Archosauromorpha evolutionary history
|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
Early Archosauromorph Evolution
The Permo-Triassic archosauromorphs represent a wonderful evolutionary succession. At one end, appearing in the late Permian period, there are reptiles that in appearance, habits, and metabolism were probably indistinguishable from modern lizards. There are the basal archosauromorphs, represented by unspecialised animals like Protorosaurus and the Prolacertiformes that superficially resembled large modern lizards (say 50 cm to 2 metres in length), differing in certain anatomical details (the most obvious being the large hind legs). Obviously there were other forms too, which lived in arid desert regions where the likelihood of fossilisation was very poor (Protorosaurus was apparently aquatic, or at least partially so).
From these lizard-like forms evolved a whole assemblage of reptiles, including several archosauromorph herbivore lineages, and more importantly, the basal Archosauriformes. The Archosauriformes (as opposed to the archosauromorphs) include more derived forms, but not the more plesiomorphic lizard-like ones. These are thecodonts, and the most basal Archosauriformes (historically, the Suborder Proterosuchia of the Order Thecodontia) first appear during latest Permian time. Their most ancestral members include the late Permian Archosaurus and the better known early Triassic Proterosuchus, whose name resembles Protorosaurus as much as Archosauriformes does Archosauromorpha. With all this resemblance of names, it is surprising anyone is not confused. But of more interest here is the fact that Proterosuchus and its relatives are much more derived animals than Protorosaurus. Of especial interest is the presence of an extra opening on each side of the skull in front of the eyes, the preorbital fenestra (which perhaps originally housed some sort of gland and is a distinguishing characteristic in almost all archosaurian skulls since, although some have secondarily lost it).
If Protorosaurus looked superficially like a large stocky lizard (a modern-day varanid might be a good analogy, at least in build), Proterosuchus was rather like a small crocodile, especially with its squat build and long toothy jaws. It would easily have been distinguished by the lack of armoured scutes; these only came later on the evolutionary tree.
Proterosuchus was "replaced" by more derived forms such as Erythrosuchus and Euparkeria. This "evolutionary succession" including further modification of the skull and (?)improved posture. All osteological modifications were inherited by the descendents of these taxa.
Euparkeria was a small facultatively bipedal animal with a double row of bony scutes along its back. The euparkeriids sister-group is the Archosauria proper or Crown Group Archosauria. Archosaurs the same relation to the Archosauriformes as the Archosauriformes do to the Archosauromorpha (think of it as like Russian dolls, each more basal/plesiomorphic or "primitive" clade includes the next more derived or "advanced" clade as among its members).
The early crown group archosaurs (the post-Proterosuchian thecodonts) were a diverse group of Triassic reptiles that dominated the land during the Middle and Late Triassic. They included small, agile two- and four-legged forms, large four-legged carnivores, armoured herbivores, and crocodile-like aquatic reptiles. They eventually gave rise to crocodiles, dinosaurs, and two rather different clades animals which achieved powered flight, the (pterosaurs and, through dinosaurs, the birds).
Early in the evolution of crown group Archosauria a dichotomous split into two major evolutionary lineages occurred, the crurotarsan line, which includes crocodilians, ateosaurs, phytosaurs and other closely related taxa, and the avemetatarsalian/ornithodiran line, 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).
In the late Triassic many clades within Crurotarsi had evolved fully erect (parasagittal) stance such as, rausuchids, prestosuchids, poposaurids and the sphenosuchians. All these groups would have been active, terrestrial hunters. Unfortunately, crurotarsians are a diverse but poorly known clade with the evolutionary relationships amongst them now being slowly reconstructed as more complete fossil specimens are found.
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).
Palaeos com MAK 991003 MAK030730; Palaeos org MAK061020); Mark T Young