REPTILOMORPHA |--LEPIDOSAUROMORPHA `--Archosauromorpha |--Rhynchosauria `--+--Prolacertiformes `--Archosauriformes |--Euparkeriidae `--Archosauria |--Ornithodira | |--PTEROSAURIA | `--DINOSAUROMORPHA `--Crurotarsi |--Phytosauridae `--Rauisuchiformes |--Rauisuchia | |--Ornithosuchidae | `--+--Prestosuchidae | `--Rauisuchidae `--Suchia |--Aetosauridae `--CROCODYLOMORPHA
|Triassic Archosauromorphs. A pair of giant predatory prestosuchids (Saurosuchus galilei) appropriate a hyperodapedontine rhynchosaur carcass (Scaphonyx sanjuanensis), causing small herrerasaurid dinosaurs and
ornithosuchids (Venaticosuchus rusconii) to flee.
Illustration by Greg S Paul, from K Padian (ed.), The Beginning of the Age of Dinosaurs. Cambridge Univ. Press.
During the Triassic period, at the start of the Mesozoic era, there lived a whole lot of animals that were no longer small and insignificant lizard-like forms, but not actually crocs, dinosaurs, pterosaurs, or birds. They used to be called "thecodonts", and the term is still used and is a very good one, even if cladistically rigid. A more precise term might be Basal or Stem Archosauromorphs. Archosauromorphs because this is a clade that includes the Archosauria or "ruling reptiles" and a number of similar types. "Basal" because they stand near the base or beginning of this great evolutionary adventure. Stem because they constitute the ancestral trunk rather than the later ramifications. They include both the ancestors of dinosaurs and other animals, and a number of independent evolutionary lines that are important in their own right. And they dominated the Triassic. For as long as they were there, no crocodile or armored herbivorous or general carnivorous dinosaur could grow very big. It was only one of the accidents in the history of life on Earth, an impact by a wandering asteroid or comet, that wiped the "thecodonts" from the land and provided their successors, the dinos and crocs, to take their place.
The name Archosauria means "ruling reptiles" and these are the creatures that dominated the world during the Mesozoic era. This great group of animals includes dinosaurs, birds, crocodiles, pterosaurs (flying reptiles) and many other groups of creatures. The archosaur skeleton has several anatomical characteristics that distinguish it from other groups of animals, such as an extra opening on each side of the skull in front of the eyes (the preorbital fenestra), and teeth set in sockets ("thecodont" teeth) which anchors them, and makes them less likely to be lost during feeding. Some archosaurs differ from other reptiles in their more upright posture which eliminated the sinuous lateral flexure of the skeleton when moving, such as is found in lizards. Archosaurs also differ from more primitive sauropsids in practice of parental care of the young and in a four-chambered heart (for more efficient distribution of oxygenated blood to the rest of the body). Some archosaurs, such as birds and coelurosaurian dinosaurs also developed large brains, indicating a higher level of intelligence. It has even been speculated that if a bolide had not seen out the dinosaurs, then they would have eventually evolved into a human-grade intelligent species, a "dinosauroid." Just because intelligent life on planet Earth is presently mammalian is no reason to suppose that it has to be necessarily so. We are truly accidents of history.
Most of the more derived archosaurs are covered in separate sections relating to crocodylomorphs, dinosaurs, birds, and pterosaurs. This unit deals with the basal forms historically known as "thecodonts", along with several of their more primitive and less well-known lizard-like cousins, generally grouped under the assemblage of "basal archosauromorphs".
A brief explanation on the terminology used here. The name Thecodontia often appears in popular books on palaeontology and evolution. But because of the prevalence of the cladistic paradigm over evolutionary systematics, the term Thecodontia is no longer much 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. It is still a useful term
as shorthand for "basal archosauromorphs," and we will use it in
that sense here. In fact, the term "thecodont" is more limited and usually refers to the somewhat more derived basal archosauriforms. These are reptiles in which the ankle has been somewhat simplified and reorganized to permit the more erect
carriage mentioned above.
