The Pachycephalosauria ("thick-headed lizards") consisted of small to medium-sized bipedal, ornithischian herbivores that measured in length up to about 15 feet (4.5 m). They are the only dinosaur group distinguished by a thick, bony skull cap. Of the 14-plus recognized genera of pachycephalosaurs, nine are known only from skulls and skull fragments. Only a single new taxon of pachycephalosaur has been named in the past 25 years (Sphaerotholus, in 2002), indicating the relative scarcity of this clade in the fossil record of dinosaurs.
Pachycephalosauria are classified as a clade within the Margino-cephalia and taxonomically defined as marginocephalians that are more closely related to Pachycephalosaurus than to Triceratops. The clade is further divided into two subgroups: the Homalocephaloi-dea, a more primitive taxa that had flattened heads, and the Pachy-cephalosauridae, having a prominent domelike skull cap. Recent studies have suggested, however, that the distinction between flattened and domed heads might merely be related to the age of the animal, the domed head being fully formed only after reaching sexual maturity. Most pachycephalosaurs have been found in North America, Europe, and Asia and lived from the Early to Late Cretaceous Epochs.
In addition to having thick, bony caps, the skulls of the Pachycephalosauria were ornamented in several unusual ways. Stegoceras (Late Cretaceous, western North America), Prenocephale (Late Cretaceous, Mongolia), and the recently discovered Sphaerotholus (Late Cretaceous, Montana and New Mexico) had a marginal ridge with bony knobs at the base of the dome in the rear of the skull. Homalocephale (Late Cretaceous, Mongolia and China) had a flat head with bony knobs on top and a similar margin of bony nodules around the back of the skull, plus small horns pointing to the rear over the neck. Stygimoloch (Late Cretaceous, western North America) had a bit of everything: a rounded skull cap, bony spikes on its nose, a protective row of bony knobs over its eyes, and several pairs of formidable spikes jutting up from the back of the head behind its ears. The various knobs and spikes adorning their skulls were probably also important ways for pachycephalosaurs to tell one individual from another and to display one's prowess to members of the opposite sex.
It is widely assumed that the domed head of the pachycephalosaurs was used for head-butting with other individuals. The same kind of behavior can be seen today in bighorn sheep. They square off and repeatedly butt heads with all of their might until one male is forced to give up and back off. Pachycephalosaurs may have done the same, squaring off with their battering-ram skulls, running headlong into each other. Because the skull cap of these dinosaurs was rounded and small, however, some paleontologists doubt whether head-butting was common among pachycephalosaurs. They would have had to have great aim to make it work. Otherwise, their heads would have just glanced off of each other. Instead, these dinosaurs may have used their heads to ram against the side of a rival, a contest that might have been a little less jarring to the brain. The backbones of pachycephalosaurs had a tightly packed, interlocking structure that was well-made to absorb the shock of such contests, and their legs were strong enough to propel them forward with considerable power. Their shock-absorber necks allowed them to engage in head- and thigh-banging contests with little danger of permanent injury.
The bipedal posture of pachycephalosaurs was aided by a spine that locked the vertebrae together for strength and flexibility. The animals walked in a leaning position, with their back parallel to the ground. Because the upper part of the pachycephalosaur leg (the femur) was longer than the lower part (the tibia and fibula), these dinosaurs were probably not fast runners.
Pachycephalosaurs had teeth and eating habits that were most similar to those of the armored and plated dinosaurs. Pachycepha-losaurs were not very tall, so they probably browsed on plants only a meter or so above the ground. Their teeth were divided into two basic groups. The small front teeth were sharp and could grasp and tear plants. The rear teeth were positioned in the cheeks and had triangular ridges for shredding vegetation. These animals were able to pluck and puncture leaves and small fruits with their front teeth and shred them with their cheek teeth. Their wide rib cage suggests that the bone-headed dinosaurs had a large gut for fermenting swallowed food.
The pachycephalosaur Stegoceras ("roofed horn") is known from more fossil remains than all other members of the clade. More than two dozen skulls and skull fragments have been found for this dinosaur. Paleontologist Ralph Chapman conducted a study of the skulls and found that they could be divided into two groups: those with a thick-walled dome and those with a thinner dome. He concluded that the ones with the thicker domes were males because of the belief that they used their heads to butt rival males.
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