Rhynchocephalia is an order of lizard-like reptiles that includes only one living genus and species, the tuatara (Sphenodon punctatus). 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 was even a successful group of aquatic sphenodontians known as pleurosaurs.
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.1 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, the Hylonomus, resembles a modern lizard.2
Tuatara were originally classified as lizards in 1831 when the British Museum received a skull.3 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.4 Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon".5Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925.5Sphenodon is derived from the Greek for "wedge" (σφηνος/sphenos) and "tooth" (δόντι/odon(t)).6 However, today Rhynchocephalia is used to include Gephyrosaurus and Sphenodontia, while Sphenodontia excludes the former.78
Classification follows Wu (1994),9 Evans et al. (2001),7 Apesteguia & Novas (2003)10 and Evans & Borsuk−Białynicka (2009).8
^ abFraser, Nicholas; Sues, Hans-Dieter; (eds) (1994). "Phylogeny" In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press. ISBN0-521-45242-2.
^"Sphenodon". Dictionary.com Unabridged (v 1.1). Random House, Inc. Retrieved 8 January 2007.
^ abcEvans, S. E., Prasad, G. V. R. & Manhas, B. K., 2001: Rhynchocephalians (Diapsida: Lepidosauria) from the Jurassic Kota Formation of India. Zoological Journal of the Linnean Society: Vol. 133, #3, pp. 309-334
^ abWu, X-C. 1994: Late Triassic-Early Jurassic sphenodontians from China and the phylogeny of the Sphenodontia. in Fraser, N. C. & Sues, H-D.. 1994: In the Shadow of the Dinosaurs. Cambridge University Press, New York. 1994
^ abApesteguía S, Novas FE (2003) Large Cretaceous sphenodontian from Patagonia provides insight into lepidosaur evolution in Gondwana. Nature, 425:609–612
Daugherty CH, Cree A, Hay JM, Thompson MB (1990). Neglected taxonomy and continuing extinctions of tuatara (Sphenodon). Nature 347, 177–179.
Evans SE. 2003. At the feet of the dinosaurs: the early history and radiation of lizards. Biological Reviews, 78:513-551. doi:10.1017/S1464793103006134
Jones MEH. 2008. Skull shape and feeding strategy in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria). Journal of Morphology. 269: 945–966. doi:10.1002/jmor.10634
Jones MEH. 2009. Dentary tooth shape in Sphenodon and its fossil relatives (Diapsida: Lepidosauria: Rhynchocephalia). In Koppe T, Meyer G, Alt KW, eds. Interdisciplinary Dental Morphology, Frontiers of Oral Biology (vol 13). Greifswald, Germany; Karger. 9–15.
Evans SE, Jones MEH (2010) The Origin, early history and diversification of lepidosauromorph reptiles. In Bandyopadhyay S. (ed.), New Aspects of Mesozoic Biodiversity, 27 Lecture Notes in Earth Sciences 132, 27-44. doi:10.1007/978-3-642-10311-7_2,