Sphenodon punctatus

Tuatara are reptiles endemic to New Zealand. Although resembling most lizards, they are part of a distinct lineage, the order Rhynchocephalia. Their name derives from the Māori language, and means 'peaks on the back'. The single species of tuatara is the only surviving member of its order, which flourished around 200 million years ago. Their most recent common ancestor with any other extant group is with the squamates (lizards and snakes). For this reason, tuatara are of interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids, a group of amniote tetrapods that also includes dinosaurs, birds, and crocodilians. 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 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), 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 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 New Zealand's North Island in over 200 years. Tuatara, along with other now-extinct members of the order Sphenodontia, belong to the superorder Lepidosauria, the only surviving taxon within Lepidosauromorpha. Squamates and tuatara both show caudal autotomy (loss of the tail-tip when threatened), and have transverse cloacal slits. The origin of the tuatara probably lies close to the split between the Lepidosauromorpha and the Archosauromorpha. Though tuatara resemble lizards, the similarity is superficial, because the family has several characteristics unique among reptiles. The typical lizard shape is very common for the early amniotes; the oldest known fossil of a reptile, the Hylonomus, resembles a modern lizard. Tuatara were originally classified as lizards in 1831 when the British Museum received a skull. The genus remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds, turtles, and crocodiles. He proposed the order Rhynchocephalia (meaning 'beak head') for the tuatara and its fossil relatives. Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a 'wastebasket taxon'. Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925. Sphenodon is derived from the Greek for 'wedge' (σφήν, σφηνός/sphenos) and 'tooth' (ὀδούς, ὀδόντος/odontos). Tuatara have been referred to as living fossils, which means that they retain many basal characteristics from around the time of the squamate–rhynchocephalian split (220 MYA). Morphometric analyses of variation in jaw morphology among tuatara and extinct Rhynchocephalian relatives have been argued to demonstrate morphological conservatism and support for the classification of tuatara as a 'living fossil', but the reliability of these results has been criticised and debated. Taxonomic research on Sphenodontia indicates that the group has undergone a variety of changes throughout the Mesozoic, and the rate of molecular evolution for tuatara has been estimated to be among the fastest of any animal yet examined. Many of the niches occupied by lizards today were then held by sphenodontians. There was even a successful group of aquatic sphenodontians known as pleurosaurs, which differed markedly from living tuatara. Tuatara show cold-weather adaptations that allow them to thrive on the islands of New Zealand; these adaptations may be unique to tuatara since their sphenodontian ancestors lived in the much warmer climates of the Mesozoic. For instance, Palaeopleurosaurus appears to have had a much shorter lifespan compared to the modern tuatara.