian but the fossil record is poor. Insects with foldable wings evolved in late Pennsylvanian and became common in the Permian Period.

Amphibians dominated terrestrial habitats during the Carboniferous time before being replaced by terrestrial reptile groups in the Permian Period. Amphibians were the only vertebrates in the Mississippian (early Carboniferous) Period. The amphibians required a moist environment and lacked teeth capable of tearing up flesh, necessitating swallowing an entire organism. Reptiles evolved from amphibians in the Pennsylvanian Period and became common in the Permian Period. They had the advantage of reproduction with eggs, allowing them to live in non-moist environments. The "amniote" egg of reptiles (and also birds which evolved in the Mesozoic, probably from the dinosaurs) contains two sacs in a nutritious yoke; one for the embryo and one for body wastes, and is covered with a shell. Later reptiles also developed another advantage over amphibians, they evolved a jaw which could apply pressure on food, capable of tearing food into smaller chunks. Eventually, the mammal-like reptiles

By Permian time, mammal-like reptiles became the dominant large animals. The primitive mammal-like (cold-blooded or ectothermic) pelycosaurs, or finback reptiles, dominated in the early Permian Period and the partly (?) warm-blooded or endothermic therapsids dominated during the late Permian Period. The therapsids had their feet positioned more vertically beneath the body than other reptiles (better for balance and increased mobility). Warm-blooded animals have greater endurance than cold-blooded animals, an inherent advantage for both predator and prey. Mammals are thought to have evolved from the therapsids in the early Mesozoic. It is somewhat surprising that dinosaurs (classed as reptiles), rather than mammals became the dominant large animals of the Mesozoic Era. Perhaps the dinosaurs were also warm-blooded, explaining their effective competition with mammals. The dinosaurs did not evolve until the Triassic Period and they evolved from the thecodonts, a reptile group that also didn't evolve until the Triassic.

Jurassic 144 to 208 m.y. ago Named after the Jura mountains in France which are part of the Alps

The similar fossil record of terrestrial animals confirms that Pangea existed as a single supercontinent throughout early and middle Triassic. Pangaea began to break apart at the end of the Triassic Period into Laurasia and Gondwanaland; however, the breakup was minimal at the end of the Triassic Period. The breakup began between Laurasia (in Eurasia) and Gondwanaland (in Africa), forming the Tethys Sea in the region of the present Mediterranean Sea in the late Triassic Period. Note that Eurasia did not include the Indian subcontinent which was still attached to Gondwanaland. This rifting produced a narrow seaway which contains Triassic-age evaporites (off the present northwestern coast of Africa). The evaporites formed from the limited sea-water circulation during early rifting.

During Jurassic time, the rifting zone extended between North America and Africa and then westward through the Gulf Coast region between North and South America. Note that the rifting did not continue to the point of separation of South America and Africa until the Cretaceous Period. The rifting between North America and Africa followed closely the old suturing region of the Allegheny (also called in Europe the Hercynian or Variscan) orogeny. An extension of the rifting to the west in the Gulf Coast region connected with the Pacific Ocean. Jurassic-age evaporites accumulated along the rift and were prominent in two regions: the area between Morocco and Nova Scotia where northwestern Africa and southeastern Canada were splitting and in the Gulf Coast Region. Jurassic-age evaporites in the Gulf Coast formed as the result of restricted sea-water circulation coming out of the west from the Pacific Ocean and out of the northeast from the Tethys Sea. These Jurassic-age salt deposits form the Louann Salt which underlies Louisiana and is exposed in the salt domes which have pierced the overlying sediments, as at Avery and Weeks Islands. Much later, Cretaceous age evaporites also formed between South America and Africa when the zone of rifting extended southward in early Cretaceous time.

Associated with the evaporite deposits forming with the rifting of a continent are red-bed deposits of continental sediments. These red beds are sediments which have been rapidly eroded and deposited in depositional basins (formed by normal faulting) along the edge of the rift. The thick red-bed sediments have not been weathered enough to remove unstable minerals and can be easily recognized in the geologic record. They are very characteristic of the early stages of rifting of a continent. The Gulf Coast red beds that coincide with the Louann Salt form the Eagle Mills Formation. The Newark Supergroup along the east coast of the United States is composed of red beds associated with the late Triassic and Early Jurassic rifting between North America and Africa, for which the associated salt deposits occur off the northwestern African coast.

The west coast of North America was built up by the accretion of exotic terranes such as that which occurred during the Sonoma orogeny in Permian-Triassic time. The Sonoma orogeny sutured an exotic terrane (Sonomia with the attached Golconda Arc) and the accretionary wedge of sediment separating the Golconda Arc from the continent (the Golconda terrain) to the west coast of North America. The suturing occurred due to the destruction of ocean floor in a subduction zone between Sonomia and North America in which the subduction zone dipped westward under the island arc. As the continent approached and reached the subduction zone (as the intervening oceanic crust was destroyed), subduction stopped and the island arc and wedge of sediment were sutured onto the west coast of North America.

Subduction zones later developed in the Triassic that dipped eastward under the west coast of North America, producing mountains equivalent to the Andes today along the western margin of the continent. The Sierra Nevada Mountains formed as the result of an easterly-dipping subduction zone in Late Jurassic in the Nevadan Orogeny. The Franciscan Formation, underlying San Francisco, was an accretionary wedge of deep-water sediment overlying this subduction zone near the North American margin.

The Great Valley sediments east of the Franciscan Formation are marine sediments from the Forearc Basin. The folded mountains in Nevada represent the fold and thrust belt of this same orogeny, and the Morrison Formation in Utah and western Colorado represents the Foreland Basin molasse deposits forming associated with the Sundance Sea.

In Europe during the Triassic, the deposition (mostly non-marine) consisted of three divisions for which the name "Triassic" originated. The sequences were the Bunter (lowest, non-marine silic-clastics); the Muschelkalk (middle, marine mussel limestone); and the Keuper (upper, non-marine). During the Jurassic, the deposition was again in three divisions: the Liassic (lower, black shales); Dogger (middle, brown iron-rich sandstones and limestones); and the Malm (upper, white limestones). The Solnhofen Limestone in which the first bird fossils were found is part of the Malm.

