Tuatara: Sphenodontidae

At a glance, each of the two species of tuatara could be mistaken for a lizard. A closer look, however, reveals how different they really are. One difference is in their teeth. Tuataras have not one, but two rows of teeth lying side by side in the upper jaw. When the mouth closes, the single row of teeth in the lower jaw fits between the two upper rows. Tuataras have ears as lizards do, but lizards have an ear opening on each side of the head and tuataras do not. Baby tuataras have another unusual feature. They have a pale patch on the top of the head, which some people have called a "third eye." The patch becomes covered with scales as the animal grows up. Scientists are unsure of the patch's purpose but believe it may allow the reptile to see light from the sun. Such information about the sun's location may help the animal find its way.

A tuatara has a large head on a sturdy body that ends in a thick tail. Its skin is wrinkly and covered with noticeable beady scales. A white crest runs along the back of the head and down the middle of the back. The tail also has a row of toothy spines down its center. Males are larger and heavier than females, and they also have larger crests on the head and back. The biggest of the two species, the northern tuatara, can grow to more than 24 inches (61 centimeters) long from head to tail and weigh at least 2 pounds (1 kilogram). The smaller females of the species usually reach 16 inches (40.6 centimeters) and 1 pound (0.5 kilograms) at most. The other species, known as Brother Islands tuatara, is slightly smaller.

The Brother Islands tuatara is often a bit greener in color than the greenish brown northern tuatara, but both are sometimes reddish to almost black in color. The two species have white and black blotches and spots, but the Brother Islands tuatara usually has more white spots. Young tuataras of both species are commonly light grayish brown with light V-shaped patterns running along the back and dark markings by the eyes.