The Hidden World of the Conodonta
W. Britt Leatham
Department Geological Sciences, California State University San Bernardino
Reprinted with permission from The Earth Scientist. The Journal of The National Earth Science Teachers Association. Volume 11, Number 1, Winter 1994, pages 17-20.
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In the fall of 1978, I was given a choice of either of two senior research projects in paleontology: classifying a suite of graptolites from Idaho, or determining whether or not a couple of bags of rock contained conodonts. That simple decision has sent me down one of the most satisfying roads in my paleontological career. Conodonts intrigue me and over 150 specialists who try to decipher their hidden world, often with little more than a very fine brush and a microscope. Although their secrets are understood by few, many if not most geologists recognize their utility as guide fossils for the Paleozoic and Triassic, as well as for predicting petroleum production on possible source rocks. What is it that makes the Conodonta so fascinating and so useful?
No Living Representatives
One of the reasons dinosaurs are so popular is because they are extinct. Except for birds, there are no living dinosaurs. Conodonts are a lot like living dinosaurs. They are not as big (by orders of magnitude) as dinosaurs, but like the dinos, there are no living models to use for reconstructing their paleobiology.
The Conodonta are an extinct phylum (Sweet, 1988) of small, bilaterally symmetrical vermiform (worm-like) animals. Almost the entire fossil record of Conodonta consists of small, isolated, microscopic, calcium fluorapatite "tooth- and jaw-like" structures, termed "elements", located in the head region of the animal. Conodont elements occur in a wide variety of shapes and are commonly found as discrete microscopic grains in most Late Cambrian-Triassic marine sedimentary rocks.
All conodont elements are basically modifications of phosphatic laminations draping a conical basal cavity. The basal cavity is usually capped by a large tooth-like projection termed the cusp, although the cusp can be relatively inconspicuous in many elements. Conodont elements grew through the external addition of layers of apatite around the basal cavity, which suggests that the elements grew within an epithelial pocket in the conodont. Unequal lateral, anteroposterior, and vertical addition of apatite laminations away from the basal cavity produced a wide array of element shapes. Many of those expansions, termed processes, are crowned with small denticles, nodes, or ridges.
Figure 1. Representative Late Ordovician conodont elements.
a) Outer circle clockwise from 12 o'clock: ramiform (Plectodina tenuis), rastrate (Culumbodina occidentalis), rastrate (Pseudobelodina kirki), ramiform (Plectodina tenuis), ramiform (Amorphognathus ordovicicus), ramiform (Phragmodus undatus), pectiniform (Plectodina tenuis), rastrate (Pseudobelodina vulgaris), ramiform (Oulodus ulrichi), ramiform (Rhipidognathus symmetricus), ramiform (Rhipidognathus symmetricus).
b) Inner circle clockwise from 12 o'clock: coniform (Coelocerodontus trigonius), coniform (Staufferella brevispinata), coniform (Scabbardella altipes), coniform (Staufferella brevispinata), coniform (Walliserodus amplissimus), coniform (Panderodus sp.).
c) Central element: pectiniform (Amorphognathus ordovicicus).
The simplest conodont elements are coniform, characterized by a single cusp that lacks denticles on either a posterior or lateral processes. Modifications of the basic conical structure include rastrate, ramiform, and pectiniform elements. Rastrate (rake-like) elements are simply large cusps with denticulated or serrated posterior margins. Ramiform (ray-shaped) elements have posterior, anterior, and/or lateral processes that are often denticulated. Pectiniform (comb-shaped) elements include not only denticulated blade-like elements that are either straight or bent, but also include plate-like, or platform elements. Recurrent collections of elements and a few clumps of grouped elements (fused clusters and bedding-plane assemblages) suggest that individual conodonts possessed more than one element type or shape, and that most conodont elements are either rights or lefts, or rarely bilaterally symmetrical. This array of elements, termed an apparatus (see Figure 2), is believed to be used for food-gathering and/or food-processing by the conodont. For many years, most interpretations of the paleobiology of conodonts was limited to interpretation of collections of isolated elements. Each element type was thought to represent a single species. Consequently, reconstruction of the apparatus resulted in a better understanding of conodont paleobiology, evolution, and paleoecology.