In the Triassic, Pangaea spanned the equator, ranging from the South Pole (Australia in Gondwanaland) to the North Pole (Siberia in Laurasia) with the Gulf Coast region and North Africa on the equator. The interior of the continent was dry because of great distances from water. Little of the interior of the continent was flooded due to low sea level. The ocean to the east of Pangaea was called the Tethys Sea and the ocean to the west was the Pacific Ocean. The fauna could be split into a tropical flora called the Euroamerican flora and two cold floras, the Siberian and Gondwanaland floras. Evaporite deposition in Early Triassic was unrelated to rifting but due to arid climate in the center of Pangaea in the equatorial regions. The continent was just so large, that its interior climate was arid. The mass extinction at the end of the Triassic appears to be unrelated to glaciation.

Sea level was low at the beginning of the Triassic Period (remember there was a time of major glaciation at the end of the Permian Period) and gradually rose throughout the Triassic, Jurassic, and Cretaceous Periods. Sea level was high in Late Jurassic, flooding most of the interior of the continents, e.g., the Sundance Sea in the western United States was one of these epicontinental seas. The marine organisms living near the northern continents lived in cooler waters (the Boreal realm) than those living near the southern continents (the Tethy realm). In particular, coral reefs characterize the Tethy realm in the late Jurassic Period, suggesting a tropical climate. The high sea level corresponded to a lack of glaciation and gentle surface temperature gradients with latitude, i.e., wasn't as cold near the poles, as today.

The most important marine invertebrates during the Triassic Period were the bivalve and gastropod mollusks which had been the groups least affected by the mass extinction at the end of the Permian Period. Sea urchins expanded in the Triassic and the Jurassic Periods. Hexacorals (modern reef corals) evolved in the Triassic but were much more abundant in the Jurassic Period. The symbiotic relationship of hexacorals with dinoflagellates did not develop until the Jurassic Period. Some of the Triassic coral reefs formed in deeper waters than later coral reefs did, because later reefs have dinoflagellates which require photosynthesis, i.e., must be in the photic zone or shallow-water zone.

During the Triassic Period, the belemnoids (which evolved in the late Paleozoic) became common (more abundant in the Jurassic Period). The belemnoids were squidlike cousins of the ammonoids and nautiloids and lacked an external shell. Instead, they had an internal gas sac for buoyancy with a stone weight which acted as a counterbalance. The stone weight had a bullet shape and often looks (as a fossil) like a large caliber shell, e.g., a 50 caliber shell.

The primitive ray-finned (bony) fish had scales; however, they didn't overlap completely as they do on modern fish. They developed a swim bladder (gas-filled bladder) from a residual lung (the same lung earlier passed on to amphibians), to help regulate buoyancy. Unlike modern bony fish, their bones were partly made of cartilage and their tails were asymmetrical (characteristics inherited from Paleozoic bony fishes). Some of the fish had rounded pegs for crushing rather than sharp teeth.

The primitive shark group, the hybodonts, were prominent in the Triassic seas and had rounded pegs for teeth for crushing.

One reptile branch returned to the sea: the marine swimming reptiles (our sea monsters) evolved and included those with armored turtle-like bodies (placodonts), not to be confused with the placoderms or armored Devonian fishes; with paddlelike limbs (nothosaurs); with winglike or whalelike limbs (plesiosaurs); and the fishlike reptiles (ichthyosaurs). The placodonts and nothosaurs became extinct by the end of the Triassic Period.

The plesiosaurs evolved from the nothosaurs in Triassic time and survived until the end of the Cretaceous. The ichthyosaurs also evolved in the Triassic and survived until the end of the Cretaceous. Terrestrial crocodiles evolved in the Triassic and developed marine forms in the Jurassic.

Other (land-water) reptile groups evolving in the Triassic were turtles and crocodiles. The amphibian group of frogs dates back to Early Triassic.

During the Late Triassic, small rodent-like mammals evolved from the therapsids (mammal-like reptiles) and the dinosaurs and flying reptiles (pterosaurs which lacked feathers) evolved from the thecodonts (a reptile group evolved in the Early Triassic which was commonly bipedal). The thecodonts became extinct at the end of the Triassic Period. The thecodonts were the dominant vertebrates in the Triassic.

Active flying (i.e., wing flapping) pterosaurs probably had to be warm-blooded in order to maintain the energy requirements of flying - otherwise, cold weather would ground them. Species that did mostly gliding, rather than flying were probably cold-blooded. The early dinosaurs were bipeds and more agile than the mammals which apparently led to their dominance over mammals. They may not have been cold-blooded, but warm-blooded? There were two types of dinosaurs: (1) bird-hipped herbivore dinosaurs and (2) lizard-hipped herbivore and carnivore dinosaurs. Dinosaurs became much more abundant in the Jurassic.

The mass extinction at the end of the Permian Period did not affect the plants to the extent of the animals. The gradual decline of the spore-bearing plants had already begun in the Permian Period. Ferns and seed ferns were abundant in the Triassic Period. Gymnosperms continued their dominance of plants, dating back to the Permian Period. The major groups were the cycads, the cycadeoids (close extinct relatives of the cycads), the conifers, and the ginkgos.

A major extinction occurred at the end of the Triassic Period, eliminating all species of the conodonts, placodonts, nothosaurs, thecodonts and most species of bivalves, ammonoids, plesiosaurs, ichthyosaurs, large amphibians, and therapsids and other mammal-like reptiles. The resulting void in large terrestrial species allowed the dinosaurs to undergo adaptive radiation.

During the Jurassic Period, the dinoflagellates (phytoplankton) underwent adaptive radiation. They replaced the acritarchs which were last important during the Devonian. Calcareous nannoplankton (phytoplankton) first evolved in early Jurassic and began to form chalks. They later became much more abundant in the Cretaceous. Globigerina foraminifera (zooplankton) also evolved in the Jurassic. Radiolarians (zooplankton) were present and formed siliceous-rich radiolarites. Note that radiolarians are not diatoms which are phytoplankton having siliceous shells that evolved in the Cretaceous.

The bivalves, ammonoids, plesiosaurs, and ichthyosaurs recovered from the end of the Triassic mass extinction and become abundant. Gastropods, belemnoids, ray-finned fish, hybodont sharks were abundant in the marine environment. Modern mackerel and tiger shark families evolved in the Jurassic Period. Some crocodiles became marine in the Jurassic Period.

