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Tuatara are reptiles endemic to New Zealand and which, although resembling most lizards, are part of a distinct lineage, the order Rhynchocephalia. Their name derives from the Māori language, and means "peaks on the back". The two species of tuatara are the only surviving members of their 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 great interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids (the group that also includes birds, dinosaurs, and crocodiles). Tuatara are greenish brown and gray, and measure up to from head to tail-tip and weigh up to with a spiny crest along the back, especially pronounced in males. Their dentition, in which two rows of teeth in the upper jaw overlap one row on the lower jaw, is unique among living species. They are even more 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 a number of unique features in their skeleton, some of them apparently evolutionarily retained from fish. Although tuatara are sometimes called "living fossils", recent anatomical work has shown that they have changed significantly since the Mesozoic era. While mapping its genome, it has been discovered that the species has between five and six billion base pairs of DNA sequence.〔(Tuatara genome mapping | Otago Daily Times Online News )〕 The tuatara ''Sphenodon punctatus'' has been protected by law since 1895; the second species, ''S. guntheri'', was not recognised until 1989.〔 Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat ''(Rattus exulans)''. They were extinct on the mainland, with the remaining populations confined to 32 offshore islands,〔 until the first mainland 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,〔(New Zealand’s ‘living fossil’ confirmed as nesting on the mainland for the first time in 200 years! ), Karori Sanctuary Trust, 31 October 2008.〕 with a hatchling found the following autumn.〔(Our first baby tuatara! ), Karori Sanctuary Trust, 18 March 2009.〕 This is thought to be the first case of tuatara successfully breeding on the New Zealand mainland in over 200 years, outside of captive rearing facilities. ==Taxonomy and evolution== 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.〔Cree, Alison. 2002. ''Tuatara.'' In: Halliday, Tim and Adler, Kraig (eds.), ''The new encyclopedia of reptiles and amphibians'', Oxford University Press, Oxford, pp. 210–211. ISBN 0-19-852507-9〕 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" (σφηνος/''sphenos'') and "tooth" (δόντι/''donti''). Tuatara have been referred to as living fossils,〔 which means they retain many basal characteristics from around the time of the squamate – rhynchocephalian split (220 MYA). However, taxonomic work on Sphenodontia has shown this group has undergone a variety of changes throughout the Mesozoic, and a March 2008 molecular study showed their rate of molecular evolution has been the fastest of any animal yet examined.〔(【引用サイトリンク】title=Fastest Evolving Creature is 'Living Dinosaur' )〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tuatara」の詳細全文を読む スポンサード リンク
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