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Two important evolutionary changes took place among the archosaurs. They changed from-sprawling, lizardlike animals to animals that walked with their legs held directly under their bodies. The other change was from a cold-blooded, lizardlike metabolism (the way a body uses energy) to a warm-blooded, birdlike metabolism. These changes did not take place in all archosaurs, but they happened in the dinosaurs. Crocodilians are the only surviving example in which those changes did not occur; birds are the only surviving group in which they did occur.
Warm-bloodedness may have appeared early in the dinosaur-bird clade, so that almost all dinosaurs were warm-blooded. This trait may have been inherited from a common ancestor dinosaurs shared with birds. Or, these changes may have happened after dinosaurs and birds separated, so that only a few advanced predatory dinosaurs were warm-blooded.
The ancestors of dinosaurs developed a stronger ankle. This kind of ankle occurs in pterosaurs, dinosaurs, and birds. Lagosuchus, Lagerpeton, and Pseudolagosuchus were small, bipedal (they walked on two legs) archosaurs with advanced ankles and other features that suggest they were closely related to dinosaurs.