Hexacoral reefs were abundant and mark the Jurassic Period as a time of major reef building. Remember that the symbiotic relationship with dinoflagellates, which began in the Jurassic, involves the dinoflagellates serving as a source of food and removing carbon dioxide from the coral polyps.

The dinosaurs expanded to fill up the void due to the elimination of thecodonts, and most large species of therapsids and other reptiles, and amphibians. Pterosaurs were abundant.

The bird-hipped dinosaurs developed into two herbivore groups: bipedal varieties such as ornithopods and quadrupedal varieties such as stegosaurs, e.g., the Jurassic-age Stegosaurus and the ceratopsians, e.g., the Cretaceous-age Triceratops. The lizard-hipped dinosaurs included both herbivores (sauropods such as Jurassic-age Brontosaurus) and carnivores (theropods such as the Cretaceous-age Tyrannosaurus). The large sauropods were generally quadrupeds because of their weight; whereas, the more nimble theropods were generally bipeds. The Morrison Formation in the western United States is famous for containing dinosaur bones. The Morrison Formation formed from the previously mentioned epicontinental Sundance Sea.

The earliest known fossils of birds are from the Late Jurassic (Archaeopteryx) Solnhofen limestone in Germany; however, some fossils from the Late Triassic may actually be of birds? Birds evolved from the dinosaurs or from thecodonts and not from the flying reptiles (pterosaurs).

The plants continued to be dominated by ferns and the gymnosperm groups: the cycad, cycadeoids, conifers, and ginkgos; however, the cycads were the major plant group in the Jurassic Period.

There was not a major extinction at the end of the Jurassic Period, however, the Stegasauria dinosaurs and the larger sauropods did not pass through to the Cretaceous Period. Also the therapsids died out during the Jurassic Period.

Cretaceous, 146 to 65 m.y. ago. Named after abundant deposits of chalk composed of calcareous nannoplankton in Europe

The fragmented continents of Pangea were still clustered together at the start of the Cretaceous Period. North America had separated from Eurasia along its southern margin by the ancestral opening of the Atlantic Ocean; however, the two continents were still attached to the north through Greenland. North America and South America were separated by a narrow extension of the Tethys Sea which also narrowly separated Eurasia from Africa. The Tethys Sea was oriented as an east-west warm-water passageway between the continents which had originally formed Laurasia and those that had originally formed Gondwanaland. Evaporites formed along the margin of the Tethys Sea during the Early Cretaceous. South America was still connected to Africa. Antarctica, Australia, and India were still attached together and possibly to Africa.

The rifting continued during Cretaceous time, connecting the Arctic and North Atlantic Oceans through rifts on both sides of Greenland, splitting South America from Africa to form the South Atlantic Ocean, and splitting Australia, India, and Antarctica together away from Africa. At the end of the Cretaceous, India separated from the Antarctica plate and moved northward towards Eurasia. Not until the Eocene Epoch in the Paleogene Period did Australia separate from Antarctica.

Sea level rose to its maximum level during the Cretaceous period. It had been rising since the end of the last previous major Ice Age at the end of Permian time. Warm temperatures during the Cretaceous may have been due to excess volcanic activity, releasing carbon dioxide to promote a greenhouse effect. High sea level flooded the interiors of continents with warm-water seas. These included the Mowry Sea in Early Cretaceous time and the Cretaceous Interior Seaway in Late Cretaceous time in western North America. In Europe, the Chalk Sea covered much of North Sea area during Late Cretaceous time. The marine deposits from these interior seas are exposed today, providing an excellent fossil record for Cretaceous time. Since the end of the Cretaceous time, sea level had gradually fallen.

The warm seas during the Cretaceous Period helped produce anoxic (low oxygen) waters in the oceans. Cold surface waters have a high density and will sink to ocean depths. This occurs today in the polar regions of the oceans. The cold sinking waters are oxygenated and bring oxygen to the deep ocean. These waters flow along the bottoms of the oceans and are eventually returned to the surface waters by upwelling. The absence of this cycle during the Cretaceous helped produce anoxic ocean waters during the Cretaceous. Even the shallow epicontinental seas were sometimes anoxic, resulting in the deposition of organic-rich black shales, e.g., the black shales deposited in the Mowry Sea during Early Cretaceous time in western North America. The sinking of cold surface waters in the oceans did not reoccur until the Eocene Epoch in the Paleogene Period.

During Jurassic time, orogenic activity (mountain building) was occurring along the western edge of North America in the Nevada Orogeny, forming the Sierra Nevada Mountains. During the Cretaceous time, the orogenic activity shifted eastward, inland from the western margin of the coast. The Cordilleran Mountain Belt, east of California and Washington and Oregon, was forming in the Sevier Orogeny as a result of a shallow-dipping subduction zone, dipping to the east under the western edge of the continental margin. The shallow dip pushed the mountain building eastward, because magma could only be generated where the subduction zone had reached sufficient depth within the mantle. The elevation of the Rocky Mountains began in Late Cretaceous time in the Laramide Orogeny but reached its climax in the Cenozoic Era.

The concept of explaining mountain building in the interior of a continent by a shallow-dipping subduction zone extending from the continental margin, is not (to your instructor) a satisfactory explanation; however, it is the only explanation consistent with plate tectonics. The explanation for the Laramide Orogeny, forming the Rocky Mountains in Colorado, is particularly weak. Colorado is so far eastward from the western continental margin, that the subduction zone would have to dip eastward to the base of the continental crust, run horizontal for a thousand kilometers, and then dip downward into the mantle.

Several groups of phytoplankton became very abundant: the dinoflagellates (without shells), the calcareous nannoplankton and globigerina foraminifera with their calcareous shells and the diatoms with their siliceous shells.

The abundant Cretaceous chalks were formed from calcareous nannoplankton (phytoplankton) shells and globigerina foraminifera (zooplankton) shells, often preserved in marine deposits of shallow epicontinental seas on the underlying continental rocks. They are thus exposed today as chalk deposits in the interior of continents. Remember that calcareous nannoplankton (phytoplankton) and globigerina foraminifera (zooplankton) had previously evolved in the early Jurassic. Diatoms (phytoplankton), which have shells of silica, evolved in the Cretaceous. Radiolarians (zooplankton), which also have shells of silica, evolved back into the Cambrian but did not become abundant until the Mesozoic.

