This primitive reptile, which is about 2 ft (60 cm) long, is thought of as a living fossil since it has survived more or less unchanged for about 140 million years. Similar reptiles became extinct along with the dinosaurs about 65 million years ago.
The tuatara's third eye is known as the parietal eye because it sits between the parietal bones that form the top and sides of the skull. It has all the structures found in an eye - a lens, a retina and nerves connecting it to the brain - and may function as a light receptor. The tuatara cannot see through it, however, because it does not send any images to the brain. But the eye is linked to the pineal gland, a gland that is present in all animals with backbones. In human beings the pineal gland is positioned quite deep inside the head.
The pineal gland produces a hormone known as melatonin, the output of which increases during the night, controlling the cycle of sleeping and waking. The gland is also thought to control the body clock responsible for timing such functions as mating and hibernating. This biological clock is triggered by changes in natural light which, in the case of the tuatara, may be registered by the third eye.