Hypsilophodon

Hypsilophodon is a genus of neornithischian dinosaur from the Early Cretaceous of England. It was long classified as an early ornithopod, though newer studies have challenged that placement. Most of its fossils come from the Isle of Wight, giving a clear picture of its anatomy.

The first remains turned up in 1849. The type species, Hypsilophodon foxii, was named in 1869. It was a small, quick, two-legged plant-eater, about 1.5 to 2 meters long and weighing 20 kilograms. Its pointed head had a sharp beak, like a modern parrot, for snipping off vegetation.

Several old, outdated studies led to mistaken ideas about Hypsilophodon—for instance, that it was armored, lived in trees, or occurred outside the Isle of Wight. Later research has disproven all of that.

**Discovery and history**

**First specimens and the debate of distinctiveness**

The first Hypsilophodon specimen was found in 1849 when workers dug up what became known as the Mantell-Bowerbank block from the Wessex Formation, part of the Wealden Group, on the Isle of Wight’s southwest coast. The larger half—seventeen vertebrae, rib fragments, a coracoid, parts of the pelvis, and hindleg pieces—went to naturalist James Scott Bowerbank. The other half—eleven tail vertebrae and more hindleg parts—went to Gideon Mantell. After Mantell died, his portion went to the British Museum; Bowerbank’s was acquired later, reuniting the two halves. Richard Owen studied both and, in 1855, published a short paper calling the specimen a young Iguanodon, not a new type. That view held until 1867, when Thomas Henry Huxley compared the vertebrae and foot bones more carefully to Iguanodon and decided it was a different animal. The next year, Huxley saw a fossil skull that William Fox had found in the same area and put on display at a British Association meeting. Fox thought it might be a young Iguanodon or a new small species in that genus. Huxley noticed its unusual teeth and toothless front upper jaw, which resembled Iguanodon’s but were clearly distinct. He concluded this skull, too, belonged to a separate animal. After losing track of the specimen for months, Huxley asked Fox for permission to study it in detail. Fox agreed, and Huxley began work.

Huxley first announced the new species in an 1869 lecture; the published text of that lecture serves as the official naming paper, since it included a sufficient description. He named the species Hypsilophodon foxii, with the Fox skull as the holotype (now NHMUK PV R 197). The next year, Huxley published a full description. In the same block as the Fox skull, a dorsal vertebra centrum was preserved, allowing comparison with the Mantell-Bowerbank block and confirming it was the same species. Fox also confirmed the block came from the same geological layer as his material. Huxley therefore described that specimen as well; it became the paratype, its two pieces now registered as NHMUK PV OR 28707 and NHMUK PV OR 39560–1. Later that year, Huxley classified Hypsilophodon in the family Iguanodontidae, alongside Iguanodon and Hadrosaurus. A common misunderstanding later arose about the name’s meaning. It is often translated as “high-ridged tooth” from Greek, but Huxley actually intended it to echo the naming of Iguanodon (“iguana-tooth”): he chose the herbivorous lizard Hypsilophus and combined its name with the Greek word for “tooth.” Hypsilophodon therefore means “Hypsilophus-tooth.” The Greek word hypsilophos means “high-crested,” referring to the lizard’s back frill, not to Hypsilophodon’s own teeth, which are not high-ridged. The species name foxii honors Fox.

Reaction to Huxley’s proposal of a new genus was mixed. The question mattered because people wanted more information on Iguanodon’s form, especially its skull anatomy. If the Cowleaze Chine fossils belonged to a different genus, they were no longer useful for that purpose. Some paleontologists, like William Boyd Dawkins and Harry Seeley, supported the distinction, but Fox rejected it and took back his skull, giving it to Owen to study. To settle things, John Whitaker Hulke returned to the Hypsilophodon fossil bed on the Isle of Wight for more material. He noted that the whole skeleton seemed present but that fragility made excavation difficult. He published a description of his new specimens in 1873, and after examining the new teeth, he echoed Fox’s doubts. Owen then published a detailed comparison of the teeth of known Iguanodon and Fox’s specimens. He agreed there were differences but found them insufficient to justify a separate genus, so he renamed the species Iguanodon foxii. By then, however, Hulke had changed his mind after obtaining two more specimens.