The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia.[1][2] The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago.[2] 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 and crocodiles).
Tuatara are greenish brown, and measure up to 80 cm (32 in) from head to tail-tip[3] 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 further unusual in having a pronounced parietal eye, dubbed the "third eye", whose current function is a subject of ongoing research. 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 taxonomic and molecular work has shown that they have changed significantly since the Mesozoic era.
The tuatara has been classified as an endangered species since 1895[4][5] (the second species, S. guntheri, was not recognised until 1989).[3] Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and the introduced Polynesian Rat (Rattus exulans). They were extinct on the mainland, with the remaining populations confined to 32 offshore islands,[2] until the first mainland release into the heavily fenced and monitored Karori Wildlife Sanctuary in 2005.[6]
The name "tuatara" derives from the Māori language, and means "peaks on the back".[7] As with many other Māori loanwords, the plural form is now generally the same as the singular in formal New Zealand English usage. "Tuataras" remains common in less formal speech, particularly among older speakers.
[edit] Taxonomy and evolution
Tuatara, and their sister group Squamata (which includes lizards, snakes and amphisbaenians), 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 a transverse cloacal slit.[8] 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, since 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.[9]
1. Tuatara
2. Lizards
3. Snakes
4. Crocodiles
5. Birds
"Lizards" are polyphyletic. Branch lengths do not indicate divergence times.
Tuatara were originally classified as lizards in 1831 when the British Museum received a skull.[11] 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.[12] Now, most authors prefer to use the more exclusive order name of Sphenodontia for the tuatara and its closest living relatives.[13]
Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon".[14] Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925.[14] Sphenodon is derived from the Greek for "wedge" (σφηνος/sphenos) and "tooth" (δόντι/odon(t)).[15]
Tuatara have been referred to as living fossils.[1] This means that they have remained mostly unchanged throughout their entire history, which is approximately 220 million years.[16] However, taxonomic work[17] on Sphenodontia has shown that this group has undergone a variety of changes throughout the Mesozoic, and a recent molecular study showed that their rate of molecular evolution is faster than of any other animal so far examined.[18][19] 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.
[edit] Species
There are two extant species: Sphenodon punctatus and the much rarer Sphenodon guntheri, or Brothers Island tuatara, which is confined to North Brother Island in Cook Strait.[20] The species name punctatus is Latin for "spotted",[21] and guntheri refers to Albert Günther. S. punctatus was named when only one species was known, and its name is misleading, since both species can have spots. The Brother's Island tuatara (S. guntheri) has olive brown skin with yellowish patches, while the colour of the other species, (S. punctatus), ranges from olive green through grey to dark pink or brick red, often mottled, and always with white spots.[6][8][22] In addition, S. guntheri is considerably smaller.[23] A third, extinct species of Sphenodon was identified in November 1885 by William Colenso, who was sent an incomplete sub-fossil specimen from a local coal mine. Colenso named the new species S. diversum.[24]
Sphenodon punctatus is further divided into two subspecies: the Cook Strait tuatara (unnamed subspecies), which lives on other islands in and near Cook Strait, and the northern tuatara (Sphenodon punctatus punctatus), which lives on the Bay of Plenty, and some islands further north.[2]
[edit] Description
The tuatara is considered the most unspecialised living amniote; the brain and mode of locomotion resemble that of amphibians and the heart is more primitive than that of any other reptile.[16]
Both species are sexually dimorphic, males being larger.[8] Adult S. punctatus males measure 61 cm (24 in) in length and females 45 cm (18 in).[8] The San Diego Zoo even cites a length of up to 80 cm (31 in).[25] Males weigh up to 1 kg (2.2 lb), and females up to 0.5 kg (1.1 lb).[8] Brother's Island tuatara are slightly smaller, weighing up to 660 g (1.3 lb).[23]
