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Hypsilophodon (; meaning "high-crested tooth") is a 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 animal with an herbivore or possibly omnivore diet, 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Some outdated studies have given rise to a number of misconceptions about Hypsilophodon, including that it was an armoured, arboreal animal, and that it could be found in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.
Immediate reception to Huxley's proposal of a new genus, distinct from Iguanodon, was mixed. The issue of distinctiveness was seen as important as more information on the form of Iguanodon was in demand, and the cranial anatomy in particular was of importance. If the Cowleaze Chine material was a distinct genus, it ceased being useful in this respect. 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. In attempt to clarify the situation, John Whitaker Hulke returned to the Hypsilophodon fossil bed on the Isle of Wight to obtain more material. He remarked that the whole of the skeleton seemed to be present, but that fragility limited excavation. He published a description of his new specimens in 1873, and based on his examination of the new teeth fossils echoed Fox's sentiments of doubt. Owen followed this with a study comparising at length the teeth of known Iguanodon and those from Fox's specimens. He agreed there were differences, but found them lacking in sufficient distinctiveness to be considered a distinct genus. As such, he renamed the species Iguanodon foxii. But Hulke had by then shifted his opinion, having obtained two new more informative specimens. Building on Huxley's comments on the Mantell-Bowerbank block, he gave focus to vertebral characters. As a result of his study, he concluded Hypsilophodon was a distinct genus related to Iguanodon. He published these findings in a supplementary note, also in 1874. 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. Fox had by this point died, and no further argument against generic distinctiveness had occurred in the intervening time.
Modern research of Hypsilophodon began with the studies of Peter Malcolm Galton, starting with his thesis of 1967.Galton, P.M., 1967, On the anatomy of the ornithischian dinosaur Hypsilophodon foxii from the Wealden (Lower Cretaceous) of the Isle of Wight, England. unpublished Ph.D. Thesis, University of London, King’s College, UK, 513 pp 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 . 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 nomen dubium 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.
Fossils from other locations, especially from the mainland of southern Great Britain, Portugal and Spain, have once been referred to Hypsilophodon. 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. This leaves the finds on Isle of Wight, off the south coast of England, as the only known authentic Hypsilophodon fossils. 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, part of the upper Wessex Formation and dating to the late Barremian, about 126 million years old. Reports that Hypsilophodon would be present in the later Vectis Formation, Galton in 2009 considered as unsubstantiated.
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.
The skull of Hypsilophodon was short and relatively large. 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 eye socket was very large. A palpebral with a length equal to half the diameter of the eye socket overshadowed its top section. A sclerotic ring of fifteen small bone plates supported the outer eye surface. The back of the skull was rather high, with a very large and high jugal and quadratojugal closing off a highly positioned small infratemporal fenestra.
A long-lived misconception concerning the anatomy of Hypsilophodon has been that it was armoured. This was first suggested by Hulke in 1874, after the find of a bone plate in the neck region.Hulke, J.W., 1874, "Supplemental note on the anatomy of Hypsilophodon foxii", Geological Society of London, Quarterly Journal, 30: 18-23 If so, Hypsilophodon would have been the only known armoured ornithopod. As Galton pointed out in 2008, the putative armour instead appears to be from the torso, an example of internal intercostal plates associated with the rib cage. It consists of thin mineralized circular plates growing from the back end of the middle rib shaft and overlapping the front edge of the subsequent rib. Such plates are better known from Talenkauen and Thescelosaurus, and were probably cartilage in origin.
In 2014, Norman resolved a monophyletic Hypsilophodontia (avoiding the name "Hypsilophodontidae" due to its complicated history). Hypsilophodon was recovered as the sister taxon to the clade containing Tenontosaurus and Rhabdodontidae.
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:
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.
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. The Hypsilophodon fossils were probably accumulated in a single mass mortality event, so it has been considered likely that the animals moved in large groups. For these reasons, the , 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.
Later research
Later, the number of specimens was increased by Reginald Walter Hooley. In 1905, Baron Franz Nopcsa dedicated a study to Hypsilophodon,Nopcsa, F., 1905, "Notes on British dinosaurs. Part I. Hypsilophodon", Geological Magazine, London, (5) 2: 203-208 and in 1936 William Elgin Swinton did the same,Swinton, W.E., 1936, "Notes on the osteology of Hypsilophodon, and on the family Hypsilophodontidae", Zoological Society of London, Proceedings, 1936: 555-578 on the occasion of the mounting of two restored skeletons in the British Museum of Natural History.Swinton, W.E., 1936, "A new exhibit of Hypsilophodon", Natural History Magazine, London, 5: 331-336 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. Although this was the largest find, new ones continue to be made.
