Pachycephalosaurus
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[Photo] Skull of Pachycephalosaurus. Photographer: User:Ballista http://en.wikipedia.org/wiki/User:Ballista Taken in the Oxford University Museum of Natural History 2006

Pachycephalosaurus (the 'thick headed lizard', from the Greek pachy-/παχυ- meaning 'thick', cephale/κεφαλη meaning 'head' and saurus/σαυρο?? meaning 'lizard') was a dinosaur of the family Pachycephalosauridae, from the Late Cretaceous Period of what is now North America. It was an herbivorous creature, which is only known from a single skull and a few extremely thick skull roofs.

Anatomy
Pachycephalosaurus was probably bipedal and was the largest of the bone-headed dinosaurs. It is famous for having a large, bony dome atop its skull, up to 25cm (10 inches) thick, which safely cushioned its brain. The dome's rear aspect was edged with bony knobs and short bony spikes were projected upwards from the snout. These features suggest that, despite their bipedal stance, they were more likely to have been relatives of the ceratopsians, rather than ornithopods.

Using data from other Pachycephalosauridae, it has been estimated that Pachycephalosaurus was approximately the length of a large car, maybe around 4.6m long (15 feet) and had a fairly short, thick neck, short fore limbs, a bulky body, long hind legs and a heavy tail, which was likely to have been held rigid by ossified tendons. Large eye-sockets that faced forward suggest that the animal had good vision and was capable of binocular vision.

Lifestyle
Scientists once suspected that Pachycephalosaurus and its dome-headed relatives were the bipedal equivalents of the big-horned sheep of today. It was thought that, in the mating season, big males would run at one another, clashing heads to decide which would dominate and mate with a herd of females. It was also thought that they might have used their domed heads for defence against predators. However, it is now believed that the Pachycephalosaurs would not have used their domes in this way. The adult head bones could not adequately have withstood pressure and impact and the skulls lacked proper shock absorption. Also, there is no evidence of scars or other damage on fossilized Pachycephalosaurus skulls.

Diet
Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, fruit and insects. The sharp, serrated teeth would have been very effective for shredding plants.

http://en.wikipedia.org/wiki/Pachycephalosaurus

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