Hypsilophodontids are a group (family) of small Ornithopod Dinosaurs. The group is called after the genus Hypsilophodon. Which dinosaurs belonged to this group has been a subject of debate for a long time. This debate has influence on which dinosaurs appear in the Jurassic Park novels Paul Sereno published a review article in which he made a classification of all Ornithopod species. For instance, he placed Dryosaurus outside of this family. Other paleotologists like Jack Horner think that Dryosaurus did belonged to the Hypsilophodontid family. More real-life information: Hypsilophodont.