Cynosphenodon ( ; "Dog Sphenodontian") is an extinct genus of rhynchocephalian in the family Sphenodontidae from the Lower Jurassic sediments of the La Boca Formation of Tamaulipas, Mexico. It is known from a largely complete lower jaw and fragments of the upper jaw. It is suggested to be among the closest known relatives of the tuatara, with both being placed in the Sphenodontinae, which is supported by among other characters, the growth pattern of the teeth.Reynoso, V. H. (2003). Growth patterns and ontogenetic variation of the teeth and jaws of the Middle Jurassic sphenodontian Cynosphenodon huizachalensis (Reptilia: Rhynchocephalia). Canadian Journal of Earth Sciences 40(4), 609–619
Cladogram after Simoes et al. 2022.
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