Orthacanthus

Orthacanthus is an orthacanthid xenacanthid cartilaginous fish from the Mississippian-Guadalupian of the United Kingdom, Germany, the United States of America, Canada, Poland, and France. It was named in 1843 by Louis Agassiz. It was one of the largest true sharks to exist during the Carboniferous period, as well as one of the most widespread freshwater predators of the time.

## Physiology

Orthacanthus resembled its semi-close cousin, Xenacanthus; it had a long, eel-like body, a small head with big jaws, two thin, rounded anal (rear) fins, a ribbon-like dorsal (back) fin, and a long spine protruding from the back of its head. However, it also had a few differences; its pectoral (arm) and pelvic ("leg") fins were more triangular, its caudal (tail) fin was shorter and triangular in shape, and its head was more rounded in profile, compared to the pointed head of Xenacanthus. Its body would have been covered in scaly skin.

## Diet

Orthacanthus was a predator, preying on smaller sharks as well as other small fishes and even small amphibians. Its teeth bore two prongs and formed a V-shape, and were used for getting a hold of struggling prey.

## Ecology

Orthacanthus was the top predator of the freshwater lakes it inhabited, feeding on a wide variety of prey within its size range; its long, eel-like body may have helped it weave around dense weeds and underwater obstacles, and it may have even used said obstacles as cover when hunting, lying in wait for prey to come by before lunging towards them and snagging them in its double-pronged teeth. In fact, it may have even fed on its own young, as indicated by teeth of juveniles inside coprolites (fossilized feces); this practice is known as "filial cannibalism", and may have been done as a response to decreased populations of other kinds of prey.[1] As well as this, the long spine on its head may have been used for recognition by other members of its species; this is shown by the fact that juveniles had small spines covered in 1-2 layers of dentine (the middle layer of teeth and shark scales), whereas subadults and adults had larger spines covered in 3-4 layers of dentine (with females having the largest spines). The distribution of these spines throughout the quarry it was found in shows that it may have been catadromous, migrating into the ocean in order to lay eggs.[2]

## References

1. Ó Gogáin, A., Falcon-Lang, H., Carpenter, D., Miller, R., Benton, M., Pufahl, P., Ruta, M., Davies, T., Hinds, S., & Stimson, M. (2016, June 27). Fish and tetrapod communities across a marine to brackish salinity gradient in the Pennsylvanian (early Moscovian) Minto Formation of New Brunswick, Canada, and their palaeoecological and palaeogeographic implications. University of Southampton Institutional Repository. https://eprints.soton.ac.uk/399371/1/Palaeontology%2520Accepted%2520Manuscript.pdf.
2. Beck, K. G., Soler-Gijón, R., Carlucci, J. R., & Willis, R. E. (2016, December 18). Morphology and Histology of Dorsal Spines of the Xenacanthid Shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and Palaeoenvironmental Implications. Acta Palaeontologica Polonica. https://bioone.org/journals/acta-palaeontologica-polonica/volume-61/issue-1/app.00126.2014/Morphology-and-Histology-of-Dorsal-Spines-of-the-Xenacanthid-Shark/10.4202/app.00126.2014.full.