Tuatara are members of the taxon Rhynchocephalia and are most closely related to squamates (i.e., amphisbaenians, lizards, and snakes). Like squamates, tuatara are elongate and shed skin in large fragments. Unlike squamates, tuatara lack paired hemipenes (male copulatory organs).

The common name "tuatara" comes from Maori words meaning "spines on back," in reference to the crest on the backs of males and females. Tuatara have a lizard-like appearance: both groups are elongate with four limbs (most lizards) and both lizards and tuatara are known to shed their tails (caudal autotomy). The groups diverge, however, on the presence or lack of a paired hemipenes, the morphology of the teeth and skull, and other important features. Tuatara are long-lived species, reaching sexual maturity at about 20 years. Two species of tuatara, Sphenodon guntheri and Sphenodon punctatus, are located in New Zealand, and are the only species known to exist.

Source: National Biological Information Infrastructure (NBII) at http://www.nbii.gov