Because current cladistic analyses now suggest that Hypsilophodontidae is a para-phyletic grouping of basal ornithopods, the use of "hypsilophodontid" is used here to describe all euornithopods outside of the clade Iguanodontia and is not meant to connote a clade.
In contrast to heterodontosaurids, hypsilophodontids are better known through complete or otherwise well-preserved skeletons of one species (Hypsilophodon) and fragments of about 10 other species. Hypsilophodontids occur in strata formed during the Middle Jurassic through to the Late Cretaceous, representing a wider geologic range than heterodontosaurids. Fossil specimens are found in regions as widespread as North America, South America, Europe, China, and Australia. Morphologically, hypsilophodontids were larger than their heterodontosaurid predecessors (about 1.5 to 4 meters long) and their robust hind limbs were slightly longer in proportion to their fore limbs, indicating better adaptations to bipedal-ism. Ossified tendons reinforced their caudal vertebrae in places, an adaptation that would have rectified the tail and formed a counterbalance during fast movement. This trait is similar to the functional morphology of dromaeosaurids (Chapter 9).
Erroneous interpretations of the foot structure of Hypsilophodon, particularly the position of the hallux, led some paleontologists, earlier in the twentieth century, to propose that these not-so-small dinosaurs had an arboreal habitat. This conclusion was based on an initial assessment of a hallux (digit I) that pointed posteriorly, in direct opposition to the anteriorly pointing digits II through to IV. This arrangement would have given it an anisodactyl foot, which is characteristic of modern birds with perching habits (Chapter 15). In perching birds, the hallux grasps a branch from behind while the other toes grasp from the front. Nevertheless, re-examinations of the foot anatomy by the mid-1970s showed that the foot was not anisodactyl, but instead had the hallux pointing anteriorly, as with other ornithopods. These insights tie in much better with other skeletal features that argue for a cursorial animal, rather than a tree climber.
Hypsilophodontid skulls underwent some slight outward modifications with respect to those of heterodontosaurids, especially in the loss of their distinctive tusks in the anterior portion. However, their skulls were among the first of herbivorous dinosaurs to show evidence of pleurokinesis. This condition was a jointing between the premaxilla and the rest of the skull that caused the maxilla to shift outward when the mouth closed. In humans, the maxilla stays in the same place when chewing and the lower jaw can move laterally. In hypsilophodontids (and all other euor-nithopods), the maxilla moved out so that a shearing motion occurred when the lower jaw occluded with the upper jaw. Pleurokinesis was accompanied by:
1 the retention of the premaxillary teeth;
2 better-developed dental batteries in the cheek teeth, which also lost their denticles; and
3 a non-ossified (horny) beak.
The latter served as an extension to the anterior part of the face and was used to crop vegetation. Overall, hypsilophodontid skulls point toward advanced adaptations to feeding that exemplified later modifications to herbivore behavior in other euornithopods.
Besides Hypsilophodon, an important hypsilophodontid is Thescelosaurus of the Late Cretaceous in North America, which retains some primitive characteristics of basal ornithopods (Fig. 11.4). This retention shows that it was a conservative form because it lived toward the end of the geologic range for basal euornithopods. In 2000, Thescelosaurus became famous from one well-preserved specimen that had a mineralized mass in the probable site of its heart. This concretion led to the hypothesis that it represented a four-chambered heart with a single aorta, which subsequently encouraged some critical scientific debate (Chapter 8).
Jurassic hypsilophodontids include the Middle Jurassic Agilisaurus and Yandusaurus, which come from the same formation in China. The similar and small (less than 1 meter long) Drinker and Othniela from the Late Jurassic Morrison Formation are representative hypsilophodontids from the western USA (Fig. 11.5). Ironically, these dinosaurs were named after rival paleontologists: Drinker was named in honor of Edward Drinker Cope, whereas Othniela was named after Othniel Charles Marsh (Chapter 3). Early Cretaceous genera from Australia are Atlascopcosaurus, Leaellynasaura, and Qantassaurus, but each genus is known from only a few fragments. Although the Early Cretaceous Hypsilophodon considerably predates Parko-saurus from the latest part of the Cretaceous, the two species closely resemble one another, indicating probable relatedness. Another Late Cretaceous species,
Orodromeus, has the best-known life history of any hypsilophodontid because of some well-preserved juvenile skeletons found in deposits of the western USA.
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