It is placed as a member of the extinct family Orthacodontidae, which is either considered to be a member of the extinct order Synechodontiformes,[1] or the modern shark order Hexanchiformes.

[2] 29 species have been described,[3] though some of these are likely synonyms, which span from the Early Jurassic (Sinemurian) to Paleocene (Danian).

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.