Name: Orthacanthus ‭(‬Vertical spike‭)‬.
Phonetic: Orf-ah-can-fus.
Named By: Louis Agassiz‭ ‬-‭ ‬1836.
Classification: Chordata,‭ ‬Chondrichthyes,‭ ‬Elasmobranchii,‭ ‬Euselachii,‭ ‬Xenacanthida,‭ ‬Orthacanthidae.
Species: O.‭ ‬buxeri‭ (‬type‭)‬.‭ ‬O.‭ ‬arcuatus,‭ ‬O.‭ ‬cylindricus,‭ ‬O.‭ ‬senckenbergianus.
Diet: Carnivore/Piscivore.
Size: Up to‭ ‬3‭ ‬meters long.
Known locations: Europe.‭ ‬North America.
Time period: Emsian of the Devonian through to the Carnian of the Triassic.
Fossil representation: Several specimens.

       Not only was Orthacanthus very similar to the smaller Xenacanthus,‭ ‬it also has a similar temporal range in the fossil record.‭ ‬The two sharks probably coexisted with only the minimum of competition between the two.‭ ‬This is because at only one meter long,‭ ‬Xenacanthus could operate in waters that were too small or densely overgrown to support a three meter long predator like Orthacanthus.‭
       As the larger of the two,‭ ‬Orthacanthus would have specialised in larger and more powerful prey that it would have seized with its double fanged teeth.‭ ‬The overall eel like body morph of Orthacanthus is a reflection of its habitat.‭ ‬Rather than being a pelagic open seas predator,‭ ‬Orthacanthus hunted in freshwater swamps and waterways that would have been densely overgrown in areas.‭ ‬By having a long body with short fins Orthacanthus could navigate these waters without getting stuck in submerged debris and vegetation.‭ ‬It's possible that Orthacanthus may have used ambush tactics like lurking within the submerged debris‭ ‬and plants waiting for prey to pass by.
       The spike that rises up from the back of the head of Orthacanthus seems to have been a defensive feature to stop other predators from clamping their jaws onto its head.‭ ‬These exact predators may still be unknown to science but they may have been other fish like Hyneria,‭ ‬or large amphibian tetrapods.

Further reading
- Recherches Sur Les Poissons Fossiles. Tome III (livr. 15-16). - Imprimérie de Petitpierre, Neuchatel 157-390 - L. Agassiz - 1843.
- Descriptions of extinct Vertebrata from the Permian and Triassic formations of the United States. - Proceedings of the American Philosophical Society 17(100):182-193 - E. D. Cope - 1877.
- Systematics and relationships among the Xenacanthiformes (Pisces, Chondrichthyes) in the light of Carboniferous and Permian French materal. - Acta Musei Reginaehradecensis S. A.: Scientiae Naturales 22:69-78 - D. Heyler, & C. Poplin - 1989.
- Occipital spine of Orthacanthus (Xenacanthidae, Elasmobranchii): Structure and growth - Journal of Morphology vol 242 issue 1, p1-45 - Rodrigo Soler-Gijon - 1999.
- Dentitions of Late Palaeozoic Orthacanthus species and new species of ?Xenacanthus (Chondrichthyes: Xenacanthiformes) from North America - Acta Geologica Polonica vol 49, issue 3 - Gary D. Johnson - 1999.
- Early Pennsylvanian Xenacanth Chondrichthyans from the Swisshelm Mountains, Arizona, USA. Acta Palaeontologica Polonica 54(4):649-688 - G. D. Johnson & D. W. Thayer - 2009.


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