
Thecodontia (meaning 'socket-teeth'), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of early
archosaurian
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s 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 While the taxon Thecodontia is obsolete, the term ''thecodont'' remains in use as an anatomical description of teeth in bony sockets; in addition to species formerly in this group (such as crocodiles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 "
wastebasket taxon" for any archosaur other than a crocodilian, a pterosaur, or a dinosaur (any
basal
Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''.
Science
* Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure
* Basal (medicine), a minimal level that is nec ...
archosaur). Because the
cladistic paradigm only recognises
monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
taxa as natural groups, and because thecodonts are a
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
group (they include among their descendants animals that are not thecodonts), the term is no longer used as a formal name by most
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
Owen
Owen may refer to:
Origin: The name Owen is of Irish and Welsh origin.
Its meanings range from noble, youthful, and well-born.
Gender: Owen is historically the masculine form of the name. Popular feminine variations include Eowyn and Owena. ...
, 1859 was divided into four suborders, the
Proterosuchia
Proterosuchia is one of the suborders of the paraphyletic group Thecodontia; containing the most primitive and ancestral forms. These were primitive, vaguely crocodile-like, archosauriforms that mostly lived during the Early Triassic epoch.
Th ...
(early primitive forms, another paraphyletic assemblage),
Phytosauria (large crocodile-like semi-aquatic animals), the
Aetosauria (armoured herbivores), and the
Pseudosuchia (see e.g.,
Alfred Sherwood Romer's ''
Vertebrate Paleontology'' and
Edwin H. Colbert
Edwin Harris "Ned" Colbert (September 28, 1905 – November 15, 2001)O'Connor, Anahad ''The New York Times'', November 25, 2001. was a distinguished American vertebrate paleontologist and prolific researcher and author.
Born in Clarinda, Iowa, he ...
'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. Pseudosuchia as used in recent literature is a
stem-based taxon that includes
crocodile
Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...
-line
archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...
s, or all archosaurs (including
crocodilians) that are more closely related to crocodilians than
birds.
Robert Carroll, in his book ''
Vertebrate Paleontology and Evolution
''Vertebrate Paleontology and Evolution'' is an advanced textbook on vertebrate paleontology by Robert L. Carroll, published in 1988 by WH Freeman. It provides a very detailed technical account of various groups of living and fossil vertebrates. ...
'' (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.
Brian Gardiner (1982) attempted to define Thecodontia within a cladistic framework, thus giving the old name to a new concept. All more recent cladistic studies (e.g.,
Jacques Gauthier
Jacques Armand Gauthier (born June 7, 1948 in New York City) is an American vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology.
Life and career
Gauthier is the so ...
1986) have confirmed that the traditional Thecodontia is indeed a paraphyletic taxon, the members of which are not united by any
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 ersion 1.0, 7 November 2005
External links
Evolution Wiki: Thecodont* {{webarchive , url=https://web.archive.org/web/20101126131401/http://palaeos.com/Vertebrates/Bones/Teeth/ToothImplantation.html , date=November 26, 2010 , title=Five types of tooth implanting in non-mammals, including thecodont type
Dinosaurs of Rio grande do Sul.
Prehistoric reptile taxonomy
Prehistoric animal orders
Paraphyletic groups
Obsolete vertebrate taxa