Ammonoids and belemnoids were common during the Cretaceous; however, they would become extinct at the end of the Cretaceous Period.

Sea floor predators became more efficient. Crabs developed with crushing claws, e.g., the brachyuran crabs, and some gastropods developed the ability to drill through shells. The result was that benthic organisms were forced to evolve into forms that could swim or burrow actively, e.g., burrowing bivalve mollusks, or to develop heavy protective shells, e.g., gastropods. The efficient predation resulted in the decline of brachiopods and crinoid species, which have not become abundant again. However, bryozoans and benthic foraminifera species expanded during the Cretaceous Period.

Bivalved mollusks evolved species of enormous sizes. Large coiled oysters existed. Rudists evolved in the Cretaceous and grew to enormous sizes, a meter in length. They looked like garbage cans with a huge lower lid and a small upper lid and were stacked on top of each other to form reefs. Rudists are thought to have had the same symbiotic relationship with dinoflagellates as did the hexacorals. Rudists became extinct at the end of the Cretaceous.

Hexacorals and calcareous algae (coralline algae) were present but were not the predominant reef-building organisms in Middle and Late Cretaceous. That position, which they had occupied in Jurassic and Early Cretaceous time, became filled by the rudists. However, when the rudists became extinct at the end of the Cretaceous, hexacorals became again the dominant reef-building organisms.

Modern ray-finned fish (bony) fish, the teleost fish, evolved with symmetrical tails, rounded scales, specialized fins, and short jaws. Modern sharks were present.

The top marine carnivores were still the marine reptiles: the fish-like reptiles called ichthyosaurs and the wing-limbed reptiles called plesiosaurs (both of which had evolved in the Triassic), and mosasaurs (marine lizards which evolved in the Cretaceous). These three groups of marine reptiles became extinct at the end of the Cretaceous.

Giant marine turtles were present, e.g., Archelon. Flightless diving birds had evolved, e.g., Hesperornis.

The cycads (gymnosperms), which had dominated land plants in Jurassic time, had been replaced by the conifers (gymnosperms) in Early and Middle Cretaceous. However, the flowering plants (angiosperms) evolved in Middle Cretaceous time and became the dominant plants by Late Cretaceous time. Today there are about 200,000 species of angiosperms and only about 550 species of conifer gymnosperms. The angiosperms have a shorter reproductive cycle than the gymnosperms, giving them an advantage in colonizing bare ground. In addition, the flowers of angiosperms attract insects, which provides an advantage in pollination and spreading seeds. Because insects often feed on flowering plants, the rapid adaptive radiation of angiosperms led to an adaptive radiation of insects. Grasses are angiosperms; however, they did not evolve until the Paleocene Epoch in the Cenozoic Eon.

The largest herbivores, the lizard-hipped sauropods, had died out in the Jurassic Period, as had the bird-hipped stegosaurs. During the Cretaceous Period, the bird-hipped, duck-billed dinosaurs were the abundant herbivores, forming great herds. The bird-hipped, rhinoceros-like triceratops were perhaps the last dinosaurs to become extinct at the end of the Cretaceous. The herbivores were eaten by the lizard-hipped carnivore dinosaurs called theropods, e.g., Tyrannosaurus. There was a general evolutionary trend in all dinosaurs towards larger body size (Cope's rule), although small dinosaurs existed. Pterosaurs were abundant of which the largest was Quetzalcoatus with a wingspread of 35 feet. The Cretaceous birds were probably shorebirds, equivalent to herons and cranes. The crocodiles approached the dinosaurs in size. Snakes evolved in the Cretaceous Period. The present-day constrictors, e.g., pythons, are descendants of the primitive snake groups.

Mammals remained small in size during the Cretaceous Period. They can be distinguished from reptiles in the fossil record on the basis of a single bone making up the lower jaw, greater complexity of cheek teeth, and a relatively larger brain case. Some important differences between mammal and reptiles include the following. Mammals stop growing as adults; however, reptiles continue to grow in size through life. Mammals are warm-blooded; whereas, reptiles are cold-blooded. Mammals are usually covered with hair for insulation, while reptiles have exposed skin or are covered with scales. Mammal young are usually born alive and nursed; whereas, reptiles usually lay eggs and do not nurse their young. The two major mammal groups, placentals and marsupials, evolved in the Late Cretaceous. The marsupials are especially diverse in Australia and the placentals are diverse everywhere else. Placentals nurture their young in the uterus through the placenta; whereas, marsupials bear their young at an early stage of development and transfer them to a pouch which contains the mother's teats.

Ammonoids, belemnoids (a few species may have survived into the Cenozoic?), dinosaurs, plesiosaurs, rudists, ichthyosaurs, mosasaurs underwent total extinction; however, the extinction was gradual through the end of the last 4 million years of Cretaceous time, called Maastrichian time. Calcareous nannoplankton suffered severe losses. A sharp pulse of extinction at the very end of the Cretaceous decreased planktonic foraminifera and seed-bearing plants, all of which later recovered in the Cenozoic Era. Plant species with smooth leaf margins (indicating tropical environments) underwent greater extinctions than those with jagged leaf margins (indicating cooler environments). This suggests gradual extinction due to a stressed environment followed by a major environmental shock. The rock record at the very end of the Cretaceous shows evidence of a bolide impact. Excess iridium with microspherules and shocked grains of quartz occur at the extinction boundary on all continents. A bolide impact would liquify rock creating microspherules as the liquid cooled and would have shocked quartz by the force of the impact. The high iridium could come from vaporization of the bolide which is rich in iridium. The impact would have thrown up all these components as dust in the atmosphere, allowing the earth's surface to cool and affecting planktonic and shallow marine organisms in tropical waters. There is a large crater near the Yucatan Peninsula which is thought to represent a large meteorite hit at the end of the Cretaceous Period.

The Paleogene Period occupies Early Cenozoic time and includes the following Epochs:

whereas the Neogene Period occupies Late Cenozoic time and includes the Miocene, Pliocene, Pleistocene Epochs.

Note that in the alternate classification of the Cenozoic Era, using the Tertiary and Quaternary Periods, the Tertiary Period includes all the above epochs except the for the Pleistocene and Recent Epochs.

