Sphenodontidae is a family within the reptile group Rhynchocephalia, comprising taxa most closely related to the living tuatara (Sphenodon punctatus). Historically the taxa included within Sphenodontidae have varied greatly between analyses, and the group has lacked a formal definition. Cynosphenodon from the Jurassic of Mexico has consistently been recovered as a close relative of the tuatara in most analyses, with the clade containing the two and other very close relatives of the tuatara often called Sphenodontinae.
Like modern tuatara, members of Sphenodontinae were likely generalists with a carnivorous/insectivorous diet.
Cladogram after Simoes et al. 2022: