Hypsilophodon (/ˌhɪpsɪˈlɒfoʊdɒn/; meaning "high-crested tooth") is a neornithischian dinosaur genus from the Early Cretaceous period of England. It has traditionally been considered an early member of the group Ornithopoda, but recent research has put this into question.
The first remains of Hypsilophodon were found in 1849; the type species, Hypsilophodon foxii, was named in 1869. Abundant fossil discoveries were made on the Isle of Wight, giving a good impression of the build of the species. It was a small, agile bipedal animal with an herbivorous or possibly omnivorous diet, measuring 1.5–2 m (4.9–6.6 ft) long and weighing 20 kg (44 lb). It had a pointed head equipped with a sharp beak used to bite off plant material, much like modern-day parrots.
Some outdated studies have given rise to a number of misconceptions about Hypsilophodon, including that it was an armoured, arboreal animal, and that it could be found in areas outside of the Isle of Wight. However, research from the following years has shown these ideas to be incorrect.The first specimen of Hypsilophodon was recovered in 1849, when workers dug up the soon-called Mantell-Bowerbank block from an outcrop of the Wessex Formation, part of the Wealden Group, about one hundred yards west of Cowleaze Chine, on the south-west coast of Isle of Wight. The larger half of the block (including seventeen vertebrae, parts of ribs and a coracoid, some of the pelvis, and assorted hindleg remains) was given to naturalist James Scott Bowerbank, and the remainder (including eleven caudal vertebrae and most of the rest of hindlegs) to Gideon Mantell. After his death, Mantell's portion was acquired by the British Museum; Bowerbank's was acquired later, bringing both halves back together. Richard Owen studied both halves and, in 1855, published a short article on the specimen, considering it to be a young Iguanodon rather than a new taxon.This was unquestioned until 1867, when Thomas Henry Huxley compared the vertebrae and metatarsals of the specimen more closely to those of known Iguanodon, and concluded that it must be a different animal entirely. The next year, he saw a fossil skull discovered by William Fox on exhibition at the Norwich Meeting of the British Associations. Fox, who had also found his fossil in the Cowleaze Chine area, along with several other specimens, considered it to belong to a juvenile Iguanodon, or to represent a new, small species in the genus.Huxley noticed its unique dentition and edentulous premaxilla, reminiscent of but obviously distinct from that of Iguanodon.He concluded this specimen, too, represented a distinct animal from Iguanodon. After losing track of the specimen for some months, Huxley requested Fox grant him permission to study the specimen to a more extensive degree. The request was granted, and Huxley began work on his new species.