At the end of the Cretaceous, the Mid-Atlantic Rift separated Greenland from both North America and Europe in two rifts forking away from the single rift in the central Atlantic Ocean. In the Paleocene Epoch, the rift separating North America from Greenland became inactive. Greenland began to move with North America away from Europe. The rifting continued between Greenland and Scandinavia, opening a deep-water connection between the Arctic Ocean and the North Atlantic during Eocene time.

North America has remained connected to Eurasia through continental crust connecting Alaska with Siberia. At present, the Bering Sea, a shallow sea, overlies this continental crust. The Bering land bridge was generally open throughout the Paleogene Period. Only in the Neogene Period, has seawater frequently flooded this corridor.

Australia separated away from Antarctica at the end of the Eocene Epoch and began to move northward with India, as part of a single plate separated by oceanic crust, to its present position. Antarctica was already over the South Pole.

The average sea level, temperature, and rainfall dropped throughout the Paleogene Periods. Superimposed upon this trend were cycles of changes in temperature and sea level. The modern psychrosphere, cold deep ocean waters, originated at the end of the Eocene Epoch when Antarctica separated from Australia. The separation disrupted warm water currents flowing off the coast of Antarctica, resulting in forming cold, dense waters near coastal Antarctica (between Antarctica and Australia). The water sank and spread northward, as recorded by the extinction of benthic foraminifera. Another factor in the origin of the psychrosphere was the further opening of the Arctic Ocean to the North Atlantic by rifting between Greenland and Scandinavia. The frigid Arctic waters entered the North Atlantic and sank, spreading southward.

The climate trend was to more arid throughout the Paleogene Period. The Oligocene Epoch was marked by savannah (grasslands) rather than the semi-tropical wet conditions characteristic of the Eocene Epoch. Part of the increase in aridity was due to the general drop in sea level which increases the distance of the continental interiors from the oceans,

The Cannonball Sea occupied the central North American continent during the Paleocene Epoch. This sea did not completely withdraw from the present Gulf Coast region until the Oligocene Epoch. The thick sequence of Cenozoic sediments deposited in the Gulf Coast region are the source rocks (shales) and the reservoir rocks (sandstones) for much of the hydrocarbon production in this region.

The Laramide Orogeny, which had begun elevating the Rocky Mountains in Colorado at the end of the Cretaceous, persisted into Eocene time. This orogeny can only be explained in terms of plate tectonics by postulating a very shallow dipping subduction zone, extending eastward from the western margin of the continent. The orogeny was marked by uplifts of blocks of continental crust, e.g., Pike's Peak is an uplifted granite block near Colorado Springs as are the Black Hills of South Dakota. Volcanism was not common along the eastern margin of the Rocky Mountains. By the end of the Eocene, the Rockies had been largely eroded away. They later rose again as uplifted blocks of granite and crustal sediments in the Neogene Period.

Along the continental margin in Washington, a subduction zone was beginning to create the Olympic Range of volcanoes (near Seattle) during the Paleogene Period.

The oil shales in western United States in the Green River formation were laid down around the margins of lakes during Eocene time as the result of the trend towards a drier climate in the Paleogene.

The Turgai Strait, a seaway paralleling the eastern margin of the Ural Mountains served to isolate Asia from Europe during much of the Eocene and Oligocene.

The North Sea flooded central Europe during the Eocene and Oligocene Epochs. The major marine depositional basin centered on the Rhine Graben (a failed rift extending to the North Sea and underlying the present Rhine River valley).

The Alps were forming in Europe in the Paleogene Period (Eocene and Oligocene Epochs) as the result of a collision between the Italian Adriatic subcontinent and the Eurasian continent. The two land masses had been separated by the Penninitic Ocean which had formed as the result of Mesozoic rifting between them. Eurasia was moving south as the Penninic Ocean was destroyed in a subduction zone which dipped to the south near the northern margin of the Adriatic subcontinent. When the two land masses fused together, the sediment along the margins of the continents was pushed up and slid to the north to form the Western Alps of Switzerland and the Eastern Alps of Austria and slid to the south to form the Dolomites or Southern Alps of northern Italy.

The Alps are formed by three large nappes, sediment layers that slide under the influence of gravity: the Pennides, the Austrides (other names are also used), and the Helvetides. The beds dip to the north in which the Pennides underlie the Austrides which underlie the Helvetides. The Pennides are exposed along the southern margin of the Alps (particularly in Switzerland), the Austrides are exposed further north (particularly in Austria), and the Helvetides are exposed along the northern margin of the Alps (particularly in Switzerland). The Alpine orogenic belt actually extends all the way to the Himalayas which were produced by the collision of the Indian subcontinent with Eurasia in the Neogene Period.

The Atlas mountains of North Africa and the Pyrenees Mountains also formed in the Eocene due to the closure of the two land masses of North Africa with Spain. This occurred earlier than the fusion of the Adriatic subcontinent with Eurasia. However, the formation of the Atlas Mountains, Pyrenees, Alps and Himalayas form a line of mountain building due to closures of land masses against Eurasia.

Reefs did not become abundant until the Eocene Epoch. The extinction of the rudists together with depletion of species of hexacorals at the end of the Cretaceous resulted in few reefs forming during the Paleocene Epoch.

Calcareous nannoplankton recovered from the partial extinction at the end of the Cretaceous and together with diatoms and dinoflagellates were the dominate autotrophs in the oceans during the Paleogene Period. Note that diatoms and dinoflagellates were not affected by the mass extinction at the end of the Cretaceous.

Whales evolved during the Eocene Epoch from carnivorous land mammals. The penguins or swimming birds also evolved in the Eocene Epoch. The top carnivores were the whales and enormous sharks. The pinnipeds which include walruses, sea lions, and seals probably evolved in Oligocene time, however, no Paleogene fossil record exist for these species.

Grasses evolved in the Paleocene and finally reached their present abundance at the end of the Oligocene. Grasses grow from the bottom of leaves rather than from the top. The early grasses did not have continuous growth of their leaves, thus they could not withstand heavy grazing by herbivores. Once they developed this ability, they spread rapidly to form grasslands. Subsequently, grasses evolved the addition of silica to their leaves to wear down the teeth of herbivores. Mammals have only one set of adult teeth. Mammal herbivores' teeth have evolved with the addition of enamel and intricate dentine structure to become more resistant to abrasion. Grasses cannot depend upon insects for pollination because of the enormous number of individual plants. Instead, grasses depend upon wind pollination and asexual reproduction known as budding.