Figure 2. SEM photomicrographs of partial apparatus of Plectodina tenuis. Four dots in circle are approximately actual size of the illustrated specimens.
Reconstructing conodont apparatuses is the best understood through analogy. Assume an alien mega-delivery/pick-up service from a nearby star system lands in Wyoming, removes all four-footed animals from Yellowstone National Park and places them into the ship's single on-board processing vat. Their soft tissues are chemically removed and the assorted skeletal remains are bagged and tagged. The assorted bones are then shipped to an alien exobiologist on its home planet for analysis. The alien biologist, never having seen a bison, moose, elk, bear, rat, mouse, shrew, rabbit, bird, lizard, frog, etc., must then attempt to reconstruct the skeletal anatomy of each animal and interpret their lifestyles, without actually seeing any living specimens or articulated skeletons. The task is very similar to interpreting discrete assemblages of conodont elements.
Recent discoveries of soft-bodied impressions, housed in an aging museum collection in Edinburgh, Scotland, verify many of the original apparatus reconstructions (Briggs and others, 1983; Aldridge and others, 1986; and Smith and others, 1987). Observations of clusters of fused elements, and of natural bedding plane assemblages, either fecal or preserved in the gut of conodontophagous (conodont-eating) predators, also vindicate previous apparatus reconstructions. Consequently, several systematic classifications are in development (e.g. Sweet, 1988) that illustrate relationships between various species.
Conodonts Were Successful
The Conodonta successfully exploited Paleozoic and Triassic oceans and seas for over 300 million years, only to disappear as the supercontinent Pangaea slowly separated to the modern configuration of continents that adorn the walls of our classrooms. Because conodonts could successfully adapt a wide variety of marine environments, very few samples of late Cambrian through late Triassic marine rock do not contain at least a few elements. Conodont elements evolved rapidly and provide a useful tool for correlating coeval marine rocks and determination of relative age, as well as for identifying questionable Paleozoic strata. Indeed, the late Cambrian through late Triassic is divisible into over 150 conodont biozones. The sheer number of conodont biozones established that they are the "Ruling Fossil Group" of late Cambrian-late Triassic marine biostratigraphy, clearly surpassing other "classical" Paleozoic index fossils (e.g. graptolites, brachiopods, trilobites, foraminifera, rugosa) in their overall biostratigraphic utility.
Remnants of the oldest conodont faunas consist primarily of "simple" coniform elements. Coniform elements are a conspicuous fraction of Late Cambrian-Early Middle Ordovician conodont faunas. By the Devonian, only a few coniform-bearing species persisted, including Icriodus, Panderodus, and Dvorakia. Coniform elements are not present in Carboniferous-Triassic faunas.
Conodonts with ramiform and pectiniform elements first appear in the late Cambrian and exploited the marine realm until their disappearance in the late Triassic. What happened to them? The simplest answer is that they became extinct. A former Chief Panderer (appointed head of the Pander Society, the organization devoted to conodont study) once remarked, "Old conodonts never die. They just lose their apatite!" If that is true, then it is only a matter of fossil preservation, although there are no close relatives in the soft-bodied modern marine fauna!
Although certain coeval conodonts occur in a wide variety of rock types representing various marine environments, others were apparently restricted to certain types of environmental conditions and consequently can be used for paleoecological and paleogeographical reconstructions and interpretations. For example, early Silurian offshore conodont faunas are dominated by coniform elements, whereas coeval onshore conodont faunas are dominated by ramiform and pectiniform elements.
In its unaltered state, conodont phosphate is typically a light amber color. Heat due to deep burial, intrusion, or regional metamorphism effectively devolatizes small inclusions of organic compounds between element laminae, making conodont elements darker than their unaltered counterparts. Comparison of elements suspected for thermal alteration with a set of either laboratory or field reference specimens is often used to identify gas and/or oil generation in potential hydrocarbon source rocks.
