The tuatara is a unique reptile endemic to the northern tip of New Zealand. Though it superficially resembles a lizard, the tuatara is a sphenodontian, a sister clade of squamates, the reptile group that includes both lizards and snakes. Sphenodontians were highly successful about 200 million years ago, including many terrestrial forms and even an aquatic form. At the time, sphenodontians occupied many of the niches that are today taken by lizards. The tuatara is the only surviving sphenodontian.
Although it is sometimes called a "living fossil," the tuatara has actually changed extensively in the last 200 million years, including developing adaptations for the colder weather of the Cenozoic era. A broad survey of molecular evolution rates across diverse animal genera revealed that the tuatara is one of the fastest-evolving species among the groups studied. Like many reptiles, the tuatara retains the basic lizard-like body plan, which dates back to the earliest reptiles living 315 million years ago, such as Hylonomus.
The tuatara is of great interest to those studying the evolution of reptiles and tetrapods in general. Of all the amniotes, the tuatara is among the least specialized. Its locomotion is considered amphibian-like, and its heart is the most primitive of all amniotes. Instead of distinct teeth that can fall out and grow back, the tuatara's teeth are direct projections from its jawbone. When they wear down, they cannot be replaced, so old tuataras must switch to softer food such as earthworms. The tuatara lacks an earhole or eardrums, the middle ear instead being filled with sensory tissue. As a result, the animal displays poor hearing.
Of all animals, the tuatara displays the best example of a parietal eye, a relict third eye on the top of its head, consisting of its own retina, cornea, lens, and a degenerated bundle of nerves to carry its information to the brain. In hatchlings, the parietal eye is clearly visible, but between four to six months, it becomes covered in opaque scales and pigment. The function of the eye is unknown, but in salamanders the parietal eye has been shown to determine the polarization of light, allowing it to find the sun even under extensive cloud cover.