Sphenodus

29 species have been described,^[3] though some of these are likely synonyms, which span from the Early Jurassic (Sinemurian) to Paleocene (Danian).^[2] Most species are only known from isolated teeth,^[2] though the species Sphenodus macer and Sphenodus nitidus from the Late Jurassic of Germany are known from skeletons. These suggest that it was relatively large, with a body length of 2–3 metres (6.6–9.8 ft), with a fusiform body with a single dorsal fin placed posteriorly without a fin spine.^[1] The teeth of Sphenodus consist of a single long, narrow central cusp, with much smaller lateral cusplets.^[2] Species of Sphenodus are thought to have been actively swimming predators, eating cephalopods.^[1]

A 2025 study considered Sphenodus nitidus to be a synonym of S. macer, and suggested that Sphenodus should be considered a nomen dubium. This is due to a lack of diagnostic characters associated with original type specimen teeth used to define the genus, with S. macer assigned to the new genus Archaeogracilidens which was also placed as a member of Orthacodontidae within the Hexanchiformes.^[4]