Pachycephalosaurus was an herbivorous (plant eating) dinosaur that lived in North America. It inhabited this region during the late Cretaceous period, between about 76 million and 65 million years ago, and was one of the dinosaurs which died out in the Cretaceous-Tertiary extinction.

Pachycephalosaurus is noted for a number of bony features around its skull. These features included a number of bony knobs on its snout, and bony knobs at the back of its head, but the most remarkable of these features is a bony done on its head that was up to 10 inches (25 centimeters) thick. Because of these bony features, many scientists believe that Pachycephalosaurus was related to other cerotopsian dinosaurs such as Protoceratops, Styracosaurus, and Triceratops. However, unlike these other cerotopsian dinosaurs, which were all quadrapeds (walked on four legs), Pachycephalosaurus was a biped (walked on two legs). Assuming Pachycephalosaurus was indeed related to the cerotopsians, this of course raises some interesting questions.

However, the biggest mystery about Pachycephalosaurus was why it had these bony features, and most especially why it had the bony dome on its head. One theory that has been suggested is that Pachycephalosaurus may have engaged in head butting contests for group dominance or mates (rather like some modern species of sheep and deer do). However, there are a number of problems with this theory - Pachycephalosaurus does not appear to have any shock-absorbing mechanism, the bony dome does not appear to be strong enough to engage in head-buttong in the absence of such a mechanism, and even if one puts aside these issues, one would expect to found tell-tale signs of head-butting damage in fossilized skulls, but such damage seems to be absent.

By S. Tanna. Discover more about Pachycephalosaurus at http://www.pachycephalosaurus.org/