LECTURE NOTES: Early Paleozoic Era: Cambrian, Ordovician and Silurian

Paleogeography of the Cambrian

Early in this period four major land masses developed after the breakup of Rodinia:

Laurentia, Baltica, Gondwana and Siberia.

PaleoMap Project (www.scotese.com)

The Cambrian (544-495ma)

Largest explosion of life occurs here dominated by "small shelly fossils". Cambrian marine life dominated by invertebrates with hard parts like trilobites and inarticulate brachiopods.

The Burgess Shale found in the Canadian Rocky Mountains contains classic examples of organism from this period.

The first jawless is found in rocks of this period.

No evidence glaciation has been found. This explains the higher sea level and formation of many inland (epieric) seas and deposition of limestone.

The Ordovician Period (495-443ma)

The largest continental land mass Gondwanaland starts moving northward

PaleoMap Project (www.scotese.com)

Taconic Orogeny

Laurentia experiences a major island arc collision resulting in the first of three mountain building events producing the Appalachian Mountains. This is referred to as the Taconic Orogeny

Life during the Ordovician

Life during this period is still dominated by marine life: trilobites, brachiopods, conodonts and graptolites.

New evidence (moss-like spores) suggests that plants may have populated land during this time

First fish fossil was found armored fish informally called ostracoderms is an abundant freshwater species, lacked a jaw for chewing & has a platy exoskeleton.

Ordovician Climate

There is also evidence that climate grew colder as Gondwanaland moved over the south pole.

Massive glaciation causes sea level to drop and drying out of the inland seas and reducing the numbers of marine species by 60%. This may have lead to the disappearance of trilobites.

At the end of the Ordovician another extinction may be explained by warming and loss of cool water species

Silurian: 440 - 410 mya

As we heard during the Case Study Project on Silurian reefs, this period is characterized by stabilizing climates a rise in sea level by melting glaciers.

In the shallow seas were coral reefs constructed by rugose and tabulate corals with stromatoparoid sponges. Over time, these structure grow in sized and become lithified by carbonate sediments.

By Mid-Silurian

PaleoMap Project (www.scotese.com)

Silurian Salt Deposits

Reduced circulation within the inland sea’s produced large evaporites (salts) deposits in Michigan Basin and under the Great Lakes.

Silurian marine and freshwater fish

Paleontologists find the first evidence for wide spread radiation of jawless fish and the first appearance of freshwater, jawed fish (acanthodians)

The earliest know land plant (Cooksonia) and animal life on land is well preserved in rocks from this period.

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