Rhynchocephalia is an order of lizard-like reptiles that includes only one living species of tuatara, which in turn has two subspecies (Sphenodon punctatus punctatus and Sphenodon punctatus guntheri), which only inhabit parts of New Zealand. Despite its current lack of diversity, the Rhynchocephalia at one time included a wide array of genera in several families and represents a lineage stretching back to the Mesozoic Era. Many of the niches occupied by lizards today were then held by sphenodontians. There were even several successful groups of aquatic sphenodontians, such as pleurosaurs and the bizarre Ankylosphenodon.
Sphenodonts, and their sister group Squamata (which includes lizards, snakes and amphisbaenians), belong to the superorder Lepidosauria, the only surviving taxon within Lepidosauromorpha. Squamates and sphenodonts both show caudal autotomy (loss of the tail-tip when threatened), and have transverse cloacal slits. The origin of the sphenodonts probably lies close to the split between the Lepidosauromorpha and the Archosauromorpha. Though they resemble lizards, the similarity is superficial, because the group has several characteristics unique among reptiles. The typical lizard shape is very common for the early amniotes; the oldest known fossil of a reptile, Hylonomus, resembles a modern lizard. R.L. Ditmars, Litt.D, says; "The Tuatara resembles in form stout-bodied modern lizards, which we might call iguanas; this resemblance is further intensified by a row of spines upon the back. It is dark olive, the sides sprinkled with pale dots. The eye has a cat-like pupil. Large specimens are two and a half feet long. While superficial resemblance might tend to group this reptile with lizards, its skeleton and anatomy show it to belong to a different part of a technical classification." A unique feature of the tuatara is a “third eye” on the top of the head. The “eye” has a retina, lens, and nerve endings, but is not used for seeing. It is visible under young tuataras’ skin but after a few months becomes covered with scales and pigment. The unique eye is sensitive to light and is believed to help the tuatara judge the time of day or season. Additionally, it has two parallel rows of teeth in its upper jaw, and the gap between these rows are where the teeth from the lower jaw fit to perform special grinding/sawing motion to crush prey. Additionally, the tuatara has a diapsid skull but lacks a complete lower temporal bar, which separates it from other species as well as its acrodont dentition and overhanging pair of incisor-like teeth. Tooth shape was originally designed for a strictly insectivore diet with piercing teeth. Later on, the teeth became more diversified for various ancestors of the tuatara, which included herbivores, carnivores, and omnivores. The teeth were complex enough to crush crab shells, while others maintained continuous growth in the lower jaw for the breakdown of plant material. Their current dental structure is specialized for grinding up prey after capture. The fossil record shows the tuatara lineage separating from squamates 250 million years ago.
Tuatara were originally classified as lizards in 1831 when the species was discovered by John Edward Gray and the British Museum received a skull. The fossil records show that they have been around since the middle of the Triassic Period, approximately 251-199 million years ago. The tuatara is often considered to be a “living fossil”, which is currently being challenged by people who consider them to be a model of evolutionary adaptation who are well adapted to their current conditions and are not an unchanging group. The name tuatara was given to the vertebrate by the Maori people, the indigenous Polynesian people of New Zealand. The word tuatara means “peaks on the back” or “spiny back” for their obvious dorsal crest of pointed scales down their head, back and tail. 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.[dead link] 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/sphen) and "tooth" (ὀδούς/odous). However, today Rhynchocephalia is used to include Gephyrosaurus and Sphenodontia, while Sphenodontia excludes the former.
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