Horned and Bone Headed Dinosaurs

One of the most familiar images in the folklore of dinosaurs is a battle to the death between Tyrannosaurus and the horned dinosaur Triceratops (Late Cretaceous, western North America). Triceratops was a large horned dinosaur, one of the last of its kind, and probably would have made a formidable foe for T. rex had the theropod been careless enough to engage it in a tussle. Weighing as much as 6 tons (5.4 tonnes), a defensive-minded Triceratops had many advantages to draw upon in battle. Its low-profile, stocky body stood on powerful limbs that were stabilized by large, broad, hooflike, four-toed feet. Unlike ornithopods, which had long tails that they used to maintain balance, Triceratops had a tail that was short, flexible, and probably of no consequence in maintaining balance. This massive horned dinosaur was naturally surefooted and so did not need its tail for balance.

Whereas most dinosaurs had four or five sacral vertebrae, Triceratops had 10, all the more to fortify its shock-absorbing vertebral column when the animal engaged in fending off predators or locking horns with rivals of its own kind. The business end of Tricer-atops was the enormous skull, measuring up to 8 feet (2.4 m) long and equipped with stout horns over the eyes and a shorter, broader horn on the nose. The perimeter of its skull was adorned with a bony extension called a frill and it protected the area behind the head. The anterior end of the neck, where it supported the heavy skull, was made up of four fused vertebrae to provide extra support and strength. Triceratops was nothing less than a living tank and it

Triceratops

can be assumed that predatory dinosaurs rarely took a chance by attacking a healthy adult of this horned dinosaur.

Triceratops is the most familiar member of the Ceratopsia, a subgroup within the clade of Cerapoda termed Marginocephalia, or "bordered heads." Marginocephalians were a group of ornithischian dinosaurs that included the Ceratopsia ("horned faces") and Pachy-cephalosauria ("thick-headed lizards"). The name Marginocephalia refers to a bony shelf at the back of the skull that is characteristic of all members of the clade. In ceratopsians such as Triceratops, this bony shelf was expanded, often enormously, to compose the frill. In pachycephalosaurs, sometimes called bone-headed dinosaurs, the shelf was less pronounced but always accompanied by a thick skull cap from which this clade gets its name.

All marginocephalians were herbivores. Ceratopsians ranged in length from about 3.3 feet (1 m) in the most basal members to nearly

30 feet (9 m) in the largest taxa such as Triceratops. Except for a few primitive forms such as Yinlong (Late Jurassic, China), Psittacosau-rus (Early Cretaceous, central and eastern Asia), Chaoyangsaurus (Late Jurassic or Early Cretaceous, China), and Liaoceratops (Early Cretaceous, China), the horned dinosaurs were obligate quadrupeds. The pachycephalosaurs, whose domed skull cap was up to several inches thick in some taxa, were small to medium-sized bipedal herbivores measuring up to 15 feet (4.5 m) long.

This chapter explores the evolution and lifestyles of the Mar-ginocephalia.

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