|ARCHOSAUROMORPHA - Archosauromorpha, Archosauria (clade), Thecodontia (grade)|
o Sauria |--Lepidosauromorpha `--o Archosauromorpha |?--Choristodera `--+--Rhynchosauria `--+--Trilophosauridae `--+--Prolacertiformes `--o--Proterosuchidae `--+--Erythrosuchidae `--+--Euparkeriidae `--o--Proterochampsidae `--o Archosauria |--o Ornithodira | |--Pterosauria | `--Dinosauromorpha `--Crurotarsi
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What is a thecodont?
The Thecodonts were a diverse grade of Triassic reptiles that included small, agile two- and four-legged forms, large four-legged carnivores, armoured 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 in current literature, 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, as well as by the absense of advanced and specialised crocodylian, pterosaurian, and dinosaurian characteristics. 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 animals that in the past were called thecodonts. They are the same animals, only the taxonomy has changed. All these animals were very similiar anatomically and clearly related to each other by common ancestry and many shared ancestral traits (plesiomorphies). However, thecodonts are nowadays no longer considered a single taxon, but rather are divided along cladistic (sister-group) lines.
o Archosauria |--Ornithodira `--o CRUROTARSI |--Phytosauridae `--o--o?--Ornithosuchidae | `--+?--Prestosuchidae (grade?) | `--+--Rauisuchidae | `--+?--Ctenosauriscidae | `?--Poposauridae `--o--Stagonolepididae `--+--Euscolosuchus `--o--Gracilisuchus stipanicicorum |?--Poposauridae `--o--Erpetosuchus granti `--Crocodylomorpha
Until quite recently the Thecodontia (following the Linenan Evolutionary-Systematic approach) were considered a valid order divided into four suborders, the Proterosuchia, Phytosauria, the Aetosauria, and the Pseudosuchia. The Proterosuchia include various ancestral types, the phytosaurs were crocodile-like forms, the aetosaurs were armoured forms remarkably convergent with some armoured ornithischian dinosaurs, and the Pseudosuchia is a sort of catch-all or "waste-basket" category for everything that doesn't fit in any of the other suborders. This last group is also considered ancestral to dinosaurs, birds, and crocodiles.
Thecodont Evolutionary History
The Thecodontian evolved as sprawling semi-aquatic predators (Proterosuchidae) in tropical Pangea during the late Permian. For several 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 centre 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 a close sister group to Archosauria proper (crown-group), and possibly a good model of the ancestral archosaur. Their descendents split into major evolutionary lineages, the crurotarsian or pseudosuchian clade which includes crocodilians, ateosaurs, phytosaurs and other closely related taxa and the avemetatarsalian or ornithodiran clade, which includes the dinosaurs, pterosaurs and related taxa.
Various morphological forms evolved within the Crurorarsi including the phytosaurs (large semi-aquatic crocodile-like forms); the rauisuchids and prestosuchids (small to very large quadrapedal terrestrial carnivores), poposaurids/shuvosaurids (bipedal Ostrich-like forms), the armoured herbivorous aetosaurs; ctenosauriscids (sail-backed quadrapedal carnivores) and a number of small cursorial forms, one of which during the late Triassic included earliest "crocodilians". The Crurotarsian or crocodile-related line constituted the dominant branch of what was considered to be thecodonts.
Whereas the Crurotarsi diversified into a variety of ecomorphological forms (many of them convergent with later dinosaurs), the avemetatarsalians remained as small bird-like forms which early in their evolution evolved total bipedality. Cursorality (agile running tendency) was well developed in Marasuchus (late Ladinian; mid-Triassic), the slim, long-legged sister-taxon of Dinosauria. Scleromochlus is at the moment the best example of what the ancetral bauplan of basal avemetatarsalians could have been (all though being late Carnian in age it is younger than the "lagosuchians"). It was a small (?facultative) biped insectivore that recent phylogenetic reconstructions have placed at the base of Avemetatarsalia (Benton 1999).
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), armoured herbivores (aetosaurs), several lines of large terrestrial carnivores, the large predatory ornithosuchids capable of running on either their hind legs or on all fours (facultatively bipedal), crocodile-like semi-aquatic predators (phytosaurids and proterochampsids) and small, active quadrapedal cursors such as the sphenosuchians and Erpetosuchus (which were considered to be thecodonts, though are now known to be either the basal most crocodilians or the sister-group to them). The thecodont "lagosuchians" however, remained small as did the first dinosaurs (with the notable exception of the sauropodomorphs which by the Norian became the dominant herbivorous clade).
Just as the terminal Permian mass extinction paved the way for the thecodont revolution with the extinction of most of the therapsids, so to the terminal Triassic extinction killed off most of the Crurotarsians, with only the sphenosuchians and protosuchian crocodilians surviving. With the vacation of many terrestrial niches dinosaurs underwent a mass-radiation; the age of the dinosaurs had begun. The dinosaurs and pterosaurs would continue to dominate terrestrial and aerial niches until the end of the Mesozoic, whilst the small terrestrial protosuchian crocodilians would give rise to the wide variaty of crocodilians that would exist throughout the Mesozoic (such as herbivores, marine finned forms and giant semi-aquatic dinosaur eaters), and eventually the semi-aquatic forms we are familar with today. It would be these forms that would share the dinosaur's world and ultimately outlast them.
Kheper MAK981113; Palaeos org MAK061022; Mark T Young