Description
Hypsilophodon was a relatively small dinosaur, though not quite as small as, for example, Compsognathus. For Hypsilophodon often a maximum length of is stated. This has its origin in a study of 1974 by Galton, in which he extrapolated a length of based on specimen BMNH R 167, a thigh bone.Galton, P.M., 1974, The ornithischian dinosaur Hypsilophodon from the Wealden of the Isle of Wight. British Museum (Natural History), Bulletin, Geology, London, 25: 1‑152c However, in 2009, Galton concluded that this femur in fact belonged to Valdosaurus and downsized Hypsilophodon to a maximum known length of , the largest specimen being NHMUK PV R 5829 with a femur length of 202 millimetres. Typical specimens are about long. In 2010, Gregory S. Paul estimated a weight of for an animal in length.Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 275
Cranial anatomy
In an example of primitive anatomy, although it had a beak like most , Hypsilophodon still had five pointed triangular teeth in the front of the upper jaw, the premaxilla. Most herbivorous dinosaurs had, by the Early Cretaceous, become sufficiently specialized that the front teeth had been altogether lost (although there is some debate as to whether these teeth may have had a specialized function in Hypsilophodon). More to the back, the upper jaw carried up to eleven teeth in the maxilla; the lower jaw had up to sixteen teeth. The number was variable, depending on the size of the animal. The teeth to the back were fan-shaped.
Postcranial anatomy
The vertebral column consisted of nine cervical vertebrae, fifteen or sixteen dorsal vertebrae, six of five sacral vertebrae and about forty-eight vertebrae of the tail. Much of the back and the tail was stiffened by long ossified tendons connecting the spines on top of the vertebrae. The processes on the underside of the tail vertebrae, the chevrons, were also connected by ossified tendons, which however were of a different form: they were shorter and split and frayed at one end, with the point of the sharp other end laying within the diverging end of the subsequent tendon. Furthermore, there were several counterdirectional rows of these, resulting in a herring-bone pattern completely immobilising the tail end.
Phylogeny
Huxley originally assigned Hypsilophodon to the Iguanodontidae. In 1882 Louis Dollo named a separate .L. Dollo, 1882, "Première note sur les dinosaures de Bernissart", Bulletin du Musée Royale d'Histoire Naturelle de Belgique 1: 1-18 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. Hypsilophodon in the modern view thus simply is a basal ornithopod.
Paleobiology
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 , 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.
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. In 1912 this line of thought was further pursued by Austrian paleontologist Othenio Abel. Concluding that the first toe of the foot could function as an opposable hallux, Abel stated that Hypsilophodon was a fully arboreal animal and even that an arboreal lifestyle was primitive for the dinosaurs as a whole.Abel, O., 1912, Grundzüge der Palaeobiologie der Wirbeltiere, E. Schweizerbart'sche Verlagsbuchhandlung Nägele und Dr Sproesser, Stuttgart Though this hypothesis was doubted by Nopcsa,V. Pietschinann (ed), 1914, Verhandlungen der Kaiserlich-Königlichen Zoologisch-Botanischen Gesellschaft in Wien, Vienna, pp 380 it was adopted by the Danish researcher Gerhard Heilmann who in 1916 proposed that a quadrupedal Hypsilophodon lived like the modern tree-kangaroo Dendrolagus.Heilmann., G., 1916, Vor nuværende Viden om Fuglenes Afstamning, Copenhagen 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,Heilmann, Gerhard, 1926, The Origin of Birds. London, Witherby but in 1927 Abel refused to accept this.Abel, O., 1927, Lebensbilder aus der Tierwelt der Vorzeit, Fisher, Jena In this he was in 1936 supported by Swinton who claimed that even a forward pointing first metatarsal might carry a movable toe. 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. However, Peter M. Galton in 1969 performed a more accurate analysis of the musculo-skeletal structure, showing that the body posture was horizontal.Galton, P.M., 1969, "The pelvic musculature of the dinosaur Hypsilophodon (Reptilia : Ornithischia)", Postilla, 131: 1-64 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,Galton, P.M., 1971, " Hypsilophodon, the cursorial nonarboreal dinosaur", Nature, 231: 159-161 concluding that Hypsilophodon was a bipedal running form.Galton, P.M., 1971, "The mode of life of Hypsilophodon, the supposedly arboreal ornithopod dinosaur", Lethaia, 4: 453-465 This convinced the paleontological community that Hypsilophodon remained firmly on the ground.
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