The Hagfish or Slime Eel, a modern Craniate related to the Conodont-bearing organism known from Cambrian-Triassic age rocks.
Myxine glutinosa L., figb0523, Historic NMFS Collection, NOAA

Invertebrate Paleontology Lab #12
Hemichordates (Graptolites), Craniates (Conodonts) and Selected Microfossil Groups  
Click on the lab title to see the University of California Museum of Paleontology web page

       Read BEFORE Coming to Lab: Benton & Harper, p. 409-423, 430-435, and 208-218 

Introduction

    This week we will explore two fascinating fossil groups that provide well known index fossils in biostratigraphy, and yet are still somewhat mysterious organisms.  At this point in our march through the invertebrate phyla, we have reached the groups known as the Hemichordate Phylum and at long last, the Chordate Phylum, which includes the vertebrates.  Both the graptolites, which are hemichordates, and the conodonts, which are remains of members of the craniate subphylum in the Chordates, were in use as major tools in biostratigraphy long before systematists had clearly identified what they were!
 

    The Hemichordate Phylum ("half-chordates") are a group of invertebrates that have certain features also shared by the Chordate Phylum, including a pharynx ("throat") with multiple openings,  a dorsal nerve chord and a ventral blood vessel, but unlike the Chordates, they have no notochord.  A notochord is a stiff supporting rod that runs dorsally, just above the dorsal nerve chord in the Chordates, and is part of the vertebral column in the vertebrates.  Instead of a notochord, Hemichordates have a tube that is developed from the gut (intestines), that may be a precursor of the notochord. Living hemichordates include the acorn worms and the pterobranchs.  The fossil hemichordates of interest to us this week are the Graptolites (Late Cambrian-Pennsylvanian, with most occurrences in the Ordovician-Silurian).

Why the fuss about graptolites?

They are ideal index fossils:  abundant, rapidly evolving, easily fossilized (carbonized), wide ranging (planktonic life style), distinctly identifiable.
They are the basis for the biostratigraphy of the Ordovician and Silurian rocks.

What are they?

Graptolites were colonial filter feeding organisms that apparently floated in surface ocean water in a range of depths.  A colony (called a rhabdosome) consisted of bunches of branching  structures called stipes that are covered with tiny tubes or cups called thecae.  In each cup was an individual zooid.  The base of the rhabdosome is the sicula, which is the first zooid theca that founded the colony.  Sometimes there is a long stem emerging from the sicula called the nema, that probably attached the stipe to a floating structure.  The branches or stipes are often found fossilized as carbonized remains. Graptolites did not produce a shell or calcium phosphate endoskeleton.  They were simply tubes and cups of chitin, connected into a branching colony and attached to a float of some kind.

    The Chordate Phylum includes several subphyla (such as the vertebrates), and one of those subphyla is that of the Craniates-those chordate animals that have a brain and clearly defined head region, although no vertebra:  instead, they have an internal skeleton of cartelage to support them.  Living craniates today include the lamprey eel and the hagfish or "slime eel" (see picture at top of page). Our interest in such craniates as the hagfish comes from the kind of teeth we find in them.  Hagfish have tiny microscopic tooth-like structures on their tongues which they use in a kind of rasping process.  The hagfish tooth structures are very similar to microfossil structures found in Cambrian-Triassic rocks called Conodonts.   However, unlike true teeth, these tiny structures were apparently always covered in tissue:  that is, they show no wear.  They are composed of calcium phosphate, a typical bone material, and have cellular bone material in them (Sansom et al., 1992).  These two features imply that they are remains from some chordate animal.  Unfortunately, the animals that produced these structures have never been definitely identified-the closest link has been to a hagfish-like fossil from the Lower Silurian in Wisconsin (Mjickulic et al., 1985; Smith et al., 1987).

Since we aren't all that sure what animal they came from, how are conodonts used?

Conodonts are extremely useful as biostratigraphic tools in correlation, because they are great index fossils.  They are very abundant in Paleozoic rocks, and widely distributed, rapidly evolving, and easily identified.

Conodonts change color with thermal metamorphism.  As the sedimentary rocks that enclose them are subjected to increasing temperature, conodonts will change from pale yellow to darker brown to black.  This means they can be used to judge the temperatures at which a potential hydrocarbon reserve has been subjected to, and ultimately, if those rocks are potential targets for hydrocarbon recovery.

MICROFOSSILS:  Examining Foraminifera, Radiolaria, and Ostracoda

   Foraminifera photo by D. B. Scott           Ostracode:  Heterocypris      Radiolarian shown with
           Centre for Marine Geology, Dalhousie    fretensis, Photo by Alison     permission from the Univ.
           Univ., Halifax, Nova Scotia, Canada       J. Smith, Kent State Univ.   Cal. Mus. Paleo. at Berkeley

Microfossils may be small, but they are tremendously useful to paleontologists.  This is because microfossils are 1) so abundant that statistics can be applied to their distributions; 2) tiny samples yield thousands of specimens, and 3) many can be used not only for paleoecological information but also for geochemical information (stable and radiogenic isotope data, trace elements, heavy metals...many possibilities here to explore). There are MANY important groups of microfossils, but we will be looking at only three groups today:  the Foraminifera and the Radiolaria (both groups are Protistans) and the Ostracoda (microscopic crustaceans).  If you are intrigued by these fossils, you might want to consider a Micropaleontology course someday.

