While all dinosaurs share these crucial features, they can be divided 3 into two fundamentally different types: the Saurischia (literally, | 'lizard-hipped') and Ornithischia ("bird-hipped'). As these names s suggest, the differences between such dinosaurs lie primarily in the structure of their hip bones, although several other more subtle anatomical features are important in helping to distinguish these two major types. The earliest members of both groups of dinosaur have been identified in rocks of Carnian age (at least 225 Ma), but it has not been possible to identify the earliest dinosaur of all or whether it was strictly a saurischian, ornithischian, or just a dinosaurian that was not yet either.

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