''Cynosphenodon'' ( ; "Dog Sphenodontian") is an extinct genus of rhynchocephalian in the family Sphenodontidae from the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

La Boca Formation of

Tamaulipas Tamaulipas, officially the Free and Sovereign State of Tamaulipas, is a state in Mexico; one of the 31 states which, along with Mexico City, comprise the 32 federal entities of Mexico. It is divided into 43 municipalities. It is located in nor ...

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

. 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.

References

Jurassic lepidosaurs Rhynchocephalia Fossils of Mexico Prehistoric reptile genera {{Jurassic-reptile-stub