The skulls of the so-called "thickheaded lizards," or boneheads, were dome-shaped, giving their owners a bizarre appearance. They had high foreheads and thick skull caps, made up of enormously thickened bones. Some species also had bony frills, knobs and spikes on the back and sides of their heads, and sometimes on their snouts.

Most paleontologists believe that these boneheaded dinosaurs had a similar lifestyle to that of modern mountain goats. Like these mammals, they would have lived together in herds, and the males would most likely have engaged in competitive, head-butting fights to establish a pecking order.

In other respects, pachycephalosaurs were like other ornithopods — 2-legged plant-eaters, with 5-fingered hands, 3-toed feet (with a tiny first toe), and a long, heavy tail. As a group they were rare, known mainly from Late Cretaceous times in North America and central Asia. But one bonehead, called Yaverlandia, has been found earlier than this — dating from Early Cretaceous rocks in southern England.

Some paleontologists think that the anatomy of the boneheads (especially their skull structure) is sufficiently different from that of the ornithopods to justify placing them in a separate group.

name: Stegoceras time: Late Cretaceous locality: North America (Alberta) size: 6 ft 6 in/2 m long

The whole body of this boneheaded dinosaur seems designed to provide the power behind the ramming head. When an animal charged a rival, its head would have been lowered at right angles, and its neck, body and tail held out stiffly in a horizontal line, balanced at the hips. The skull cap was thickened into a high dome of solid bone, and the small brain was well protected inside. This domed area would have absorbed the main impact as the animal crashed head-on against its opponent. A full-grown Stegoceras could have weighed 120 lb/54-4 kg.

The "grain" of the bone in the dome was angled perpendicularly to the surface, indicating that it was built to withstand great impact.

name: Prenocephale time: Late Cretaceous locality: Asia (Mongolia) size: 8 ft/2.4 m long

A truly bulbous dome surmounted the head of Prenocephale, and a row of bony spikes and bumps surrounded the solid skull, back and sides. The females probably had smaller, thinner skulls than the males, just like the modern, female big-horned sheep of the Rocky Mountains in the USA have smaller horns than the males.

Like other boneheads, Prenocephale probably had large eyes and a keen sense of smell. It lived in upland forests, browsing on leaves and fruits.

name: Homalocephale time: Late Cretaceous locality: Asia (Mongolia) size: 10 ft/3 m long

Homalocephale means "even head," and refers to the fact that this pachyceph-alosaur did not have a dome on top of its skull. It had a rather flat, wedge-shaped head, although the bones of the skull cap were greatly thickened. There were numerous pits and bony knobs scattered all over the head. This has led some paleontologists to think that rival male Homalocephale fought the same kinds of ritualistic, head-butting battles as do the modern marine iguanas of the Galapagos Islands.

Homalocephale had particularly broad hips, and paleontologists interpret this feature in different ways. Some say that the hips could have been part of the impact-absorbing system when rival males fought together. Others postulate that the broad hips could indicate that this bonehead gave birth to live young.

name: Pachycephalosaurus time: Late Cretaceous locality: North America (Alberta) size: 15 ft/4.6 m long

This was a giant among boneheads. Although it is known only from its skull, this measured 2 ft/60 cm in length. The enormous dome on top of the head was made of solid bone, some 10 in/25 cm thick. Like a great crash helmet, the thick skull could have absorbed tremendous impact as rival males butted each other, head-on.

Pachycephalosaurus was not only the biggest member of the bonehead family; it was also the last member to exist before all its plant-eating relatives and its carnivorous cousins became extinct at the end of the Cretaceous period.

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