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).

Some outdated studies have given rise to a number of misconceptions about Hypsilophodon, including that it was an armoured, arboreal animal, and that it could be found in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.

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.

[1][2] 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.

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.

[3] Huxley first announced the new species in 1869 in a lecture; the text of this, published the same year, forms the official naming article, because it contained a sufficient description.

The species was named Hypsilophodon foxii, and the holotype was the Fox skull (which today has the inventory number NHM NHMUK PV R 197).

The Greek ὑψίλοφος, hypsilophos, means "high-crested" and refers to the back frill of the lizard, not to the teeth of Hypsilophodon itself, which are not high-ridged in any case.

[2][9] In attempt to clarify the situation, John Whitaker Hulke returned to the Hypsilophodon fossil bed on the Isle of Wight to obtain more material.

[10] Finally, in 1882 he published a full osteology of the species, considering it of great importance to properly document the taxon as such a wealth of specimens had been discovered and comparison with American dinosaurs was necessary.

[14][clarification needed] Most known Hypsilophodon specimens were discovered between 1849 and 1921 and are in the possession of the Natural History Museum that acquired the collections of Mantell, Fox, Hulke and Hooley.

The specimen was found in 1900 in the Black Hills of South Dakota, United States, by George Reber Wieland, who the species was named after.

This species was seen at the time as indicative of a probable late land bridge between North America and Europe, and of the dinosaur fauna of both continents being similar.

However, in 2009 Galton concluded that the specimens from Great Britain proper were either indeterminable or belonged to Valdosaurus, and that the fossils from the rest of Europe were those of related but different species.

[5] The fossils have been found in the Hypsilophodon Bed, a one-metre thick marl layer surfacing in a 1200 metre long strip along the Cowleaze Chine parallel to the southwest coast of Wight,[15] part of the upper Wessex Formation and dating to the late Barremian, about 126 million years old.

Numerous anatomical features aided this, such as: light-weight, minimized skeleton, low, aerodynamic posture, long legs, and stiff tail — immobilized by ossified tendons for balance.

The snout was triangular in outline and sharply pointed, ending in an upper beak of which the cutting edge was markedly lower than the maxillary tooth row.

The following cladogram is reproduced from this study:[29] Hypsilophodon Marginocephalia Gideonmantellia Elasmaria Burianosaurus Muttaburrasaurus Rhabdodontidae Tenontosaurus Dryosauridae Camptosaurus Iguanodon Ouranosaurus In one analysis in her 2022 review of iguanodontian phylogenetic relationships, Karen E. Poole recovered a large Hypsilophodontidae as the sister taxon of Iguanodontia, which consisted of several "traditional" hypsilophodontids, as well as Thescelosauridae.

The Bayesian topology of her phylogenetic analyses is shown in the cladogram below:[30] Othnielosaurus Jeholosaurus Haya Hypsilophodon Leallynasaura Gasparinisaura Macrogryphosaurus Zephyrosaurus Orodromeus Parksosaurus Thescelosaurus spp.

They suggested that Vectidromeus and Hypsilophodon represented the only members of the Hypsilophodontidae, since other taxa previously assigned to the group had subsequently been moved to other clades.

[31] Due to its small size, Hypsilophodon fed on low-growing vegetation, in view of the pointed snout most likely preferring high quality plant material, such as young shoots and roots, in the manner of modern deer.

The structure of its skull, with the teeth set far back into the jaw, strongly suggests that it had cheeks, an advanced feature that would have facilitated the chewing of food.

In 1882 Hulke suggested that Hypsilophodon was quadrupedal but also, in view of its grasping hand, able to climb rocks and trees in order to seek shelter.

[32] Though this hypothesis was doubted by Nopcsa,[33] it was adopted by the Danish researcher Gerhard Heilmann who in 1916 proposed that a quadrupedal Hypsilophodon lived like the modern tree-kangaroo Dendrolagus.

[13] As Swinton was a very influential populariser of dinosaurs, this remained the accepted view for over three decades, most books typically illustrating Hypsilophodon sitting on a tree branch.

[37] In 1971 Galton in detail refuted Abel's arguments, showing that the first toe had been incorrectly reconstructed and that neither the curvature of the claws, nor the level of mobility of the shoulder girdle or the tail could be seen as adaptations for climbing,[38] concluding that Hypsilophodon was a bipedal running form.

[15] The Hypsilophodon fossils were probably accumulated in a single mass mortality event,[23] so it has been considered likely that the animals moved in large groups.