Hypsilophodon (; 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 herbivore, measuring long and weighing . 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Some outdated studies have given rise to a number of misconceptions about Hypsilophodon, including that it was an armoured, arboreal animal, and that it could be found in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.
Discovery and history
First specimens and the debate of distinctiveness
thumb|left|Illustration of the complete Mantell-Bowerbank block. Mantell had received the bottom half, and Bowerbank the top.
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. The next year, Huxley published the expanded full description article. There would later be a persistent misunderstanding as to the meaning of the generic name, which is often translated directly from the Greek as "high-ridged tooth". In reality Huxley, analogous to the way the name of the related genus Iguanodon ("iguana-tooth") had been formed, intended to name the animal after an extant herbivorous lizard, choosing for this role Hypsilophus and combining its name with Greek ὀδών, odon, "tooth". Hypsilophodon thus means "Hypsilophus-tooth". 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. The specific name foxii honours Fox. Whilst some palaeontologists such as William Boyd Dawkins and Harry Seeley supported distinction, Fox rejected Huxley's proposal of a distinct genus and subsequently took back his skull and gave it to Owen to study.
Later research
left|thumb|Specimens NHMUK PV R 5829 and NHMUK PV R 5830 at the Natural History Museum, London
Later, the number of specimens was increased by Reginald Walter Hooley. In 1905, Baron Franz Nopcsa dedicated a study to Hypsilophodon, and in 1936 William Elgin Swinton did the same, on the occasion of the mounting of two restored skeletons in the British Museum of Natural History. 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. These represent about twenty individual animals. Apart from the holotype and paratype, the most significant specimens are: NHM R5829, the skeleton of a large animal; NHMUK PV R 5830 and NHMUK PV R 196/196a, both skeletons of juvenile animals; and NHMUK PV R R2477, a block with a skull together with two separate vertebral columns. He and James Jensen briefly described a left femur, AMNH 2585, in 1975, and in 1979 formally coined a second species, Hypsilophodon wielandi, for the specimen. The femur was diagnosed with two supposed minor differences from that of H. foxii. 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. Geologically, it comes from the Lakota Sandstone. 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. Spanish Palaeontologist José Ignacio Ruiz-Omeñaca proposed that H. wielandi was not a species of Hypsilophodon but instead related to or synonymous with "Camptosaurus" valdensis from England, both species being dryosaurids. Galton refuted this in his contribution to a 2012 book, noting the femurs of the two species to be quite different, and that of H. wielandi to be unlike those of dryosaurs. He, as well as other studies before and after Ruiz-Omeñaca's proposal, considered H. wielandi a dubious basal ornithopod, with H. foxii the only species in the genus. Galton elaborated on the invalidity of the species in 2009, noting that the two supposed diagnostic characters were variable in both H. foxii and Orodromeus makelai, making the species dubious. He speculated that it may belong to Zephyrosaurus, from a similar time and place, as no femur was known from that taxon. However, in 2009, Galton concluded that this femur in fact belonged to Valdosaurus and downsized Hypsilophodon to a maximum known length of ,
Like most small dinosaurs, Hypsilophodon was bipedal: it ran on two legs. Its entire body was built for running. 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. In light of this, Galton in 1974 concluded it would have been among the ornithischians best adapted to running. Despite living in the last of the periods in which non-avian dinosaurs walked the earth, the Cretaceous, Hypsilophodon had a number of seemingly "primitive" features. For example, there were five digits on each hand and four on each foot. With Hypsilophodon the fifth finger had gained a specialised function: being opposable it could serve to grasp food items. If so, Hypsilophodon would have been the only known armoured ornithopod.
Phylogeny
thumb|[[O. C. Marsh's restoration in tripod-pose]]
Huxley originally assigned Hypsilophodon to the Iguanodontidae. By the middle of the twentieth century that had become the accepted classification but in the early twenty-first century it became clear through cladistic analysis that hypsilophodontids formed an unnatural, paraphyletic group of successive off-shoots from throughout Neornithischia.
In 2017, Daniel Madzia, Clint Boyd, and Martin Mazuch reassessed Hypsilophodon outside of Ornithopoda altogether, placing it in a more basal position, as the sister taxon to the Cerapoda; several other "hypsilophodontids" have undergone similar reclassifications. The following cladogram is reproduced from this study:
thumb|right|[[Life restoration]]
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:
In 2023, Longrich et al. described Vectidromeus as a new coeval genus of ornithopod closely related to Hypsilophodon. 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.
Paleobiology
thumb|left|1894 restoration by [[Joseph Smit showing the animal in both reptilian and kangaroo-like postures]]
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. There were twenty-three to twenty-seven maxillary and dentary teeth with vertical ridges in the animal's upper and lower jaws which, due to the fact that the tooth row of the lower jaw, its teeth curving outwards, fitted within that of the upper jaw, with its teeth curving inwards, appear to have been self-sharpening, the occlusion wearing down the teeth and providing for a simple chewing mechanism. As in almost all dinosaurs and certainly all the ornithischians, the teeth were continuously replaced in an alternate arrangement, with the two replacement waves moving from the back to the front of the jaw. The Zahnreihen-spacing, the average distance in tooth position between teeth of the same eruption stage, was rather low with Hypsilophodon, about 2,3. Such a dentition would have allowed to process relatively tough plants.
thumb|Restoration of [[Eotyrannus chasing Hypsilophodon, with other dinosaurs from the Wessex Formation in the background]]
Early paleontologists modelled the body of this small, bipedal, herbivorous dinosaur in various ways. 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. Though this hypothesis was doubted by Nopcsa, it was adopted by the Danish researcher Gerhard Heilmann who in 1916 proposed that a quadrupedal Hypsilophodon lived like the modern tree-kangaroo Dendrolagus. In 1926 Heilmann had again changed his mind, denying that the first toe was opposable because the first metatarsal was firmly connected to the second, but in 1927 Abel refused to accept this. In this he was in 1936 supported by Swinton who claimed that even a forward pointing first metatarsal might carry a movable toe. 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, concluding that Hypsilophodon was a bipedal running form. This convinced the paleontological community that Hypsilophodon remained firmly on the ground.
The level of parental care in this dinosaur has not been defined, nests not having been found, although neatly arranged nests are known from related species, suggesting that some care was taken before hatching. so it has been considered likely that the animals moved in large groups. For these reasons, the hypsilophodonts, particularly Hypsilophodon, have often been referred to as the "deer of the Mesozoic". Some indications about the reproductive habits are provided by the possibility of sexual dimorphism: Galton considered it likely that exemplars with five instead of six sacral vertebrae — with some specimens the vertebra that should normally count as the first of the sacrum has a rib not touching the pelvis — represented female individuals.