Kingdom Protista
Phylum Sarcomastigophora
Subphylum Sarcodina
Class Granuloreticulosa
Order Foraminiferida  Foraminifera Cambrian-Recent

The forams are marine protists with a fossil record that extends back to the Cambrian.  These single-celled organisms are an important part of the food chain in the oceans-they are zooplankton, feeding on diatoms and other microorganisms, and some groups also harbor symbiotic algae.  Most are benthic, but an important group, the Order Globigeriina, are planktonic and used in paleoclimate and oxygen isotope studies of the ocean.  Fossil forams from Cambrian rocks had shells composed simply of an organic tube with various sand grains glued to it (agglutinated shell).  However, by late Paleozoic time, several types of shell appeared, including a very large form, rice-grain shaped, belonging to the Fusulinid forams (be sure to look at these in the type collection).  Foram protists move and capture prey using pseudopodia called reticulopodia.  The shell, known as a "test" is species specific in its design, and is the basis for an enormous literature on the biostratigraphic applications of forams.  Most foram shells are composed of calcium carbonate or agglutinated sand grains, but the wall structures and designs are quite elaborate and varied.  Looking carefully at a foram, you will be able to see the opening in the shell (the aperture), and the individual chambers and suture lines of the shell.  Note that some are planispirally coiled, others are trochispiral, as in snail shell coils, and still others are composed of linear rows of chambers (uniserial-one row, biserial, 2 rows, or triserial, 3 rows).  In life, the cellular material extends throughout the entire test, and continues in a layer around the outside of the test as well.  Also, in life many of these forams have long, delicate spines that are later broken off after death.
 
 

Examine the forams in the teaching collection and draw any two slides/specimens, labelling the aperture, chamber(s), and suture lines

Foram shell (test) shapes, image shown with permission from the
University of California Museum of Paleontology

Photo Credit:  D.B. Scott, Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada
 
 
 

Kingdom Protista
Phylum Sarcomastigophora
Subphylum Sarcodina
Class Granuloreticulosa
Subclass Radiolaria Cambrian-Recent

    The Radiolaria are also microscopic marine protists that range throughout the world oceans.  However, the shells of these protists are not made of calcium carbonate, but instead are composed of opaline silica (hydrated silica).  These are about half the size of forams, ranging in size from 60 to about 200 microns. Radiolarians show strong vertical stratification in ocean waters to depths of 1,000 m, and are also highly abundant:  enough so to produce what is known as "radiolarian ooze" sediments on the ocean floor!  Their abundance is strongly tied to deep ocean upwelling, where nutrients and dissolved silica are abundant.  Some radiolarians can live in nutrient poor water by keeping symbiotic algae (just as some forams do).  Two common radiolarian Orders are the Spumellaria (radial-spherical symmetry) and the Nasellaria (conical symmetry).  Radiolarians are important microfossils in paleoclimate analysis, and have been used to reconstruct oxygen isotope records of the past conditions of the oceans, just as has been done with forams.
 
 

Examine the radiolarians in the teaching collection and draw a radiolarian shown on the Radiolarian strewn slides.  Indicate whether it is a Nasellarian or Spumellarian radiolarian.

Nasellarian            Spumellarian 
 

Here is a Nasellarian radiolarian (conical symmetry) and a Spumellarian (spherical symmetry).  Images shown with permission from the University of California Museum of Paleontology.
 
 

Kingdom Animalia
Phylum Arthropoda
Subphyum Crustacea
Class Ostracoda Ordovician-Recent

    The ostracodes are microcrustaceans that produce a hinged calcite shell, often ornamented, in which they can completely close themselves up.  They live in every aquatic environment, from marine to freshwater to groundwater to wetlands, with the exception of very acidic water.  They are slightly larger than the forams (typically 500-700 microns), and can also be used as both paleoecological and geochemical data in paleoenvironmental and paleoclimatic reconstructions.  The shell is commonly preserved, although often disarticulated into two valves.  When examining these bean shaped valves, look for the adductor muscle scars.  Other scars are sometimes visible, including the dorsal muscle scars.  Ostracodes are sensitive to temperature and salinity in the deep ocean, and are used to map the changing position of water masses through time.  In fresh water environments, ostracodes are sensitive to water composition as well as temperature.  Their rapid speciation, wide distribution, and commonly preserved valves make them excellent index species.
 
  To see images of non-marine ostracodes in the NANODe (North American Non-Marine Ostracode Database, click here.
 

References

Mjickulic, D.G., Briggs, D.E., and Kluessendorf, J., 1985.  A Silurian soft-bodied biota, Science, 228-714-717.

Sansom, I.J. and others, 1992.  Presence of the earliest vertebrate hard tissues in conodonts.  Science, 256, p 1308-1311.

Smith, M.P., Briggs, D.E., and Aldridge, R.J., 1987.  A conodont animal from the lower Silurian of Wisconsin, U.S.A. and the apparatus architecture of panderodontid conodonts, p 91-104 in Aldridge, R.J., (ed.) Palaeobiology of Conodonts, Ellis Horwood, Chichester.