Adaptive radiation of mammals and birds occurred in the Paleocene to replace the void left by the extinction of the dinosaurs and pteropods (pterosaurs) at the end of the Cretaceous. The mammals included ancestors of bats, primates, horses, cattle, rodents, and modern placental carnivores.

The modern hoofed herbivores began to appear in the Eocene Epoch, known as ungulates and are divided into odd-toed ungulates (horses, tapirs, and rhinos) and even-toed (cloven hoof) ungulates (cattle, sheep, goats, pigs, bisons, deer, camels). In the Eocene Epoch, the odd-toed ungulates outnumbered the even-toed ungulates; however, the situation was permanently reversed in the Oligocene Epoch. Odd-toed ungulates have only one toe or finger that has developed into a hoof; whereas, even-toed ungulates have two toes or fingers that have developed into a hoof. The evolution to hoofed animals was an adaption to gain speed.

Ancestors of elephants, dogs, cats, weasels evolved in the Eocene Epoch and underwent adaptive radiation in the Oligocene Epoch.

Huge flightless birds, the diatrymas, evolved in Eocene time but became extinct by the end of the Eocene. Most of the birds present were shore birds. Song birds did not evolve until the Neogene.

In Oligocene time, the largest land mammal to ever live on the surface of the earth evolved from the rhino family, the Indrichotherium. Also present in Oligocene time were the titanothere, rhinolike animals with blunt horns rather than sharp ones. The Indrichotherium and titanotheres became extinct at the end of the Oligocene Epoch.

Monkeys and apelike primates evolved during the Oligocene Epoch. Earlier primate ancestors had evolved by the Eocene.

Reptiles and amphibians were relatively inconspicuous during the Paleogene Period. Modern frogs first appeared in the fossil record in the Eocene Epoch.

The decrease in smooth-margin leaf plants, relative to jagged-margin leaf plants implies that the temperature became colder during the Late Paleogene Period. Globigerina foraminifera suffered a major extinction by the end of the Eocene in which more species of the spiny (tropical environment) globigerina foraminifera became extinct than did species of the spineless (colder environment) globigerina foraminifera. Also many species of calcareous nannoplankton, which prefer tropical waters, became extinct during Late Paleogene time.

The colder temperatures occurred after early Eocene time in several pulses. The temperature remained cold in the Oligocene Epoch through the Neogene Period. This colder climate corresponded to the expansion of polar ice, particularly in Antarctica in the Oligocene Epoch and to the formation of the psychrosphere at the end of the Eocene. The mass extinctions during the Paleogene Period did not occur at the end of the Oligocene Epoch but occurred in about 5 pulses from Middle Eocene through Middle Oligocene. The formation of the psychrosphere (cold deep ocean water) resulted in the extinction of some benthic organisms, such as deep-water foraminifera.

Chapters 19 and 20, The Neogene World (p. 518-557) and The Holocene (p.559-583)

In the alternate classification, the Miocene, Pliocene, Pleistocene Epochs are part of the Tertiary Period and the Pleistocene and Recent (also called Holocene) Epochs are part of the Quaternary Period.

India's northward movement resulted in the subcontinent being sutured on to Asia in Middle Miocene time, creating the Himalayan Mountains which are still forming today. The movement of India was as part of the northward movement of the Australian plate which also contained both Australia and New Guinea and part of New Zealand. While India was suturing on to Asia, Australia and New Guinea were colliding with the Asian plate in the vicinity of Indonesia. The collision brought the distinctly different Australian biota into contact with the Eurasian biota by late Miocene time. The zone of contact of biota, where intermingling has occurred, is south of the Indonesian Island of Sulawesi (Celebes), and is called Wallace's line.

The present Andes have been forming since the Pliocene Epoch as the result of subduction along the western margin of South America.

The Tethys Sea gradually closed in the Miocene with the suturing of Africa with Arabia onto Eurasia. This caused the Tethy Sea to separate into two branches without an outlet into the present Indian Ocean. The southern branch became the Mediterranean Sea and the northern branch, the Paratethy Sea, is represented today by the present Black Sea, Caspian Sea, and the Aral Sea.

During the Pliocene Epoch, isostatic uplift produced block faulting which elevated the Sierra Nevada Mountains in eastern California (originally formed in the Jurassic Period and subsequently eroded down) and during the Miocene Epoch, isostatic uplift raised the Rocky Mountains and the Colorado Plateau (both had originally formed in the Eocene Epoch and had been eroded down). A Miocene extension of rifting from the East Pacific Rise, running up the Gulf of California under the western United States, may have formed the Basin and Range Province of Nevada and the basaltic Columbia Plateau in eastern Washington and western Oregon. The present volcanoes of the coastal Cascades are Pliocene through Recent in age; however, volcanism, associated with the subduction zone along the western coast of northern California to northern Washington, has occurred throughout the Neogene Period. The San Andreas transform fault has been active since the Oligocene Epoch along the western boundary of southern California, connecting the East Pacific Rise in the Gulf of California with the subduction zone north of San Francisco that has produced the Cascades. The fact that the San Andreas has been active throughout the Neogene Period casts some doubt on a Miocene extension of the East Pacific Rise to form the Basin and Range Province and the Columbia Plateau.

The Appalachians were elevated in the Miocene Period by block faulting in response to isostatic uplift.

The initial forming of the present Andes corresponded to the forming of the Isthmus of Panama 3.5 m.y. ago, an event that isolated the Caribbean plate from the Pacific Plate by joining North America to South America. The isthmus of Panama was an island arc formed by subduction (dipping to the east). The occurrence of the Isthmus of Panama allowed plants and animals to migrate between the two continents. South America, like Australia, contained some interesting marsupials and also placentals which had developed since the continent had been isolated from Africa by the end of the Cretaceous Period. North America was not so isolated due to the Bering land bridge with Asia in the Paleogene Period. This same land bridge was sometimes present in the Neogene Period.

Africa was sutured to Eurasia by way of the Arabian Peninsula in early Miocene time, closing the eastern end of the Tethy Sea. At present, Africa is splitting away from the Arabian Peninsula with the opening (rifting) of the Red Sea and the Arabian Sea. This rifting began during the Miocene Epoch. A "dead" zone of rifting, the East African Rift Valley, connects with the active rifting at the intersection of the Red and Arabian Seas and extends southward downs the eastern margin of Africa.

