page_title: Rhynchocephalia
section_title: Differences from lizards
hierarchical_section_title: Rhynchocephalia / Differences from lizards
caption_attribution_description: English: Life reconstruction of Priosphenodon avelasi
context_page_description: Rhynchocephalia is an order of lizard-like reptiles that includes only one living species, the tuatara of New Zealand, which is thought to have two subspecies the latter was recognised as a separate species from 1989 to 2009.
  Despite its current lack of diversity, the Rhynchocephalia at one time included a wide array of genera within multiple families, and represents a lineage stretching at least as far back as the Middle Triassic. Most rhynchocephalians belong to the group Sphenodontia, which are characterized by a hooked beak-like structure at the tip of the snout formed by enlarged premaxillary teeth.
  Many of the niches occupied by lizards today were held by sphenodontians during the Triassic and Jurassic, although lizard diversity began to overtake sphenodontian diversity in the Cretaceous. While the modern tuatara is primarily carnivorous, there were also sphenodontians with omnivorous, herbivorous, and durophagous lifestyles. There were even several successful groups of aquatic sphenodontians, such as pleurosaurs and Ankylosphenodon.
context_section_description: 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 juvenile tuatara 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. Furthermore, 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 approximately 240 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 Triassic, approximately 240 million years ago. The tuatara is traditionally called a "living fossil". This statement has been challenged, as some studies have considered it a model of evolutionary adaptation well adapted to its current conditions, rather than an unchanging remnant of the rest of Sphenodontia. A further major difference from lizards is the row of spine-like structures running part way down the back at least in the New Zealand species of Tuatara.
  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". These include the superficially similar (both in shape and name) but unrelated rhynchosaurs, which lived in the Triassic. 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.
caption: Life reconstruction of Priosphenodon avelasi
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