Thecodontia

Thecodontia (meaning 'socket-teeth'), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of early

archosaurian reptiles that first appeared in the latest Permian period and flourished until the end of the Triassic period. All of them were built somewhat like crocodiles but with shorter skulls, more erect pose and usually somewhat lighter. The group includes the ancestors of dinosaurs, pterosaurs, and crocodilians

, as well as a number of extinct forms that did not give rise to any descendants. The term thecodont is still used as an anatomical description of the tooth morphology seen in these species and others.

Definition

Thecodonts are characterized by certain shared primitive features, such as the antorbital 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. While the taxon Thecodontia is obsolete, the term thecodont remains in use as an anatomical description of teeth in bony sockets; in addition to species formerly in this group (such as crocodiles and dinosaurs), mammals also possess thecodont dentition, which evolved independently.

They constitute an evolutionary grade of animals, a "

paleontologists

, but it can still be found in older (and even fairly recent) books as a convenient shorthand for the basal archosaurs.

Taxonomic history

Traditionally, the order Thecodontia

stem-based taxon that includes crocodile-line archosaurs, or all archosaurs (including crocodilians) that are more closely related to crocodilians than birds

.

Robert Carroll, in his book Vertebrate Paleontology and Evolution (1988), replaces Pseudosuchia with Rauisuchia, Ornithosuchia, and the traditional category incertae sedis (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognizes the taxon Thecodontia, as it uses a traditional Linnaean based taxonomy.

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 As the association of the name with the outdated concept proved to be very strong, it is now considered a historical term only, and its current usage has been abandoned.

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. Stem Archosauria—TaxonSearch [version 1.0, 7 November 2005]

External links

Evolution Wiki: Thecodont
Five types of tooth implanting in non-mammals, including thecodont type at the Wayback Machine (archived November 26, 2010)
Dinosaurs of Rio grande do Sul.