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 rhynchocephalia ...

s that first appeared in the latest

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

period and flourished until the end of the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

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

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

s, and

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...

ns, 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 antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with archosauriforms, first appearing during the Triassic Period. Among extant archosaurs, bird ...

(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 Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

" for any archosaur other than a crocodilian, a pterosaur, or a dinosaur (any basal 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 gr ...

taxa as natural groups, and because thecodonts are a

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

group (they include among their descendants animals that are not thecodonts), the term is no longer used as a formal name by most

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

s, 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 (early primitive forms, another paraphyletic assemblage),

Phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent g ...

ia (large crocodile-like semi-aquatic animals), the

Aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivoro ...

ia (armoured herbivores), and the

Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

(see e.g.,

Alfred Sherwood Romer Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...

's ''

Vertebrate Paleontology Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by us ...

'' 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 Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

that includes

crocodile Crocodiles (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 include all extant me ...

-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-avia ...

s, or all archosaurs (including

ns) that are more closely related to crocodilians than

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s. Robert Carroll, in his book '' Vertebrate Paleontology and Evolution'' (1988), replaces Pseudosuchia with

Rauisuchia "Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians ...

, Ornithosuchia, and the traditional category ''

incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...

'' (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