|- |(unranked)||Archosauriformes

|- |(unranked)||Archosauria Crurotarsi
    Crocodylia (crocodiles)
Ornithodira
    Pterosauria
    Dinosauria
       Aves (birds)
Archosaurs (Greek for "ruling reptiles") are a group of diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian (Lower Triassic). Archosaurs are set apart by having socketed teeth (a feature that inspired the traditional name, "thecodonts", for the Triasisc forms) and four-chambered hearts, among other characteristics. Most early forms were carnivores, with narrow serrated meat-tearing teeth. Their "reptilian" metabolism seem to have given them a clear advantage over the mammal-like therapsids that were their contemporaries in the arid interiors and strong monsoon climates that were the natural result of the single world-continent, Pangaea. Thus, whereas the Permian was dominated by synapsids, the Triassic came to be dominated by sauropsids.

There are two primary groups of archosaurs — the Ornithodira which were insignificant during the Middle Triassic but in the Late Triassic radiated as the dinosaurs and pterosaurs; and the Crurotarsi, which were the predominant group at this time, and included a number of purely Triassic groups like the rauisuchians, the phytosaurs, and the herbivorous aetosaurs , as well as the ancestors of the crocodylians.

A number of these archosaur groups - chiefly those large Crurotarsi that are in pre-cladistic books called the Thecodonts - became extinct 195 million years ago, during the Triassic-Jurassic extinction event. The survivors - the Dinosaurs and the Pterosaurs among the Ornithodira, and first the Sphenosuchia and Protosuchia then their descendants the Crocodylia among the Crurotarsi - flourished during the Jurassic and Cretaceous periods. The dinosaurs dominated the land, the pterosaurs and later another archosaurian group, the birds, the air, and the crocodiles the rivers and swamps and even invading the seas (the Teleosaurs and Metriorhynchidae ).

So complete is the archosaurian supremacy during this time that the Mesozoic should not be called the "Age of Reptiles", but "the Age of Archosaurs"

Most of these taxa perished 65 million years ago, during the Cretaceous-Tertiary extinction event. The only groups of archosaurs to continue through to the Tertiary, and ultimately to the present day, are the birds, which are descended from the dinosaurs, and crocodylians, which include all modern crocodiles, alligators, and gharials.

Birds are traditionally treated as a separate class, Aves, while the rest of the archosaurs are treated as a subclass or infraclass, Archosauria, within the class Reptilia. More recently, with the cladistic method dominating Biology, only monophyletic groups are considered valid, and birds are included within the division Archosauria.

External links

• UCMP

• Paleos

References

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd

• Carroll, R. L. 1988, Vertebrate Paleontology and Evolution, W. H. Freeman and Co. New York