HYPSILOPHODON

a genus of small to medium-sized herbivorous dinosaurs that flourished from Late Jurassic to Early Cretaceous times (163 to 97.5 million years ago). Hypsilophodon gave its name to the family of ornithopods known as Hysilophodontidae; these primitive types ultimately gave rise to two other major groups of ornithopodsthe hadrosaurs, or duck-billed dinosaurs, and the iguanodonts. Hypsilophodon itself was about 2 m (6.5 feet) long and weighed about 60 kg (130 pounds). It had short arms with five fingers on each hand and was equipped with much longer hind feet bearing four toes apiece. The animal had high, grooved, self-sharpening cheek teeth adapted for grinding up plant matter, as well as incisor-type teeth in its horny beak that were used to nip off vegetation. Hypsilophodon may also have had a ridge of bony scales running down its back, though this remains uncertain. For many decades paleontologists thought that Hypsilophodon's long fingers and toes had enabled it to live in trees, but the dinosaur is now recognized to have been a swift-running, ground-dwelling plant-eater that lived in herds and depended on speed to outrun predatory carnivores.