Sphenodus

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Sphenodus is an extinct shark genus in the family Orthacodontidae. It lived from the Early Jurassic (Sinemurian) to the Paleocene (Danian). It was about 2–3 meters long, with a streamlined body and a single dorsal fin toward the back, without a fin spine. Its teeth have one long main cusp with much smaller side cusplets. Most species are known only from teeth, but two Late Jurassic German species, Sphenodus macer and Sphenodus nitidus, are known from skeletons and suggest it swam actively and ate cephalopods. There are about 29 described species, though some may be synonyms. A 2025 study argues that Sphenodus nitidus is the same as S. macer, and that the genus Sphenodus may be a nomen dubium because the original teeth lack clear diagnostic features. It places S. macer in a new genus, Archaeogracilidens, within Orthacodontidae and Hexanchiformes. The exact position of Orthacodontidae is debated, with some classifications placing it in the extinct order Synechodontiformes and others in the modern order Hexanchiformes.