Hypsilophodontids

Hypsilophodontids are a historically recognized group of small, bipedal, herbivorous dinosaurs within Ornithopoda. They are known from the Late Jurassic to Early Cretaceous of Europe and North America, with the type genus Hypsilophodon providing the best-known example. Members were generally small, with slender skeletons, long hind limbs, and a beaked mouth; teeth were leaf-shaped and low-crowned, suited to processing plant material. They were likely fast runners and adapted to terrestrial life on low vegetation.

Taxonomic history: The group was once treated as a distinct family, Hypsilophodontidae, containing several small ornithopods. However, cladistic analyses in the late 20th and early 21st centuries showed the group to be paraphyletic; many members have since been reassigned to other ornithopod lineages, and Hypsilophodon is now usually regarded as a basal ornithopod outside the major clades Iguanodontia and Hadrosauriformes. As a result, Hypsilophodontidae is often considered a historical wastebasket taxon rather than a natural evolutionary unit.

Paleobiology: Hypsilophodontids were probably primarily herbivorous, with their small size and light skeleton suggesting a diet of low-lying vegetation. Their limb proportions imply they could move quickly, potentially aiding in predator avoidance. They inhabited temperate habitats of Europe and North America, with fossils dating to the Jurassic–Early Cretaceous interval.

Fossil record and significance: The best-known fossils come from Hypsilophodon on the Isle of Wight, England, providing crucial insight into the anatomy and locomotion of small ornithopods. The historical classification of hypsilophodontids illustrates how dinosaur taxonomy has evolved alongside new data and methods.