Animals similar to Hypsilophodon appeared early in the history of dinosaurs and persisted until the last dinosaurs became extinct 65 million years ago. They were small, fast-running herbivorous dinosaurs, related to the larger hadrosaurs and Iguanodonts. Hypsilophodon lived in Europe, but fossils of similar animals are known from every continent, including Antarctica and Australia.
Hypsilophodon ran on its long hind legs, with its body held horizontal. Its long tail accounted for half its body length and was stiffened by bony tendons. It is presumed that this helped to hold the tail off the ground while it was running. Recent research suggests that these tendons aided the efficiency of Hypsilophodon when running.
Hypsilophodon, like other ornithopods, had a small beak, broad chiselled teeth that formed a continuous cutting edge, and cheek pouches for storing food for a short time while it was chewed.Early palaeontologists thought that it looked like a tree kangaroo, and for a period thought that it may have perched in trees. These ideas have now been discounted and Hypsilophodon is thought to have lived very successfully on the ground as a small, fast sprinter.