Tuatara is known scientifically as a 'living fossil', a distinction shared with a few other species including the coelacanth fish, the horseshoe crab, and the native frogs of New Zealand, which have apparently changed little from extraordinarily ancient origins. There are at least two Tuatara species, one of which has only 400 members, and extinction is a real possibility. The order to which Tuatara belong, Spenodontida, has no other living representatives.

The Tuatara, (old spiny back in Maori) differs from lizards, which it superficially resembles, by extra holes in the skull, boney processes on the ribs, the lack of a copulatory organ in males, and the presence of a third eye, known as the parietal or pineal eye, which contains a rudimentary lens and retina and is connected to the brain by a nerve. However, the whole organ is covered with opaque scales and the formation of an image would be impossible. Some scientists believe that this third eye may function as a light sensor, influencing the amount of time a tuatara spends basking. It is particularly noticeable in hatchlings which have a patch of white scales at top centre of the skull.

       

Tuatara teeth, also, are different from those of other reptiles. They have a single row of teeth in the lower jaw, and a double row in the upper jaw, the bottom row fitting neatly between the two upper rows.Little more than serrations of the jaw, they are not replaced when worn out or damaged, and some old Tuatara are virtually toothless, chewing their food between smooth jaw bones.

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