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. Evolutionary history The earliest hybodont remains are from the latest Devonian (Famennian) of Iran, belonging to the genus Roongodus. Although the first fossils of hybodonts are from the latest Devonian, they likely branched off from neoselachians (modern sharks) during the early Devonian. Hybodont samples have been recovered from Permian deposits from Oman, indicating that hybodonts lived in the Neotethys Ocean during the Permian Period. This study combined with others from Texas suggest that hybodonts were well established, and in some places dominant, during the Permian. In general, the Permian record of hybodonts is limited. It was initially hypothesized that hybodont diversity was not significantly impacted by the end-Permian extinction, instead it was thought that diversity of Permian hybodonts declined over the 50 million years before the end-Permian extinction. However, recent samples found in Oman suggests that Permian hybodont diversity extended until the end-Permian, suggesting the extinction was more impactful than previously thought. Fossils from the Lower Triassic Vega-Phroso Siltstone Member of the Sulphur Mountain Formation of Alberta, Canada show well preserved specimens of Wapitiodus aplopagus which survived the extinction and was abundant in the Early Triassic. Maximum hybodont diversity is observed during the Triassic. During the Triassic and Early Jurassic, hybodontiforms were the dominant selachians in both marine and non-marine environments. A study of Middle Jurassic fossils from England analyzed 20 species from 11 genera suggesting that hybodonts flourished at that time. A shift in hybodonts was seen during the Middle Jurassic, a transition between the distinctly different assemblages seen in the Triassic – Early Jurassic and the Late Jurassic – Cretaceous. As neoselachians (group of modern sharks) diversified further during the Late Jurassic,