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Any of the various advanced reptiles in the subclass Archosauria (ruling reptiles), including all thecodonts, pterosaurs, dinosaurs, birds, and crocodilians. The first known archosaurs appeared 242–227 million years ago during the Triassic Period. All early archosaurs possessed an ankle specialization that aided in upright posture, and most had long hindlegs and short forelimbs. Unlike the teeth of earlier reptiles, which were set in a shallow groove, the teeth of archosaurs were (and are) set in sockets.
This entry comes from Encyclopædia Britannica Concise. For the full entry on archosaur, visit Britannica.com.