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Even this distinction is not completely clear-cut — for example, humans other than infants normally walk and run in biped fashion, but almost all can crawl on hands and knees when necessary.There are even reports of humans who normally walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as Uner Tan syndrome — very rare genetic neurological disorders rather than normal behavior.Radiometric dating of fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time.
In the Triassic period some groups of archosaurs (a group that includes the ancestors of crocodiles) developed bipedalism; among their descendants the dinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds descended from one group of exclusively bipedal dinosaurs.
A larger number of modern species intermittently or briefly use a bipedal gait.
All dinosaurs are thought to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor.
Bipedal movement also re-evolved in a number of other dinosaur lineages such as the iguanodons.
Some animals commonly stand on their hind legs, in order to reach food, to keep watch, to threaten a competitor or predator, or to pose in courtship, but do not move bipedally.