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PAK - ee - KEF - AH - loh - SAW - rus
Name Means: "Thick-headed lizard"
Incomplete dinosaur skulls featuring centrally thickened domes have been known from North America since the early years of the 20th century, but the nature of the group of dinosaurs to which these skulls belonged continued to be a mystery to paleontogists until 1940. In that year the first well-preserved complete skull-belonging to Pachycephalosaurus, the largest member of the group-was found in Montana. This skull had thick, spiky nodules of bone around the rim of the smoothly domed head and smaller protuding horns on the snout. Pachycephalosaurus is known only from its skull. No other fossils of this dinosaur have so far come to light.
Studies of the anatomy of Pachycephalosaurus skulls, and those of close relatives such as Stygimoloch and Prenocephale, indicate that these dinosaurs probably used their skulls in vigorous head-butting competitions during their courtship battles, in much the same was that modern sheep and goats do. Alternatively, they may have engaged in head-to-head or head-to-flank pushing battles. The central part of the head consisted of very thick bone that would have acted like a helmet, protecting the dinosaur's small brain by carrying any shock waves away from the area of impact, down the sides of the head, to the backbone. The numerous ossified ligaments that strengthened the backbone would then have dissipated the effects of the shock.
The skull, which was almost 2 feet (60 cm) long, was nearly 8 inches (20 cm) thick at the central part of the dome. Pachycephalosaurus had triangular teeth with coarse serrations along the edges for shredding tough plant matter.