The main thing that distinguishes dinosaurs from other reptiles is the way their body is supported by their legs. The legs of ancient and existing reptiles stick out sideways, so the body drags on the ground for most of the time. It is raised briefly when the animal runs. This is very obvious with crocodiles, which can run very fast for short distances. The skeletons of dinosaurs developed so the legs were beneath the body, raising the whole body off the ground. This allowed them to stand erect, and to walk and run with long strides. Predators developed that could run faster than more primitive reptiles, which soon became extinct.
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