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HIP - sill - OFF - oh - don
Name Means: "High-ridged tooth"
One of the most famous of small dinosaurs, Hypsilophodon was also one of the earliest to be studied. It was discovered in 1849, in the same Wealden rocks in southern England that had yielded fossils of the ornithopod Iguanadon 20 years earlier. Indeed, at first Hypsilophodon was thought to have been a juvenile Iguanadon. In 1869, the English zoologist Thomas Henry Huxley recognized that it was a different animal and named it after the strong ridges that were visible on its teeth. Huxley realized that this small animal would have been agile and would have moved principally on its hind legs. This was a suprising view, considering that at the time most scientists considered dinosaurs to be ponderous creatures that moved about on all fours.
Early reconstructions of Hypsilophodon's foot mistakenly showed it to have a reversed hallux (first toe). Such a reversal is common in animals that live in trees, as it allows them to grip branches while they perch. As a result, early reconstructions of a complete Hypsilophodon showed it perched up in branches, rather like an ancestral tree kangaroo.
Eventually the foot was reconstructed correctly and Hypsilophodon was brought to ground as a fast-running ornithopod, similar in many respects to the Jurassic Dryosaurus. Like Dryosaurus, Hypsilophodon had long hind legs that were well suited for running at high speed. Hypsilophodon's tail, like Dryosaurus's, was long and was held stiffly off the ground to counterbalance the weight of its body.
Hypsilophodon's teeth and jaws were well adapted for grinding the tough plant matter on which it fed. It had a fleshy cheek, where food could be stored before it was chewed, and a horny beak that cropped food as it entered the mouth. Unlike Dryosaurus, and most other ornithopods, Hypsilophodon still retained some front teeth in the upper jaw. In this respect, Hypsilophodon was probably more primitive than Dryosaurus, even though it lived 300 million years after the more advanced ornithopod had become extinct.