Collection and Preparation
Conodont elements are ideal fossils for Earth Science classrooms. Not only are they intriguing because of their fantastic shapes and barely microscopic size, they lack recognizable close descendants in the modern biosphere (that's why they are an extinct phylum and how many "toothed worms" have you seen recently)! They can be readily extracted from almost any Paleozoic-Triassic marine rock (simply consult a geological map for such areas near your campus) using simple, inexpensive and comparatively safe laboratory procedures. Because of their size, hundreds to thousands can be stored on one cardboard microslide.
Preparing rock residues for conodont studies is typically a three-step process: 1) disaggregating or decomposing the entombing rock; 2) sieving the wet residue: 3) concentrating the conodont elements from the dried residue. Conodont elements are perhaps easiest to extract from carbonate rocks (i.e. limestones and dolostones). It is usually best to work with about 3 kilograms of rock, especially if you are not sure of the abundance of conodont elements in any particular sample. A three kilogram sample may yield 10's of thousands of elements, although most samples yield 10-100 elements.
Carbonate rock matrix is easily digested by some weak acids, such as acetic acid (HCOOCH) or vinegar, without appreciably decomposing the phosphatic elements. Do not use hydrochloric acid, because it dissolves conodont elements. Crush the rock to about 1-2 cm chunks using a hammer, or large iron mortar and pestle, to increase the surface for more rapid dissolution. Immerse the crushed rock in a 10-15% acetic acid solution in large wash buckets or floor wax containers from the local custodian (5 gallons). About 15-20 milliliters of solution are required to dissolve 1 gram of carbonate, and about 3 kilograms of rock can be dissolved in 8 liters of acid solution.
Wet sieve the insoluble residue on a 100 to 140 mesh (holes per inch) screen, sediments, using a gentle stream of water and being careful not to squirt the residues out of the screen. Thoroughly rinse the residues into folded industrial paper towels, coffee filters, or filter paper and dry them in a warm oven.
Insoluble rock residues can be examined immediately if they are comparatively small. Several methods for concentrating conodont elements (e.g. differential density using "heavy liquids" and magnetic separation) from the insoluble residue as outlined in (Feldman and others, 1989; and Austin, 1987) may speed the picking process considerably.
Using a binocular dissecting microscope, carefully the spread the residue one grain deep on a picking tray. Paint a portion of a cardboard microslide with envelope glue (use an unused envelope) and allow it to dry. Use a wetted 000 to 00000 sable hair brush (nylon does not work well) to remove elements from the tray. The capillary action of the brush causes elements to cling to the bottom, and they can be placed on the glued microslide. Remove any other interesting clasts you find as well. Picking is actually an enjoyable activity, which I equate with fossil hunting in the laboratory!
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Sweet, W.C. (1988). The Conodonta: Morphology, taxonomy, paleoecology, and Evolutionary history of a long-extinct animal phylum. Oxford Monographs on Geology and Geophysics No. 10. Oxford, England: Clarendon Press, 212 pp.
Aldridge, R.J. (editor). (1987). Paleobiology of Conodonts. Chichester, England: Ellis Horwood, 180 pp.
Feldman, R.M., Chapman, R.E., and Hannibal J.T. (editors). (1989). Paleotechniques. The Paleontological Society Special Publication No. 4. 358 pp.
Austin, R.L. (editor). (1987). Conodonts: investigative techniques and applications. Chichester, England: Ellis Horwood, 422 pp.
Briggs, D.E.G., Clarkson, E.N.K., and Aldridge, R. (1983). The conodont animal. Lethaia, 16, 1-14.
Aldridge, R.J., Briggs, D.E.G. Clarkson , E.N.K., and Smith, M.P. (1986). The affinities of conodonts- new evidence from the Carboniferous of Edinburgh, Scotland. Lethaia, 19 (4), 279-291.
Smith, M.P., Briggs , D.E.G. and Aldridge, R.J. (1987). A conodont animal from the lower Silurian of Wisconsin, USA and the apparatus of panderodontid conodonts. in R.J. Aldridge (ed.), Paleobiology of Conodonts, p. 91-104, Chichester, England: Ellis Horwood.
Reprinted with Permission from The Earth Scientist. Figure 2 is different from the printed version because of problems encountered with scanning detailed lines. The background is a tiled embossed photomicrograph of the conodont species Cahabagnathus sweeti from the middle Ordovician of central Nevada.