Hybodontiformes

Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays (Neoselachii) as part of the clade Euselachii. They are distinguished from other chondrichthyans by their distinctive fin spines and cephalic spines present on the heads of males. An ecologically diverse group, they were abundant in marine and freshwater environments during the late Paleozoic and early Mesozoic, but were rare in open marine environments by the end of the Jurassic, having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of the Cretaceous, before going extinct. The term hybodont comes from the Greek word ὕβος or ὑβός meaning hump or hump-backed and ὀδούς, ὀδοντ meaning tooth. This name was given based on their conical compressed teeth. Hybodonts were first described in the nineteenth century based on isolated fossil teeth (Agassiz, 1837). Hybodonts were first separated from living sharks by Zittel (1911). Although historically argued to have a close relationship with the modern shark order Heterodontiformes, this has been refuted.