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The tuatara (Sphenodon punctatus) is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is part of a distinct lineage, the order Rhynchocephalia. The name tuatara is derived from the Māori language and means "peaks on the back".

The single extant species of tuatara is the only surviving member of its order, which was highly diverse during the Mesozoic era. Rhynchocephalians first appeared in the fossil record during the Triassic, around 240 million years ago, and reached worldwide distribution and peak diversity during the Jurassic, when they represented the world's dominant group of small reptiles. Rhynchocephalians underwent a great decline during the Cretaceous with their youngest records outside New Zealand dating to the Paleocene. Their closest living relatives are squamates (lizards and snakes). Tuatara are of interest for studying the evolution of reptiles.

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 able to hear, although no external ear is present, and have unique features in their skeleton.

Tuatara are sometimes referred to as " living fossils ", which has generated significant scientific debate. This term is currently deprecated among paleontologists and evolutionary biologists. Although tuatara have preserved the morphological characteristics of their Mesozoic ancestors (240–230 million years ago), there is no evidence of a continuous fossil record to support this. The species has between 5 and 6 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, the Brothers Island tuatara S. guntheri, (Buller, 1877), was recognised in 1989, but since 2009 it has been reclassified as a subspecies (S.p. guntheri). 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 Wildlife Sanctuary (now named "Zealandia") in 2005.

During routine maintenance work at Zealandia 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 New Zealand's North Island in over 200 years.