|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
The 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 archosaurians that still survive today are the Crocodylia, which are ectothermic. The Dinosauria were, or include some representatives that were at least partial endotherms (warm blooded) that flourished during the Mesozoic and which still do today as the fully endothermic birds (Aves). Some scientists reject the endothermic dinosaur hypothesis, although Bob Bakker (who re-invigorated the warm/cold-blooded dinosaur debate) states that many thecodonts were also warm-blooded. However, Triassic archosaurs were much like crocodiles except they were fully terrestrial.
Crocodilians, which are ectothermic are the last surviving crurotarsians. Although it is thought to be unlikely that an endothermic animal would loose the metabolic mechanisms and revert to ectothermy, it has been hypothesised to have happened in crocodilian evolution. The late Triassic small, terrestrial cursorial forms called the sphenosuchians have been postulated to be endotherms (See Nature 14th April 2005). In the Jurassic, with the evolution of a sit-and-wait ambush strategy endothermy was lost, with the evolution of the unique shunting mechanism of the crocodilian heart to name one adaptation.
The supremacy of the archosaurs over the most-likely (?partially) endothermic near-mammalian therapsids during the Triassic used to be seen as the result of the superiority of the reptilian (ectothermic) metabolism has in hot dry conditions, which characterised much of Pangea during the Triassic. However, archosaurian dominance of terrestrial niches occurred in phases throughout the Triassic, with archosaurs only becoming "dominant" in the Norian, after the End-Carnian mass extinction.
The Neogene history of Australia is another interesting case, here the large terrestrial predator niche was not held by mammals but gigantic varanid lizards (Megalania) and crocodilians (Quinkana). Again, arguing that ectothermy is "superior" to endothermy in hotter environments is not supported. Australasia is unique with its absence of eutherian (placental) mammals which dominat the terrestrial carnivore niche on other continents today. Interestingly, marsuipal carnivores seemed to co-exist with the large ectothermic carnivores of the late Pliocene-Plestiocene.
Birds however, which are the descendents of dinosaurs, are warm-blooded. Several theropod species have been found with fossilised impressions of feathers or, as with the spectacular Chinese fossils with structural detail. It has been assumed that some dinosaurs, especially the maniraptorans, were endothermic.
Palaeos com MAK 991003 MAK030730; Palaeos org MAK061020; Mark T Young