The present Mediterranean Sea dates back to the Miocene Epoch with the closure of the eastern edge of the Tethys by the suturing of Africa and the Arabian Peninsula to Eurasia. The sea then dried up between 6 and 5 m.y. ago, producing evaporite deposits, at the end of the Miocene Epoch as the result of closing of the narrow connection at Gibraltar with the Atlantic Ocean through a natural dam and the drop in sea level due to Antarctica glaciation known as the Messinian Event (discussed below).

Remember that sea level has gradually fallen since the end of the Cretaceous Period. At the end of the Miocene Epoch (about 5 m.y. ago), sea level apparently fell about 50 meters in the Messinian event as the result of major accumulation of glacial ice in Antarctica. Glaciation began about 3 m.y. ago, in the Pliocene Epoch and extended throughout the Pleistocene Epoch, causing sea level to fluctuate by more than 100 meters, i.e., dropping as ice accumulated and rising as ice melted in the interglacial periods. The Recent Epoch is an interglacial period that would probably lead to another ice age without the intervention of man through global warming produced by the greenhouse effect.

The Ice Ages began (as mentioned above) in the Pliocene Epoch about 3 m.y. ago, shortly after North America and South America were connected through the Isthmus of Panama (3.5 m.y. ago). This connection may have something to do with the beginning of the Ice Ages. The modern Gulf Stream (flowing clockwise between Europe and North America in the Atlantic) was strengthened (by the closure of the Panama seaway) and brought more warm water to the north Atlantic, resulting in more evaporation and consequently more snow to fall in the northern hemisphere. As snow accumulated and formed ice on land, the temperature was lowered through the reflection of solar heat (albedo effect) back to space.

Remember that there are several factors, besides ice, which have an effect on the earth's surface temperature. The cyclic nature of the earth's orbit affects the amount of solar heat reaching the earth's surface. An increase in atmospheric volcanic dust will lower the temperature by blocking solar radiation from the sun. An increase in atmospheric carbon dioxide will raise the temperature by preventing solar heat reflection. Carbon dioxide in the atmosphere should increase during an ice age as plants die back from the colder temperatures, helping to end an Ice Age. Remember that plants (in photosynthesis) use up the carbon dioxide produced by animals (in respiration).

The major centers of accumulation of glacial ice were on Antarctica, Northern North America, Greenland, and in Scandinavia. Continental ice sheets did not occur in South America, Australia, and Africa. Today we still have continental ice sheets on Antarctica and Greenland. The pulses of continental glaciation occurred about every 100,000 years with smaller cyclic variations every 20,000 to 25,000 years. These cycles often appear to conform to astronomical cycles corresponding to the 92,500 year cycle in the shape of the earth's orbit as it varies from elliptical to nearly circular and a 22,000 year cycle relative to the orientation of the earth's rotational axis as it varies where it points (the north end presently points at Polaris). There is also another cycle of about 41,000 years which reflects changes in the angle of the earth's rotational axis with the orbital plane of the earth, i.e., changing the position of true north in the sky. There does appear to be a relationship between the earth's astronomical cycles and the earth's surface temperature. However, something else began the sequence of Ice Ages because the Ice Ages have only occurred at infrequent times throughout the earth's history. The earth's surface temperature was only about 5^oC (9^oF) colder on the average, than today, during the periods of glaciation.

The time periods of glaciation have been recorded in the oxygen 18 to oxygen 16 ratio in the shells of marine life, preserved in deep sea sediment. The shells of globigerina foraminifera (zooplankton) have been used to date periods of glaciation as a function of age. The ¹⁸O/¹⁶O ratio increases in shells during times of glaciation on the continents. This is because the heavy oxygen 18 isotope remains preferentially behind in ocean water during the preferential removal of the lighter oxygen isotope in water vapor evaporated from the oceans. The glacial ice on land forms primarily from water vapor from the oceans, increasing the ratio of oxygen 18 to oxygen 16 in the ocean water.

Preserved pollen of terrestrial plants and different beetle species (found in fine-grained lacustrine sediments) have also been used to define the changes in climate on the continents during the Ice Ages.

The last major interval of glaciation in the Pleistocene Epoch is called the Wisconsin and it was separated by the second to last major interval of glaciation, the Illinoisan, by the Sangamon Interglacial Interval. Both the Wisconsin and the Illinoisan Glacial Periods consisted of two pulses of glaciation.

Fresh-water diatoms (phytoplankton), which had evolved in the Paleogene, expanded for the first time in the Miocene Epoch, leaving their silica shells (which look like tiny pill boxes) in the non-marine rock record.

Remember that diatoms like colder waters. Thus the marine diatoms tended to expand while the calcareous nannoplankton (phytoplankton) retrenched when the marine surface waters became colder during the Ice Ages of the Pliocene and Pleistocene Epochs.

Globigerina foraminifera (zooplankton) expanded in the Miocene Epoch after their major extinction at the end of the Eocene. Interestingly, the new species resembled the extinct Eocene species, an example of iterative evolution, in which the basic body structure limits the direction of evolution, so that the organism may re-evolve along a similar (but not identical) path.

Calcareous algae or coralline algae evolved into forms that could cover the seaward edge of a reef with an algae reef, allowing coral reefs to grow along coasts pounded by heavy surf.

Modern whales underwent adaptive radiation in the Miocene Epoch, and dolphins (which are whales) first appeared in this epoch.

Herbs, which are non-woody plants that die back to the ground after releasing seeds, underwent adaptive radiation in the Neogene Period, in response to the continuing trend towards a drier climate that started in the late Paleogene. Note that grasses are non-woody plants with hollow jointed stems and narrow, sheathing leaves. Remember that grasses evolved in the Paleogene Period and first underwent adaptive radiation in Oligocene Epoch in response to the climatic trend towards drier conditions.

The trend in the Paleogene Period towards a drier climate continued in the Neogene Period through the end of the Pliocene Epoch, in which grasslands were favored at the expense of woodlands. The cooling trend in the Paleogene Period, leveled off in the Miocene and Pliocene Epochs with the climate only becoming slightly cooler until the Ice Ages in the Pleistocene Epoch.

