Rhynchocephalia

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Rhynchocephalians
Temporal range:
Middle Triassic - Holocene,[1] 240–0 Ma
Homoeosaurus.jpg
Homoeosaurus fossil
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Rhynchocephalia
Günther, 1867
Families

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.[2] 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.[3]

Classification[edit]

Tuatara, Sphenodon punctatus

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.[4] 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.[5] R.L. Ditmars, Litt.D, says; "The Tuatara resembles in form stout-bodies 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. "[6] A unique feature of the tautara 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 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.[7]

Life reconstruction of Priosphenodon avelasi
Life reconstruction of Gephyrosaurus brindensis

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. [8] 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][9] Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon".[10] Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925.[10] Sphenodon is derived from the Greek for "wedge" (σφήν/sphen) and "tooth" (ὀδούς/odous).[11] However, today Rhynchocephalia is used to include Gephyrosaurus and Sphenodontia, while Sphenodontia excludes the former.[12][13]

The following is a cladogram of Rhynchocephalia after Rauhut et al., 2012.[14]

Rhynchocephalia

Gephyrosaurus


Sphenodontia

Diphydontosaurus




Planocephalosaurus





Homoeosaurus




Brachyrhinodon



Clevosaurus





Pleurosauridae

Palaeopleurosaurus



Pleurosaurus






Kallimodon



Sapheosaurus




Sphenodontidae

Sphenodon (Tuatara)




Oenosaurus




Cynosphenodon



Zapatadon





Opisthodontia

Opisthias


Eilenodontini

Toxolophosaurus




Priosphenodon



Eilenodon












References[edit]

  1. ^ Jones, M. E.; Anderson, C.; Hipsley, C. A.; Müller, J.; Evans, S. E.; Schoch, R. R. (2013). "Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara)". BMC Evolutionary Biology. 13: 208. doi:10.1186/1471-2148-13-208. PMC 4016551Freely accessible. PMID 24063680. 
  2. ^ Ditmars, Raymond L., "Reptiles of the World" The MacMillan Co., New York, 1936, p. xii
  3. ^ Reynoso, V. H. (2000). "An unusual aquatic sphenodontian (Reptilia: Diapsida) from the Tlayua Formation (Albian), central Mexico". Journal of Paleontology. 74: 133–148. doi:10.1017/s0022336000031310. 
  4. ^ Cree, Alison. 2002. Tuatara. In: Halliday, Tim and Adler, Kraig (eds.), The new encyclopedia of reptiles and amphibians, Oxford University Press, Oxford, pp. 210–211. ISBN 0-19-852507-9
  5. ^ "Hylonomus lyelli". Symbols. Province of Nova Scotia. May 2003. Retrieved 24 May 2007. 
  6. ^ Ditmars, 1936, p. xiv
  7. ^ http://animals.sandiegozoo.org/animals/tuatara
  8. ^ Lutz 2005, p. 42.
  9. ^ Lutz 2005, p. 43.
  10. ^ a b Fraser, Nicholas; Sues, Hans-Dieter; (eds) (1994). "Phylogeny" In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press. ISBN 0-521-45242-2. 
  11. ^ "Sphenodon". Dictionary.com Unabridged (v 1.1). Random House, Inc. Retrieved 8 January 2007. 
  12. ^ Evans, 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
  13. ^ Susan E. Evans & Magdalena Borsuk−Białynicka (2009). "A small lepidosauromorph reptile from the Early Triassic of Poland" (PDF). Paleontologica Polonica. 65: 179–202. 
  14. ^ Rauhut, O. W. M.; Heyng, A. M.; López-Arbarello, A.; Hecker, A. (2012). Farke, Andrew A, ed. "A New Rhynchocephalian from the Late Jurassic of Germany with a Dentition That is Unique amongst Tetrapods". PLoS ONE. 7 (10): e46839. doi:10.1371/journal.pone.0046839. PMC 3485277Freely accessible. PMID 23118861. 
  • Daugherty, CH; Cree, A; Hay, JM; Thompson, MB (1990). "Neglected taxonomy and continuing extinctions of tuatara (Sphenodon)". Nature. 347: 177–179. doi:10.1038/347177a0. 
  • 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. PMID 14700390. 
  • 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. PMID 18512698. 
  • 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,

External links[edit]