The tuatara (Sphenodon) is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of an ancient lineage. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Tuatara are of great interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids (birds, dinosaurs and crocodiles). Tuatara are greenish brown, and measure up to 80cm from head to tail-tip and weigh up to 1.3kg with a spiny crest along the back. The name "tuatara" derives from the Māori language, and means "peaks on the back". Their dentition, in which two rows of teeth in the upper jaw overlap one row on the lower jaw, is unique among living species. They are further unusual in having a pronounced photo-receptive eye dubbed the "third eye", that are thought to function in setting circadian and seasonal cycles.