Unit 270: 

The Vertebrates


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Archosauromorpha: Overview

Abbreviated Cladogram


270.000 Overview
270.100 Archosauromorpha
270.200 Rhynchosauria
270.300 Prolacertiformes
270.400 Archosauriformes
270.500 Archosauria
270.600 Rauisuchiformes
270.700 Derived Rauisuchia
270.800 Suchia (Aetosauridae)
270.900 Other Suchia


The Early Archosauromorphs - Masters of the Triassic

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 Archosauria

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".

"Thecodontia" or "Archosauria"?

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.  

Four Suborders

Until quite recently the Order Thecodontia (adopting here the Linnean Evolutionary-Systematic approach) were considered a valid order divided into four suborders, the Proterosuchia, Phytosauria, the Aetosauridae, and the "Pseudosuchia" (more or less equivalent to the Crurotarsi, minus the more derived forms).   The Proterosuchia included various primitive ancestral types, the phytosaurs were crocodile like forms, the aetosaurs specialized armoured creatures, remarkably like the armoured ornithischian dinosaurs, and the Pseudosuchia were a sort of catch-all or "waste-basket" category for everything that didn't fit in any of the other suborders.  This last group was also considered ancestral to dinosaurs, birds, and crocodiles.  Although very similar anatomically and clearly related to each other by common ancestry and many shared ancestral traits (plesiomorphies),  the thecodonts are no longer considered a single taxon, but rather are divided along cladistic (phylogenetic) lines.   


The Thecodontian Archosaurs were for the most part were armoured scaly animals, probably ectothermic, which flourished during the warm dry desert conditions of the Triassic.  The only truly ectothermic archosaurians that persisted beyond the Triassic and still survive today are the Crocodylia.  The Dinosauria were or include some representatives that were at least partial endotherms (warm blooded) which also flourished during the Mesozoic and which still flourish as the fully endothermic birds (Aves).  Many scientists still reject the endothermic dinosaur hypothesis, although Bob Bakker (who started the whole warm-blooded dinosaur thing) says that many thecodonts were also warm-blooded.  However, Triassic thecodonts were much like crocodiles except they were fully terrestrial.  As descendents of the crurotarsan  thecodonts, crocodiles are ectothermic.  Since it is unlikely that a large warm-blooded animal will loose this metabolic characteristic and become cold-blooded, there is a solid case that Triassic thecodonts were also ectotherms.  In fact, the supremacy of the archosaurian thecodonts over the certainly partially endothermic proto-mammalian therapsids during the Triassic can be seen as the result of the superiority the reptilian metabolism has in hot dry conditions, such as characterized much of Pangea during the Triassic. In fact, for much of Australia's Plio-Pleistocene history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic varanid lizards (Megalania) and land crocs, (and possibly also, as in South America, giant flightless birds)

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. warm-blooded.

Early Archosauromorph Evolution

ProtorosaurusThe 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).

ProterosuchusFrom 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.

EuparkeriaProterosuchus 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.  

TicinosuchusThe 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.

LagosuchusWhereas 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.

PoposaurusThe 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.

The End of the Thecodont Dynasty

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) 

Thumbnail credits - illustrations from Encyclopaedia of Dinosaurs and Prehistoric Animals, except for Poposaurus, which is by Robert Gay
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