Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (Sphenodon punctatus) of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a diverse group including a wide array of morphologically distinct forms. The oldest record of the group is dated to the Middle Triassic around 238 to 240 million years ago, and they had achieved a worldwide distribution by the Early Jurassic. Most rhynchocephalians belong to the group Sphenodontia ('wedge-teeth'). Their closest living relatives are lizards and snakes in the order Squamata, with the two orders being grouped together in the superorder Lepidosauria. 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, and they had disappeared almost entirely by the beginning of the Cenozoic. While the modern tuatara is primarily carnivorous, there were also sphenodontians with omnivorous (Opisthias), herbivorous (Eilenodontinae), and durophagous (Oenosaurus) lifestyles. There were even several successful groups of aquatic sphenodontians, such as pleurosaurs and Ankylosphenodon. History of discovery Tuatara were originally classified as agamid lizards when they were first described by John Edward Gray in 1831. They 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. In 1925 Samuel Wendell Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives. Sphenodon is derived from Greek 'wedge' and 'tooth'. 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. These were resolved after use of computer based cladistics, which showed the core sphenodontian grouping to be monophyletic. Classification and anatomy 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 sphenodontians have a number of shared traits (synapomorphies), including fracture planes within the tail vertebrae allowing caudal autotomy (loss of the tail-tip when threatened), transverse cloacal slits, an opening in the pelvis known as the thyroid fenestra, the presence of extra ossification centres in the limb bone epiphyses, a knee joint where a lateral recess on the femur