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) and Craniates (Conodonts)
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 (represented in the fossil record by graptolites)
 

    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 (represented by the Vertebrates, but also by the CRANIATES, which left tooth-like structures called CONODONTS)

    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.

http://www.afsc.noaa.gov/race/media/photo_gallery/photos/Myxinidae/hagbill.jpg

Living hagfish-this species is Eptatretus deani, being dealt with bravely by an employee of the Alaska Fisheries Science Center-NOAA

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.