 Most of the earlier parareptile and synapsid megafauna disappeared, being replaced by the true reptiles, particularly archosauromorphs.These were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles.The archosaurs became the dominant group during the Triassic period, though it took 30 million years before their diversity was as great as the animals that lived in the Permian.