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Dinosauria Literally, ‘terrible lizards’ (from the Greek deinos, ‘terrible’, and sauros, ‘lizard’), but in fact the dinosaurs were not lizards. They were diapsid reptiles whose closest living relatives are the crocodilians and birds. Dinosaurs first appeared in the Triassic, became the dominant group of land animals by the Middle Jurassic, and produced an astonishing array of different types and sizes before becoming extinct at the end of the Cretaceous. The two groups of dinosaurs, Saurischia and Ornithischia, are not usually thought to be more closely related to each other than to other archosaurs (Archosauria), so the concept of ‘dinosaur’ is a heterogeneous one.