Tuatara are greenish brown and grey, and measure up to 80 cm (31 in) from head to tail-tip and weigh up to 1.3 kg (2.9 lb) with a spiny crest along the back, especially pronounced in males. They have two rows of teeth in the upper jaw overlapping one row on the lower jaw, which is unique among living species. They are also unusual in having a pronounced photoreceptive eye, the third eye, which is thought to be involved in setting circadian and seasonal cycles. They are able to hear, although no external ear is present, and have unique features in their skeleton, some of them apparently evolutionarily retained from fish. Tuatara are sometimes referred to as "living fossils", which has generated significant scientific debate. While mapping its genome, researchers have discovered that the species has between five and six billion base pairs of DNA sequence, nearly twice that of humans.
The tuatara Sphenodon punctatus has been protected by law since 1895. A second species, S. guntheri, was recognised in 1989,but since 2009 it has been reclassified as a subspecies. Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat (Rattus exulans). Tuatara were extinct on the mainland, with the remaining populations confined to 32 offshore islands until the first North Island release into the heavily fenced and monitored Karori Sanctuary in 2005.
During routine maintenance work at Karori Sanctuary in late 2008, a tuatara nest was uncovered, with a hatchling found the following autumn. This is thought to be the first case of tuatara successfully breeding in the wild on the New Zealand North Island in over 200 years.