John R. Horner of Montana State University holds a cast of a baby fossilized Hypsilophodont over a clutch of eggs. Hypsilophodont were among the fastest dinosaurs. Their jaws were also better built to crop and chew vegetation than those of the early ornithopods. Hypsilophodontids flourished for 100 million years (from mid Jurassic to end of Cretaceous ), perhaps longer than any other dinosaur family, and are widely distributed through Europe, western North America, China, Australia and Antarctica.