The tuatara's family first appeared about 225 million years ago - long before the first birds, long before the first mammals, long before most kinds of dinosaur. All of their relatives went extinct millions of years ago, except this one species, which lives only in New Zealand.
Tuatara have something very unusual on the top of their head: a 'third eye'. It is a small spot that can sense light and dark, even though they can't really see with it. Scientists think it helps tuatara work out when it's day or night, and what season it is. As they grow up, the third eye gets covered with scales.
Tuatara are extremely slow at growing up. They keep growing until they are about 35 years old, and they can live for over 100 years. One famous tuatara named Henry became a dad for the first time when he was 111. Imagine being a great-great-great-grandfather and having a baby of your own.
They like cool weather. Most reptiles enjoy hot sun, but tuatara are happy in temperatures that would send a lizard to sleep. They live in burrows, often borrowed from seabirds, and come out at night to hunt insects, worms and even baby seabirds. They have two rows of teeth on top and one on the bottom - which line up perfectly when they bite.