Birds however, which are the descendents of dinosaurs, are warm-blooded; and theropod dinosaurs are extremely
similar to birds. Several advanced types (protobirds) have even been found
fossilized with a coating of feathers or feather-like structures. It can be assumed
therefore that some dinosaurs (especially the small advanced coelurosaurs) were basically like modern-day birds, i.e.
The Permo-Triassic Archosaurs 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 unspecialized animals like Protorosaurus and the Prolacertiformes, which superficially resembled from large modern lizards (say 50 cm to 2 meters in length), differing in certain anatomical details (the most obvious being the large hind legs, but many differences in the skull and skeleton show that these were not the animals that gave rise to modern lizards. Obviously there were other forms too, which lived in arid desert regions where the likelihood of fossilization was very poor (Protorosaurus was actually 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 somewhat more advanced forms, but not the more primitive 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 primitive, ancestral members include the late Permian Archosaurus and the better known early Triassic Proterosuchus, whose name resembles Protorosaurus as much as Archosauriformes does Archosauromorphs. 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 advanced 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 however by the lack of armoured scutes; these only came later on the evolutionary tree.
Proterosuchus was replaced in turn by even more advanced forms, such as Erythrosuchus and Euparkeria, which constituted even further evolutionary succession, including further modification of the skull, and improved posture. Again, all these characteristics were carried on to their descendents.
Euparkeria was a small active animal with a double row of bony scutes along its back. In time its descendents formed a new monophyletic clade, the Archosauria proper or Crown Group Archosauria (or at least the most basal and ancestral members of the taxon), which have the same relation to the Archosauriformes as the Archosauriformes do to the Archosauromorphs (think of it as like Russian dolls, each more basal or "primitive" clade includes the next more advanced clade as among its members) Archosauriformes does Archosauromorphs
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, armored herbivores, and crocodile-like aquatic reptiles. They eventually gave rise to crocodiles, dinosaurs, and two quite different breeds of flying reptiles (pterosaurs and, through dinosaurs, the birds).
At a very early stage, these basal crown group archosaurs split into two major evolutionary lineages, the crurotarsian line, which includes crocodiles and a number of other groups, and the ornithodiran line, which includes the ancestors of dinosaurs, birds, and pterosaurs.
The forms that evolved from early Crurotarsian ancestors included the phytosaurs, large semi-aquatic crocodile-like phytosaurs; the terrestrial carnivorous prestosuchids; the armoured herbivorous aetosaurs; the ctenosauriscids, strange sail-backed forms; and other little known types. This pseudosuchian 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 similar forms, three other lines, all of which, although soon becoming extinct themselves curiously presaged later evolutionary lines.
Whereas the Crurotarsi quickly diversified and evolved, the ornithodires remained small, bird-like forms, which tended more and more towards total bipedality. This cursorial, or agile running tendency, was continued further in Marasuchus ( = Lagosuchus, late Ladinian; mid-Triassic), the slim, long-legged ancestors of the dinosaurs, and possibly of the Pterosaurs as well.
The highest development of the Thecodontia was represented by the late Triassic Rauisuchia, advanced Crurotarsian types like the large carnivorous rauisuchids and poposaurs which replaced the prestosuchids, and a number of small running forms, which included the direct ancestors of the crocodiles. They represent a diverse but poorly known assemblage; active, fully terrestrial, with upright dinosaurian and mammalian type stance. The difficulty of determining evolutionary relationships among them is the result of mostly incomplete material. Hopefully this will be resolved in the future as more complete specimens are studied.
By the late Triassic then, the
thecodont archosaurs had reached their maximum diversity.
They included both large and small dinosaur-like bipeds (long tailed animals
that ran on their hind legs), armored aetosaur herbivores, 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 (phytosaurs and proterochampsids), and active little
four-legged runners (the Sphenosuchia, variously considered to be crocodiles and thecodonts). The ornithodire line meanwhile had already produced the
first dinosaurs and pterosaurs, but these early ornithodires, although common, for the most part remained in the shadow of 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 lineage evolved the large semi-aquatic crocodilians that would share the dinosaur's world, and ultimately outlive them. (MAK 991003 & 030730)