Fossil of the month: Orthacanthus?

Orthacanthus Banner
This month's fossil is an extremely tiny type of fossil shark tooth. Orthacanthus is one of several types of ancient, strange-looking sharks called xenacanthids. Xenacanthids inhabited some of the ponds and streams of the ancient coal swamps that formed Kentucky's coal beds. Because shark skeletons are composed of cartilage, rather than bone, theirem/1.5 teeth are often the only fossil evidence of these ancient sharks. 

Description. Orthacathus is the name of an ancient shark, the teeth of the shark and the spine of the same shark. Orthacanthus belongs to an extinct group of ancient sharks called xenacanthids. Xenacanthid teeth are usually tricuspid, which means they have three cusps on their crown. Usually, the two outer cusps are the largest. They may be spike-like or gently curving. The cusps are positioned on the labial (outside of mouth) side of a broad tooth base. Details of the cusps and the base are used to identify different genera and species of xenacanthid teeth.

Orthacanthus teeth are tiny to small (2 mm to 3 cm). They have two relatively large cusps on the outside of the base, and a much smaller medial cusp between. Often one of the lateral cusps is larger than the other (called a primary and secondary cusp). In some teeth, the smaller medial cusp is missing or so small it is difficult to see. The larger cusps are lenticular in cross section and often exhibit a tiny serration along their edges. Some xenacanthids have lenticular cusps, while others are more rounded. The lingual (inside of mouth) side of the base has a small, raised mound on top, called an apical (or coronal) button, and a raised button or ridge on the base, called the basal tubercle. Tiny holes in the base are termed foramens (Hotton, 1952; Lund, 1970; Johnson, 1999; 2012; Hampe, 2003).

Orthacanthus Tooth Parts
Terms used to describe parts of xenacanthid teeth, with views of Orthacanthus as an example. Compare the parts to the picture of the possible Orthacanthus? specimen from eastern Kentucky. The Kentucky specimen is one of several found in a dark gray shale in the Four Corners Formation, Breathitt Group in Perry County. Terms and general diagrams based on illustrations in Schneider (1996), and Johnson (1999; 2012).

Different types (genera) of xenacanthid sharks. Rare fossilized cartilage and soft-part preservation of xenacanthid skeletons is known from other areas, so scientists have a good understanding of what several genera of these ancient sharks lookedem/1.5 like (e.g., Schneider, 1996). Xenacanthids were characterized by (1) a spine (called an occipital or dorsal spine) that stuck out of the back of their heads, (2) a dorsal (back) fin that was a long, low-lying continuous fin on their back, rather than the tall triangular dorsal fins most people picture when they think of sharks, and (5) an eel-like tail (among other characteristics). Different genera had slight differences in their teeth and dorsal spines. More information about different distinguishing xenacanthid genera.

The teeth shown here from eastern Kentucky are tentatively identified as Orthacanthus? but they could be Xenacanthus [which is why we put a question mark at the end of the genus name]. Both Orthacanthus and Xenacanthus have apical buttons of similar shapes with similar positions. Orthacanthus typically has one very small medial cusp, while Xenacanthus may have multiple medial cusps, or a single medial cusp that is larger than the medial cusps in Orthacanthus (e.g., Johnson, 1999). Differences in the underside of the base can help identify these two genera, but the specimens shown here are in matrix rock, so the undersides of their bases are hidden.

Other Specimen of Orthacanthus
Several tiny teeth have been found in the same deposit as the fossil of the month. Several appear to be Orthacanthus? but others could belong to different xenacanthid genera. For now we list them as unspecified xenacanthid teeth. The fossils are tiny (mm=millimeters). You can see how easy it would be easy to overlook these fossils. B=bivalve fragments, c=cristae (ridges, lines) ornamentation.

Species. Several species of Orthacanthus are known from Pennsylvanian and Permian rocks in the United States. It is difficult enough to identify the genera of some xenacanthid teeth, let alone species! We haven't tried to identify the species of teeth shown here. The most common Middle Pennsylvanian species from strata in central and eastern North America is probably O. compressus (Desmoinesian through Stephanian stages), which has been reported from Illinois, Ohio, and Pennsylvania. Orthacanthus platypternus and O. texensis also occur, but are most common in the Late Pennsylvanian and Permian (Newberry, 1856; Lund, 1975; Johnson, 1999; 2012), which is younger than the Kentucky specimens.

Range and geographic occurrence. Orthacanthus is found in Lower Pennsylvanian through Triassic strata worldwide (Schneider, 1999; Johnson, 1999; Johnson and Thayer, 2009), although it is perhaps most common in Late Pennsylvanian and Permian. The possible Orthacanthus? fossils shown here are from the Haddix coal zone of the Four Corners Formation, which is early Middle Pennsylvanian (Bolsovian, Westphalian C stage) in age, and approximately 315 million years old!

Paleoecology. The Four Corners Formation was deposited in coastal environments during the Pennsylvanian Coal Age. Coals were deposited in vast peat swamps. The Orthacanthus? teeth were found in a dark gray shale above a coal bed. Gray shales can be deposited in marine, brackish, and freshwater environments. The xenacanthids, Orthacanthus and Xenacanthus, have both been interpreted as inhabiting fresh- to possibly brackish-water environments (Compagno, 1990; Schneider, 1996; Johnson, 1999; Fischer and others, 2013). In the Kentucky example, the gray shale with the tiny shark teeth also contained pyritized, fossil bivalve shells. The bivalves are Anthraconaia(and possibly Naiadites). Both were small, freshwater bivalves, so the shale was likely deposited in a freshwater pond or lake.> If the shale was a freshwater deposit, the shark likely was as well.

