Fossils indicate that the jaw mechanism began evolving at least 200 million years ago. This mechanism allows the jaws to shear through chitin and bone. The structure of the jaw joint allows the lower jaw to slide forwards after it has closed between the two upper rows of teeth. This specific tooth arrangement is not seen in any other reptile although most snakes have a double row of teeth in their upper jaws, their arrangement and function is different from the tuatara's. There is a single row of teeth in the lower jaw and a double row in the upper, with the bottom row fitting perfectly between the two upper rows when the mouth is closed. The tip of the upper jaw is beak-like and separated from the remainder of the jaw by a notch. : 113 However, the lower temporal bar (sometimes called the cheek bone) is incomplete in some fossil Rhynchocephalia, suggesting its presence in the tuatara is a distinctive ( autapomorphic) feature rather than one inherited from a common ancestor. The skull of the tuatara has a similar structure, with both upper and lower temporal openings. This makes for a very rigid, inflexible construction. The upper jaw is firmly attached to the posterior of skull. The ancestor of diapsids had a skull with two openings in the temporal region – upper and lower temporal fenestra on each side of the skull bounded by complete arches. 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. ĭuring routine maintenance work at Zealandia in late 2008, a tuatara nest was uncovered, with a hatchling found the following autumn. 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. Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat (Rattus exulans). 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). The tuatara ( Sphenodon punctatus) has been protected by law since 1895. The species has between 5 and 6 billion base pairs of DNA sequence, nearly twice that of humans. 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. This term is currently deprecated among paleontologists and evolutionary biologists. Tuatara are sometimes referred to as " living fossils", which has generated significant scientific debate. 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. They have two rows of teeth in the upper jaw overlapping one row on the lower jaw, which is unique among living species. 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. 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 (including birds) and crocodilians. Their closest living relatives are squamates (lizards and snakes). Rhynchocephalians originated during the Triassic (~250 million years ago), reached worldwide distribution and peak diversity during the Jurassic and, with the exception of tuatara, were extinct by 60 million years ago. The single extant species of tuatara is the only surviving member of its order. The name tuatara is derived from the Māori language and means "peaks on the back". Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. Tuatara ( Sphenodon punctatus) are reptiles endemic to New Zealand. Symbols may represent up to seven islands. Current distribution of tuatara (in black): Circles represent the North Island tuatara, and squares the Brothers Island tuatara.
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