The Hypsilophodontidae was a family of medium-sized ornithopod dinosaurs, somewhat resembling the fabrosaurids outwardly. They did not appear until about the Middle Jurassic, however, so the similarity between the two taxa must have been due to convergent evolution. One of the best known of these small ornithopods, Hypsilophodon (Fig. 111) from Europe and North America, was first described by T.H. Huxley in 1870. Because of the similarity of its shape to that of modern tree kangaroos (Dendrolagus spp.), reconstructions of Hypsilophodon have for almost a century illustrated this dinosaur perched, bird-like, in a tree. Not until 1974 was the skeleton reassessed and the conclusion reached that there was no evidence to indicate that it was arboreal. On the contrary, it is now known to have been a speedy terrestrial biped. Other well known genera of hypsilophodontids included Dryosaurus, Othnielia, Parkosaurus and Thescelosaurus (Palmer 1999).

The lifestyle of the hypsilophodonts resembled that of the fabrosaurids, as mentioned above, but the hypsilophodonts had evolved several adaptive anatomical modifications. One of the most important of these was the development of retaining cheeks that prevented food from falling from the sides of the mouth. In addition, the dentition was better adapted for grinding. The upper and lower teeth met (or occluded) as regular rows and did not interlock alternatively as did those of the fabrosaurids. This would have provided a more efficient grinding surface. At the same time, however, the hypsilophodontids retained more digits, a primitive character, than did the fabrosaurids. They had strong arms with five fingers each and powerful hind legs with four toes. The tail was stiff, and used for stabilisation when the animal was running.

Dryosaurus had sharp, rigid cheek teeth but there were no teeth at the front of the jaw. A horny beak at the end of the lower jaw met a toothless pad on the upper jaw. In Hypsilophodon there were incisor-like teeth on the upper jaw which closed against a toothless horny beak on the lower, while in Parkosaurus which survived until the end of the Cretaceous period, a horny beak replaced all the front teeth. This dinosaur probably foraged close to the ground, nipping off delicate shrubs from among the low-growing vegetation. Except for Tenon-tosaurus (Fig. 111) from North America, all the hypsilophodonts were relatively small, lightly built bipedal dinosaurs. Tenontosaurus, in contrast, was very much larger. Over half of its total length was made up of an extremely thick and heavy tail, while its front legs were longer and stouter than those of other hypsilophodontids, and it probably spent most of the time on all fours. The discovery of a skeleton surrounded by those of five Deinonychus (Figs. 86, 87) may have been the result of an encounter with a pack of these formidable predators, although it is more likely that the bodies were brought together by chance - perhaps in a flash flood. Like Parkosaurus and also from North America, Thescelosaurus survived until the end of the Cretaceous. It was larger than most hypsilophodontids and showed many characters resembling those of the iguanodonts (Sect. 10.5.3),as for that matter did the very much larger Tenonto-saurus. Whereas the latter measured ca. 7.3 m in length, Thescelosaurus was only about 3.5 m long. It, too, was a slow-moving creature but the bony scales on its back would have offered some defence against predators. Unlike other hypsilophodonts, which had three or four toes on each hind foot, Thescelo-saurus had no less than five (Palmer 1999). As Benton (2004) wrote, the hypsi-lophodontids had typical elongated feet with hoof-like claws and adapted for running. The overall limb proportions of most genera were similar to those of a fast-moving antelope, especially in regard to the very long shin and foot.

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