Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like chondrichthyans, which existed from the late Devonian to the Late Cretaceous. They form the group of Elasmobranchii closest to neoselachians, the clade of modern sharks and rays. Hybodonts were named and are distinguished based on their conical tooth shape. They are also noted for the presence of a spine on each of their two dorsal fins. 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.
Etymology
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.
Taxonomic history
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). Hybodontiformes are a type of elasmobranch, and the sister group to Neoselachii. Hybodontiformes are classified in the Elasmobranchii with Xenacanthiformes, Ctenacanthiformes and Neoselachii. Hybodonts are divided into a number of families, including Hybodontidae, but the higher level taxonomy of hybodonts, especially Mesozoic taxa, is poorly resolved.
Morphology and teeth
Hybodonts reached a maximum size of in length, and had a relatively robust bodyform. Due to their cartilaginous skeletons usually disintegrating upon death like other chondrichthyans, hybodonts are generally described and identified based on size and shape of teeth and fin spine fossils, which are more likely to be preserved. Rare complete skeletons are known from areas of exceptional preservation.
Hybodonts are recognized as having teeth with a prominent cusp which is higher than lateral cusplets. Hybodont teeth are often preserved as incomplete fossils because the base of the tooth is not well attached to the crown. Hybodonts were initially divided into two groups based on their tooth shape. One group had teeth with acuminate cusps that lacked a pulp cavity; these are called osteodont teeth. The other group had a different cusp arrangement and had a pulp cavity, these are called orthodont teeth. For example, the hybodont species Heterophychodus steinmanni have osteodont teeth with vascular canals of dentine which are arranged vertically parallel to each other, also called ‘tubular dentine’. The crowns of these osteodont teeth are covered with a single layer of enameloid. Hybodont teeth served a variety of functions depending on the species, including grinding, crushing (durophagy), tearing, clutching, and even cutting.
Hybodonts are characterized by having two dorsal fins each preceded by a fin spine with a specific shape. The fin spines shape is used to distinguish hybodonts from other shark groups and different hybodont species. The fin spines are elongate and gently curved toward the back of the animal. Male hybodonts had either one or two pairs of cephalic spines on their heads. Hybodonts had a fully heterocercal tail fin, where the upper lobe of the fin was much larger than the lower one due to the spine extending into it.
Habitat
Hybodont fossils are found in depositional environments ranging from marine to fluvial (river deposits). Many hybodonts are thought to have been euryhaline, able to tolerate a wide range of salinities. Hybodonts inhabited freshwater environments at least since the Permian. Based on isotopic analysis, some species of hybodonts are likely to have permanently lived in freshwater environments, while others may have migrated between marine and freshwater environments.