Tuataras are unusual, lizard-like animals that are the only living representatives of the order Sphenodonta of the vertebrate class Reptilia. The lineage of the sphenodonts is an ancient one, with a fossil record extending back 200 million years, prior even to the evolution of dinosaurs and the lizards. Until the discovery of tuataras in New Zealand, biologists had believed that this reptilian lineage had been extinct for 100 million years. Hence, the tuatara became celebrated as a "living fossil."
Tuataras have a number of unusual anatomical features. The arrangement of bones and cavities in the skull of Tuataras is considered to be a primitive character. Tuataras also have a relatively well developed, light sensitive organ on the top of their head, called the pineal organ or median eye, capable of sensing light and darkness. This primitive structure has similarities to a true eye, and even has a lens and retinal tissue. The function of the pineal eye is not understood by physiologists, but it appears to regulate activity of the pineal gland. This organ occurs in the forebrain of vertebrates, and secretes the hormone melatonin into the blood, which changes skin coloration in some animals, regulates diurnal or 24-hour biological rhythms, and possibly influences seasonal reproductive cycles. A unique characteristic of tuataras, not shared with other reptiles, is the fusion of teeth into the jawbones, rather than being set in sockets in the bone.
Tuataras are rather long-lived animals, which can exceed 75 years in age, and reach about 28 in (70 cm) in body length. Tuataras are slow moving, sluggish animals, which mostly eat large insects. These reptiles live in burrows, dug by themselves, which they often share with nesting petrels (seabirds).
The two surviving species of tuatara, Sphenodon punctatus and S. guntheri, are very rare animals, living only on a few isolated small islands off New Zealand. These species used to have much broader ranges, but they were widely extirpated from the mainland and many islands of New Zealand after European colonization. The catastrophic decline of tuataras was mainly due to predation of adults and eggs by introduced mammals, such as cats, foxes, rats, and others. It is significant that these predators do not occur on the few, closely protected islands where the world's last tuataras live.
The initial survival of tuataras was due to the early isolation of New Zealand from other continents, where ecologically more capable types of reptiles, birds, and mammals evolved and came to dominate animal communities. In other parts of Gondwanaland (the southern continent which New Zealand was originally a part of) tuataras and their relatives were incapable of competing with the better adapted creatures, and so became extinct. Tuataras are truly extraordinary survivors, relics from a bygone era of the evolution of animal forms on Earth. However, tuataras are now critically endangered as a result of recent influences by humans. The continued, though precarious, survival of tuataras now depends on their strict and perpetual protection on a few small islands.