The Neogene Period was characterized by adaptive radiation of frogs and toads (amphibians), snakes (reptiles), songbirds (birds), and rodents (mammals). The adaptive radiations can be related to the expansion of new species of grasses and herbs as the climate became drier. Rodents eat the plants and are in turn eaten by the snakes. Frogs and toads eat the insects which expanded to take advantage of new species of grasses and herbs. Songbirds evolved to eat insects feeding on the plants and the seeds of the plants.

The odd-toed ungulates have undergone a gradual decline in numbers of species during the Neogene Period. The even-toed ungulates continued to expand in the Miocene Epoch in response to the increase in grasslands but have since undergone a gradual decline in species. Carnivorous animals which fed on the ungulates underwent adaptive radiation in the Miocene Epoch as their food supply increased.

Elephant species were diverse during the Miocene and Pliocene Epochs, including the mastodons (with long jaws, tusks in both jaws, low-crowned teeth) and mammoths (with shorter jaws, tusks in only the upper jaw, and fewer grinding teeth). Modern elephants evolved from the mammoths. Mammoths and mastodons only became extinct near the end of the Pleistocene Epoch (10,000 years ago), perhaps due to overhanging by early humans.

During the Pleistocene Epoch, the large size of the mammals may have evolved due to the cooler climate. The larger the animal, the smaller the heat loss because large animals have smaller surface area for their volume than do smaller animals.

The elevation of the Isthmus of Panama occurred about 3.5 m.y. ago during the Pliocene Epoch, allowing a great faunal exchange between the North America and South America. South American marsupials, e.g., opossum, and unusual placentals, e.g., anteaters and armadillos, moved northward. The North American species moving south included most modern placental animals.

Monkeys were present in the Oligocene Epoch. The Old World monkeys lived in Eurasia and Africa. The New World monkeys appeared in South America just before the end of the Oligocene Epoch and possessed prehensile (or grasping tails). Another interesting and not well-known difference is that New World monkeys have wet noses and Old World monkeys have dry noses. Both Old World and New World monkeys underwent adaptive radiation in the Neogene Period. Apelike primates were also present in the Oligocene Epoch and flourished in the Miocene Epoch.

Apes and hominids separated from orangutans about 10-11 m.y. ago. Apes separated from hominids separated from each other about 5 m.y. ago,

Humans belong to the same superfamily, the Hominoidea, as do apes. The Hominidae family contains today only the single human species (Homo sapiens) and has a fossil record extending back about 4 m.y. ago in the Pliocene Epoch, beginning with Australopithecus afarensis which evolved into Australopithecus africanus about 3 m.y. ago that lived until about 2.5 m.y. ago. The Australopithecus species had a heavy brow and a low sloping facial region below the eyes and much smaller brains than modern humans; however their pelvis was designed to support an upright body. Their teeth were intermediate between apes and humans. The fossil record of the Australopithecus species are confined to Africa.

The Homo genius was derived from one of the later Australopithecus species (probably Australopithecus africanus) and has fossil records extending back two million years. Most of the species of this genius also appear to have evolved in Africa. The oldest species was Homo habilis which had a relatively large brain, 650 to 800 cubic centimeters, as contrasted with the smaller brains of the Australopithecus and the much larger 1330 cubic centimeters of modern humans. The evolution of a larger brain lies in continued growth of the brain cavity after birth. Other primates are born with a similar brain-size cavity at birth (relative to the total body size); however, their brain cavity growth is minimal after birth. Members of the Homo genius had the ability to make tools, a skill supposedly lacking in the earlier Australopithecus species; however, this may not have been the case.

Homo habilis had similar facial features as the Australopithecus species and eventually evolved into the larger Homo erectus about 1.6 m.y., about the time that it became extinct. Homo erectus had a larger brain size, ranging from 800 to 1300 cubic centimeters. They produced magnificent tools such as hand axes. Their tool culture is called Acheulian. The body structure of Homo erectus contains a narrow pelvis, suggesting endurance during locomotion. Their fossil record is extensive throughout Eurasia and Africa, extending to 300,000 years ago. Their fossils have been known by the terms: Java man, Peking man, and Pithecanthropus.

Modern humans (Homo sapiens) apparently evolved from Homo erectus. The pelvis of modern humans is wider in order to allow for the birth of children with much larger heads than those of Homo erectus. We have sacrificed locomotion skills for brain power during evolution.

The modern human fossil record of Homo sapiens extends back about 100,000 years with Neanderthal (Homo neanderthalensis or Homo sapiens neanderthalensis) and Cro-Magnon (Homo sapiens). Unfortunately, the poor fossil record between 400,000 and 100,000 years ago has obscured the evolutionary record of Homo sapiens from Homo erectus. Homo sapiens are thought to have originated about 250,000 years ago.

Neanderthals may have been a different species or a subspecies of modern humans. Neanderthals partially overlapped with Cro-Magnon. Their extensive fossil record in Eurasia suggests they evolved from Homo erectus in Europe. The early fossil record of Cro-Magnon suggests evolution from Homo erectus in Africa. Neanderthals were physically more massive individuals than Cro-Magnon with long, low, sloping foreheads, having a prominent brow ridge, a projecting mouth, and a receding chin. They also had a slightly larger brain (on the average) than Cro-Magnon and had religion. They did not appear to interbreed with Cro-Magnon and eventually died out during the last inter-glacial time about 35,000 years ago. The Neanderthal stone culture is known as Mousterian. Neanderthals did not make it across the Bering land bridge into North and South America.

Cro-Magnon has a fossil record extending as far back as Neanderthal. Their culture is known as Late Neolithic and they produced magnificent cave paintings. Cro-Magnon are our direct ancestors. Humans were present in the New World at least 30,000 years ago, having crossed the Bering land bridge. The extinction of many of the large mammals existing at the end of the Pleistocene (about 10,000 years ago), such as mastodons, mammoths, elephant-size bison, giant beaver may be related to hunting pressure from humans using newly developed weapons.

Chapters 19 and 20 - Review Questions - Neogene Period and Quaternary Period (Holocene Epoch)

Chapter 1	Chapter 11
Chapter 2	Chapter 12
Chapter 3	Chapter 13
Chapter 4	Chapter 14
Chapter 5	Chapter 15
Chapter 6	Chapter 16
Chapter 7	Chapter 17
Chapter 8	Chapter 18
Chapter 9	Chapter 19
Chapter 10	Chapter 20