Hypsilophodontids belong to the family Hypsilophodontidae, a group of ornithopod dinosaurs that ate plants and walked on their hind legs. Ranging in size from 3-8ft, hypsilophodontids lived right from the Mid Jurassic to the Late Cretaceous period, and their remains have been found as far afield as North America, Europe, Asia, Australia and Antarctica. Example genera were Hypsilophodon, Dryosaurus, Orodromeus, Othnielia, Pisanosaurus and Thescelosaurus.

Hypsilophodon means 'high-ridged tooth', a name derived from the ridges found on its grinding teeth. An Early Cretaceous dinosaur whose remains have been excavated on the Isle of Wight, England in the nineteenth century by the Reverend William Fox and later in the USA and Portugal, Hypsilophodon was a 4-7ft long fast-running herbivore with a horny beak. It also had cheek pouches, short arms with five-fingered hands, four-toed feet, and a stiff tail.

Dryosaurus means 'oak lizard', a name given because the top of its grinding cheek teeth were shaped a little like oak leaves. This 10-13ft long vegetarian dinosaur lived in the Mid to Late Jurassic period, and remains have been found in England, Romania, Tanzania, and the USA. This dinosaur was similar to, but larger than Hypsilophodon.

Named after Othniel Charles Marsh, a paleontologist of the nineteenth century, Othnielia was a small 2.5-5ft long hypsilophodontid herbivore of the Late Jurassic period. Its remains have been excavated in Colorado, Utah and Wyoming, USA. This dinosaur had a small head, big eyes, a beak-like jaw, teeth enameled on both sides, short arms ending in five fingered hands, long legs and a long tail.