Period: Early Cretaceous

Order, Suborder, Family: Ornithischia, Ornithopoda, Hypsilophodontidae

Location: Europe (England)

Length: 7 1/2 feet (2.3 meters)

Hypsilophodon was the first small ornithopod dinosaur studied by paleontologists. It is also the best-known member of the family Hypsilophodontidae. The first remains of Hypsilophodon were discovered in 1849 from Early Cretaceous rocks on the Isle of Wight, England. It was first described as a juvenile Iguanodon by Dr. Gideon Mantell and later by Sir Richard Owen. But more fossils convinced Thomas H. Huxley that it was a new dinosaur. He named it Hypsilophodon, which means "high-ridged tooth."

Hypsilophodon was long-legged, swift, and agile. Its short arms and five-fingered hands were not used for running, but probably were used to grasp, much like other hypsilophodontids. The long tail, which was stiffened by bony tendons, was a balance for the front of the body.

Scientists once thought Hypsilophodon lived in trees. This curious idea came from some of the early studies of the animal, based on its fingers and toes. Particular attention was paid to how similar Hypsilophodon was to the present-day tree kangaroo of Australia. For example, it was said that the claws were highly curved, the toes were very long, the arm was moveable at the shoulder, and the first toe of the foot may have been able to grasp branches. It took nearly 100 years for scientists to take Hypsilophodon out of the trees. Many of those features were incorrect. The big toe probably could not grasp, the claws were not strongly curved, and it had little movement at the shoulder. Finally, the long toes worked with the long legs to make Hypsilophodon a very fast runner.

Hypsilophodon, together with about seven other small ornithopods, form the family Hypsilophodontidae. This group was very successful, both in how long it lived and in its geographic range. The earliest hypsilophodontids are known from the Middle Jurassic, such as Yandusaurus, and the family continues to the end of the Cretaceous. Most of these animals were found in Europe, North America, and eastern Asia, but others have been discovered in parts of Africa, South America, and Australia.