Locality North America Utah and

Wyoming) size: 4 ft 6 in/1.4 m long This dinosaur is named after the famous 19th-century American fossil-hunter and Yale professor, Othniel Charles Marsh (see pp. 104-105). He had originally called it Nanosaurus in 1877, but it was renamed exactly 100 years later, in 1977, to commemorate his pioneering fieldwork in the study of dinosaurs.

Othnielia was a typical hypsilopho-dont, with long legs and tail, a lightweight body, and short arms with 5-fingered hands. Only its teeth differed. They were proportionally smaller than those of other hypsilophodonts, and were completely covered in enamel, not just on their grinding faces.

Maybe Othnielia ate tougher plants than usual, hence the protective tooth enamel to avoid excessive wear. A tough, fibrous diet would also have meant that Othnielia had to grind its food down more finely before it could be digested. As an aid to this, Othnielia, like all its relatives, had cheeks that retained the food in the mouth while it was being chewed.

parksosaurus

thescelosaurus tenontosaurus parksosaurus thescelosaurus tenontosaurus name: Hypsilophodon time: Early Cretaceous locality: Europe (England and

Portugal) and North America

(South Dakota) size: 5 ft/1.5 m long About 20 perfect skeletons of this small hypsilophodont were found in one particular bed of Lower Cretaceous rocks in the Isle of Wight, off southern England. This find probably represents a herd of animals that lived and died together, perhaps overwhelmed by the rising tide of the shallow seas that lay over northern Europe some 120 million years ago.

Hypsilophodon is the classic representative of the family, and has given its name to the whole group. Its name means "high ridge tooth," and refers to the tall, grooved cheek teeth typical of all hypsilophodonts. The upper and lower teeth met to form a flat surface for grinding up plant food.

Oddly enough, in comparison with its earlier relatives of the Late Jurassic (see Dryosaurus, p. 140), Hypsilophodon had certain primitive features. For example, it had 4 toes on each foot, and there were incisor-type teeth at the front of the upper jaw. When these were closed on the toothless, horny beak of the lower jaw, they formed an effective device for cropping vegetation.

Hypsilophodon may also have been armored, with 2 rows of thin, bony scales running down either side of its back. But paleontologists are uncertain about this feature.

The study of Hypsilophodon since its discovery in the 19th century is a classic story of paleontological research. When it was first described, by Thomas H. Huxley in 1870, paleontologists of the day were struck by the similarity of its build to that of a modern tree kangaroo. For almost a century, classic illustrations of Hypsilophodon showed it perched in a tree, 3 of its 4 toes gripping the branch, birdlike, and the fourth toe directed backward. This reconstruction neatly filled an ecological niche not yet occupied by any dinosaur, and so, the idea of a tree-dwelling, plant-eating ornithopod was conceived.

It was not until 1974 that the skeleton of Hypsilophodon was reassessed. Paleontologists concluded that there was no evidence to show that this dinosaur lived in trees. In fact, it was a perfectly adapted terrestrial animal, capable of running rapidly on 2 legs.

name: Tenontosaurus time: Early Cretaceous locality: North America (Arizona,

Montana, Oklahoma and Texas) size: 24 ft/7.3 m long This dinosaur is so uncharacteristically large compared with other hypsilophodonts that some paleontologists class it with the iguanodonts (see pp. 142-145). Indeed, its skull is very similar in shape to that of an iguano-dont, but its teeth and their arrangement in the jaws place it firmly in the hypsilophodont family.

Over half the total length of Tenontosaurus was made up of tail, which was enormously thick and heavy. The animal is estimated to have weighed about 1 US ton/900 kg, and probably spent much of its time on all-fours. Its arms were much longer and stouter than those of other hypsilophodonts.

A remarkable find in the rocks of Montana consisted of a skeleton of Tenontosaurus surrounded by 5 complete specimens of Deinonychus, a ferocious predator of the day (see p. 112). Although these bodies were most likely brought together by chance after death, perhaps in a flash flood, it is interesting to speculate that this mass-burial could have been the outcome of an encounter between the bulky plant-eater and a pack of predators.

Although Deinonychus was only about 10 ft/3 m long, it had sharp, meat-shearing fangs, and great, daggerlike claws on its feet. The bulky Tenontosaurus could have put up a good fight, kicking out with its heavy-clawed feet or using its great tail as a whiplash. But these were paltry defenses when compared with the lethal weapons of the agile carnivores.

name: Parksosaurus time: Late Cretaceous locality: North America (Alberta) size: 8 ft/2.4 m long

Parksosaurus was one of the last of the long-lived hypsilophodont family to survive before the mass-extinction of the dinosaurs at the end of the Cretaceous period. It was similar in build to all other hypsilophodonts, but there were minor differences in the skull, notably its large eyes.

Parksosaurus probably foraged close to the ground, snuffling about among the low-growing undergrowth, and selectively nipping off its preferred food with its narrow, beaked jaws.

name: Thescelosaurus time: Late Cretaceous locality: North America (Alberta,

Montana, Saskatchewan and

Wyoming) size: 11 ft 6 in/3.5 m long Thescelosaurus was discovered in the topmost rocks of the Late Cretaceous period in western North America. It seems to have been bulkier and bigger-boned than its closest relatives, the typically small, lightweight hypsilophodonts. In fact, Thescelosaurus could be a member of the iguanodont family.

Several features distinguish Thescelosaurus from other hypsilophodonts. It had teeth in the front of its upper jaw. It had 5 toes on each foot (in contrast to the hypsilophodonts, which had 3 or 4). And its thigh bones were as long as its shin bones (the shins of the agile hypsilophodonts were always longer than the thighs).

The structure of the legs suggests that Thescelosaurus was not a gazelle-type sprinter like its relatives, but rather a slower-moving creature. Perhaps to compensate for this, there were rows of bony studs set in the skin of its back, which would have offered some measure of protection against the carnivorous dinosaurs that preyed on it.

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