Thecodont 414272 207914132 2008-04-24T18:04:09Z RussBot 279219 Robot-assisted [[WP:DATE|date formatting]] [[Image: MUSEU1X.JPG | right | 250px | thumb | Girl looks at Thecodont (''[[Karamuru]] vorax''). Source: [[paleorrota]]]] '''Thecodont''' ("socket-toothed" reptile), now considered an obsolete term, was formerly used to describe a diverse range of early [[archosaur]]s that first appeared in the Latest [[Permian]] and flourished until the end of the [[Triassic]] period. The group includes the ancestors of [[dinosaur]]s (including [[bird]]s), and ancestors of [[pterosaur]]s, and [[crocodilia]]ns, as well as a number of extinct forms that did not give rise to any descendants. ==Definition== Thecodonts are defined by certain shared primitive features, such as the [[suborbital fenestra]] (an opening on each side of the skull between the eye sockets and the nostrils) and teeth in sockets. The name Thecodont is Greek for "socket-tooth," referring to the fact that thecodont teeth were set in sockets in the jawbones; an archosaurian characteristic that was inherited by the dinosaurs. They constitute an evolutionary grade of animals, a "[[wastebasket taxon]]" for any archosaur that wasn't a crocodilian, a pterosaur, or a dinosaur (i.e. any [[Basal (phylogenetics)|basal]] archosaur). Because the [[clade|cladistic paradigm]] only recognises [[monophyletic]] taxa as natural groups, and because Thecodonts are a [[paraphyletic]] group (i.e. they include among their descendants animals that are not thecodonts), the term is no longer used by most [[paleontologist]]s, although it can still be found in older (and even fairly recent) books. ==Taxonomic History== Traditionally, the order '''Thecodontia''' <small>[[Richard Owen|Owen]], 1859</small> was divided into four suborders, the [[Proterosuchia]] (early primitive forms, another paraphyletic assemblage), [[Phytosaur]]ia (large crocodile-like semi-aquatic animals), the [[Aetosaur]]ia (armoured herbivores), and the [[Pseudosuchia]] (see e.g. [[Alfred Sherwood Romer]]'s ''[[Vertebrate Paleontology (Romer)|Vertebrate Paleontology]]'' and [[Edwin H. Colbert]]'s ''[[Evolution of the Vertebrates]]''). Of these, only Phytosaurs and Aetosaurs constitute monophyletic groups, and the term Pseudosuchia was simply a catch-all term for any species that didn't fit in one of the other three sub-orders. [[Robert L. Carroll|Robert Carroll]] in his book ''[[Vertebrate Paleontology and Evolution]]'' ([[1988]]) replaces Pseudosuchia with [[Rauisuchia]], [[Ornithosuchidae|Ornithosuchia]], and the traditional category ''[[incertae sedis]]'' (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognises the taxon Thecodontia, as it uses a traditional [[Linnaean taxonomy|Linnaean based taxonomy]]. [[Brian Gardiner]] (1982) attempted to define Thecodontia within a cladistic framework, thus giving the old name to a new concept. All recent cladistic studies (e.g. [[Jacques Gauthier]] 1986) have confirmed that the traditional Thecodontia is indeed a paraphyletic taxon, the members of which are not united by any [[synapomorphy|shared derived characteristics]]. As the association of the name with the outdated concept proved to be very strong, it is now considered a historical term only, and its current usage has been abandoned. All current [[vertebrate paleontology]] textbooks, (e.g. [[Michael Benton]]'s ''[[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]]'' (first ed. [[1990]], 2nd ed. [[1997]]), follow the [[cladistic]] approach, and so the name [[Archosaur]]ia is used instead. This includes both the Thecodonts and all their descendants. ==See also== *[[Paleorrota]] ==External links== * [http://wiki.cotch.net/wiki.phtml?title=Thecodont Evolution Wiki: Thecodont] * [http://www.palaeos.com/Vertebrates/Bones/Teeth/ToothImplantation.html Five types of tooth implanting in non-mammals, including thecodont type.] *[http://www.royalsul.com.br/paleo/galeria.asp Dinosaurs of Rio grande do Sul.] ==References== * Benton, M. J. 1997, ''Vertebrate Paleontology'', Blackwell Science Ltd * Carroll, R. L. 1988, ''Vertebrate Paleontology and Evolution'', W. H. Freeman and Co. New York * Colbert, E H. 1969, ''Evolution of the Vertebrates'', John Wiley & Sons Inc (2nd ed.) * Gardiner, BG (1982). Tetrapod classification. ''Zool. J. Linn. Soc. London'' 74: 207-232. * Gauthier, J., 1986. Saurischian monophyly and the origin of birds. In: K. Padian, ed. The Origin of Birds and the Evolution of Flight. Memoirs California Academy of Sciences 8. pp. 1–55 * Sereno, P. C. 2005. [http://www.taxonsearch.org/Archive/stem-archosauria-1.0.php Stem Archosauria—TaxonSearch] [version 1.0, 7 November 2005] [[Category:Prehistoric reptiles]] [[Category:Archosaurs]] [[de:Thecodontia]] [[es:Thecodontia]] [[fr:Thecodontia]] [[it:Thecodontia]] [[he:תקודונטיה]] [[nl:Thecodontia]] [[pl:Tekodonty]] [[pt:Tecodonte]] [[ru:Текодонты]] [[zh:槽齒目]]