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Archosauria:
Archosaurs (Greek for 'ruling lizards') are a group of diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian (Lower Triassic Period). They are represented today by birds and crocodiles. Archosaurs are set apart by having socketed teeth (a feature that inspired the traditional name, 'thecodonts', for the Triassic 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 crocodilians. 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 Crocodilia among the Crurotarsi - flourished during the Jurassic and Cretaceous Periods. The dinosaurs dominated the land, the pterosaurs and later another archosaurian group, the birds, dominated the air,and the crocodiles dominated the rivers and swamps and even invaded the seas (the Teleosaurs and Metriorhynchidae). 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 the crocodylia (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. |
