Orthacanthus is an extinct genus of fresh-water xenacanthid sharks, named by Louis Agassiz in 1836, ranging from the Upper Carboniferous until the Lower Permian. Orthacanthus had a nektobenthic life habitat, with a carnivorous diet. Multiple sources have also discovered evidence of cannibalism in the diet of Orthacanthus and of "filial cannibalism" where adult Orthacanthus preyed upon juvenile Orthacanthus. The genus Orthacanthus has been synonymized with Dittodus (Owen, 1867), Didymodus (Cope, 1883), and Diplodus (Agassiz, 1843). About 260 million years ago, Orthacanthus was the apex predator of freshwater swamps and bayous in Europe and North America. Mature Orthacanthus reached nearly 3 meters (10 feet) in length. Orthacanthus teeth have a minimum of three cusps, two principal cusps, and an intermediate cusp, where the principal cusps are variously serrated, with complex base morphology. Additionally, Orthacanthus can be diagnosed by major transverse axes of proximal ends at a 45 degree angle to and often almost parallel to the labial margin of the base between the cusps. Deformed teeth are characteristic of the xenacanth sharks and of Orthacanthus. Description Teeth The larger teeth of Orthacanthus compressus and Orthacanthus texensis are differentiated by a more pronounced basal tubercle in O. compressus. The basal tubercle of a typical tooth file is on the apical button of the underlying tooth. The larger adult teeth of O. compressus have a wider rather than longer base, similar to O. texensis, and tend to have serrations on both carinae of each cusp, while the medial carinae of smaller adult teeth are not serrated. The juvenile teeth of O. compressus are longer than wide, have a thinner base, and lack serrations, similar to O. platypternus teeth. Orthacanthus platypternus from the Craddock Bonebed shark layer in Texas, USA, shows evidence of resorption, and the equivalent of an "enamel pearl." Some of the teeth specimens found at this location show evidence of resorption, which has not been previously observed in other faunas at the same location. Where the superjacent basal tubercle is expected to be resorbed if the teeth were to undergo resorption, the apical button is resorbed instead. Sexual Dimorphism The difference in characteristics between the large and small O. compressus adult teeth might indicate sexual dimorphism. The spines of O. platypternus showing 3 to 4 dentine layers are interpreted to be subadults or young adults, and are separated into two size classes where females have the largest spines in comparison to