Orthacanthus and Naiadites
Pyritized fossil Anthraconaia (and possibly Naiadites) bivalves found with the xenacanthid teeth from the Four Corners Formation, Perry County, Kentucky. Learn more about these bivalve fossils at the Anthraconaia fossil-of-the-month page

The tiny teeth of this month's fossil suggests the host shark was very small, possibly only one to two feet in length. Further study might determine if they belonged to a juvenile or adult. Many xenacanthids were small, but some genera could grow to more than 6 ft in length (Heidtke, 1982; Johnson, 1999; Schneider, 1996). Both Orthacanthus and Xenacanthus likely would have had a diet consisting of smaller creatures in the lake or pond they inhabited (Olsen, 1977; Johnson, 1999). A famous fossil specimen from the Permian of southwest Germany preserved the stomach contents of Triodus Sessilis, a similar type of freshwater xenacanthid shark. It contained two smaller amphibian-like creatures (called temnospondyls), and one of them had an even smaller fish in its stomach (Kriwet and others, 2008)! No small fish or temnospondyls have been found in the Four Corners Formation in Kentucky, but small boney fish called paleoniscoids, and temnospondyls have been found in Appalachian coal fields in other areas so are known to have inhabited ponds, lakes and streams during the Pennsylvanian Coal Age.

Orthacanthus Illustration by Steve Greb
Illustration of what a small Orthacanthus freshwater shark might have looked like in a Middle Pennsylvanian lake or pond. The reed-like plants are Calamites. The small shells are the freshwater bivalve Anthraconaia (and possibly Naiadites ). Smaller paleoniscoid fish also lived in these lakes. Copyright © 2023, Stephen Greb.

Learn more about Calamites plants

Learn more about lycopod trees and their roots, Stigmaria

Learn more about Naiadites and similar freshwater bivalves


Compagno L.J.V., 1990, Alternative life-history styles of cartilaginous fishes in time and space: Environmental Biology of Fishes, v. 28, p. 33-75.

Hampe, O., 1997, Dental growth anomalies and morphological changes in the teeth of the Xenacanthida (Lower Permian; Saar-Nahe Basin, SW Germany): Modern Geology, v. 21, p. 121-135.

Hampe, O., 2003, Revision of the Xenacanthidae (Chondrichthyes: Elasmobranchii) from the Carboniferous of the British Isles: Transactions of the Royal Society of Edinburgh, Earth and Environmental Science, v. 93, p. 191-237.

Hansen, M.C., 1996, Chapter 21-Phylum chordata-Vertebrate fossils, in Feldman, R.M., and Hackathorn, M., eds., Fossils of Ohio: Ohio Division of Geological Survey, Bulletin 70, p. 288-369.

Heidtke, U., 1982, Der Xenacanthide Orthacanthus senckenbergianus aus dem pfälzischen Rotliegenden (Unter-Perm):em/1.5 Bad Dürkheim, Germany, Mitteilungen der Pollochia, v. 70, p. 65-86.

Hotton, N. 1952, Jaws and teeth of American xenacanth sharks: Journal of Paleontology, v. 26, p. 489-500.

Johnson, G.D., 1999, Dentitions of late Palaeozoic Orthacanthus species and new species of Xenacanthus (Chondrichthyes: Xenacanthiformes) from North America: Acta Geologica Polonica, v. 49, no. 3, p. 215-266.

Johnson, G.D., 2012, Possible origin of the xenacanth sharks Orthacanthus texensis and Orthacanthus platypternus in the Lower Permian of Texas, USA: Historical Biology, v. 24, no. 4, p. 369-379.

Kriwet, J., Witzmann, F., Klug, S. and Heidtke, U.H., 2008, First direct evidence of a vertebrate three-level trophic chain in the fossil record: Proceedings of the Royal Society B: Biological Sciences, v. 275, no. 1631, p. 181-186.

Lund, R., 1970, Fishes from the Duquesne Limestone (Conemaugh, Pennsylvanian), Part I: Fossil fishes from southwestern Pennsylvania: Pittsburgh, Annals of Carnegie Museum, v. 41, p. 231-261.

Lund, R., 1975, Vertebrate fossil zonation and correlation of the Dunkard Basin, in Barlow, J.A., and Burkhammer, S., eds., The Age of the Dunkard: West Virginia Geological and Economic Survey, p. 171-182.

Newberry, J.S., 1856, Description of several new genera and species of fossil fishes from the Carboniferous strata of Ohio: Proceeding of the Academy of Natural Sciences of Philadelphia, v. 8, p. 96-100.

Schneider, J., 1988, Grundlagen der Morphogenie, Taxonomie, und Biostratigraphie isolieter Xenacanthodier-Zähne (Elasmobranchii): Leipzig, Germany, Freiberger Forschungshefte, v. C 419, p. 71-80.

Schneider, J. 1996, Xenacanth teeth—a key for taxonomy and biostratigraphy: Modern Geology, v. 20, p. 321-340.

Soler-Gijón, R., 2004, Development and growth in xenacanth sharks: new data from Upper Carboniferous of Bohemia, in Arratia, G., Wilson, M.V.H., and Cloutier, R., (eds.), Recent Advances in the Origin and Early Radiation of Vertebrates: Munich, Verlag Dr. Friedrich Pfeil, p. 533-562.

Text and illustrations by Stephen Greb

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