The Thecodonts were a diverse group of Triassic reptiles that included small, agile two- and four-legged forms, large four-legged carnivores, armored herbivores, and crocodile-like aquatic reptiles. They gave rise to crocodiles, dinosaurs, and probably flying reptiles (pterosaurs).
Because of the prevaliance of the cladistic paradigm over evolutionary systematics, the term Thecodontia is no longer used, as they are a paraphyletic group. The term still appears however in older books and in various on-line taxonomic lists based on them.
The Thecodontia are defined by certain shared primitive or ancestral 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 actually Latin 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.
Thecodontia therefore is an evolutionary grade of animals, rather than a clade. They represent a "grab-bag" taxon for any archosaur that wasn't a crocodilian, a pterosaur, or a dinosaur. Most palaeontologists nowadays use the term "basal archosaur" to refer to Thecodonts.
The Thecodontia are generally divided into four suborders, the Proterosuchia, Phytosauria, the Aetosauria, and the Pseudosuchia. However the Pseudosuchia constitute an artificial group, having the same "grab-bag" status within the Thecodontia as the Thecodontia have within the Archosauria. Robert Carroll in his book Vertbrate Paleontology and Evolution replaces the Pseudosuchia with the Rauisuchia and the Ornithosuchia.
See Thecodontian Systematics - changing perspectives for more on this.
The Thecodontia were for the most part were armoured scaly archosaurs, probably ectothermic, which flourished during the warm dry desert conditions of the Triassic. The only truely ectothermic archosaurians that persisted beyond the Triassic and still survive today are the Crocodylia. The dinosauria were at least partial endotherms (warm blooded) which also flourished during the Mesozoic and still flourish as the bird kingdom (class or superorder Aves). Many scientists still reject the endothermic dinosaur hypothesis, while Bob Bakker (who started the whole warm-blooded dinosaur thing) says that many thecodonts were also warm-blooded. I get the feeling though that Triassic thecodonts were a lot like crocodiles except they were fully terrestrial. As descendents of the "Pseudosuchian" (or Dromaeosuchian) thecodonts, crocodiles are ectothermic. Since it is unlikely that a warm-blooded animal will loose it's metabolic advantage and become cold-blooded, I think there is a pretty good case that Triassic thecodonts were also ectotherms. Birds however, which are the descendents of dinosaurs, are warm-blooded; and as at least from their skeletons theropod dinosaurs are extremely similiar to birds, and several advanced types (protobirds) have even been found fossilised with a coating of feathers or feather-like structures. It can be assumed therefore that some dinosaurs were basically like modern-day birds, i.e. warm-blooded.
The Thecodontian evolved as sprawling semiquatic archosaurian preditors (Proterosuchidae) in tropical Pangea during the late Permian. For some fifteen million years they remained rare, undergoing very little evolutionary change. The terminal Permian catastrophe, which killed off 95% of all types of life, cleared the world of all large Therapsids and allowed the Proterosuchids to take center stage as the top carnivore. Within the space of five million years the Proterosuchids had evolved into a wide variety of terrestrial and semi-aquatic carnivores.
Early Triassic thecodonts like Euparkeria were the source of a further evolutionary radiation. Their descendents split into major evolutionary lineages, the Curotarsian or Pseudosuchian line, which includes crocodiles and a number of other groups, and the Ornithodiran line, which includes the ancestors of dinosaurs and crocodiles. Thuis is shown in the following diagram
Great
Triassic Assemblages Pt 1 - The Chinle and Newark - Professor Paul
Eric Olsen.
This has Crurotarsi as an unresolved tricotomy consisting of Phytosaurs,
Ornithosuchuia, and the Suchia as the most advanced and specialised group.
Note: there is still some disagreement whether the very dinosaur-like Ornithosuchia
belong to the dinosaur or the crocodile lineage. For more info see
the following webpage (slow to load because of the graphics - scroll down
about 5 screen/page lengths):
Great
Triassic Assemblages Pt 1 - The Chinle and Newark
- Professor P. E. Olsen.
The forms that evolved from the Euparkeriids included the Parasuchids (or Phytosaurs), large semi-aquatic crocodile-like forms; the Rauisuchids, Prestosuchids, and Poposaurs, large to very large terrestrial carnivores; the Ctenosauriscids, strange sail-backed forms; and a number of distinct lines of small running forms, one of which was ancestral to the crocodiles. The Psuedosuchian or crocodile-related line constituted the dominant branch of Thecodonts. Among their members they included, along with the small bird-/dinosaur-like crocodile-ancestors and similiar forms, three other lines, all of which, although soon becoming extinct themselves curiously presaged later evolutionary lines.
Whereas the Crurotarsi (Psuedosuchians) quickly diversified and evolved, the Ornithodirans remained as small bird-like forms, which tended more and more towards total bipedality. This cursorial, or agile running tendency, was continued further in Lagosuchus (late Ladinian; mid-Triassic), the slim, long-legged ancestors of the dinosaurs, and possibly of the Pterosaurs as well.
By the late Triassic the thecodonts had reached their maximum diversity. They included both large and small dinosaur-like bipeds (long tailed animals that ran on their hind legs), armored herbivores (aetosaurs), several lines of large terrestrial carnivores, the large predatory Ornithosuchids, capable of running on either their hind legs or on all fours, and crocodile-like semi-aquatic predators (parasuchians and proterochampsids), and active little four-legged runners (the Sphenosuchia, variously considered to be crocodiles and thecodonts). The Lagosuchids meanwhile had already evolved into the first dinosaurs and pterosaurs, but these early dinosaurs were very much subordinate to the large thecodonts.
Just as the terminal Permian mass extinction paved the way for the Thecodont revolution by eliminating most of the Therapsids, so the terminal Triassic extinction killed off all but a few Sphenosuchids, allowing their fellow archosaurs the Dinosaurs to take over as the dominant life-form. The Dinosaurs and pterosaurs would continue to dominate terrestrial and aerial niches until the end of the Mesozoic, while the small terrestrial Sphenosuchids evolved into the large semi-aquatic crocodilians that would share the dinosaur's world, an dultimately outlive them..
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Rauisuchia |
Aetosauria |
Dromaeosuchia |
Ornithosuchia |
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| primitive semi-aquatic ancestral forms - transitional
between Thecodontia and Protorosauria.
Pangean distribution |
large to very large semi-aquatic carnivores
Pangean distribution |
medium-sized semi-aquatic carnivores, limited to Western Gondwanaland | small active terrestrial forms, ancestral to all higher Archosaurs |
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| large crocodile-like forms |
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Family Ornithosuchidae |
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medium to large terrestrial carnivores |
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large terrestrial carnivores |
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Palaeos Page (incorporates some of this material, plus a lot of additional material)
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