With fewer than ten species discovered, the bonehead group is among the smallest of the main dinosaur groups. Most notable for the dome on the top of its head that was up to 10 inches thick; the function of this dome is uncertain, and has been the subject of considerable debate among experts since the first remains of these animals were found in the 1940.
Head-bashing for defense is the main explanation for the greatly thickened roof of the skull. The neck bones were also sturdy and the neck itself was short, to withstand stress. The spine and rear limbs were also reinforced, to help absorb the shock of impact and transmit the force down into the ground.
There are flaws with this idea. In some species of Pachycephalosaurs, the skull roof was curved or domed. This means that, when two heads clashed, unless the skulls were exactly in line they would slip past each other and jerk the animals neck sideways.
Perhaps the boneheads did not butt head to head, but rammed opponents on the soft tissues on the sides of their bodies. Another theory suggests that these animals used their heads to bash predators, as a bull or rhino lowers its head to charge at an enemy.
One major problem with the head-butting theory is that none of the skulls found to date show the kind of damage that would have been sustained in such of engagement. Also, all pachycephalosaur skulls are not equally thick: on some, the bony dome is only a few centimeters thick, far too weak for ramming assaults.
Most of what we know about this family is based on the range of skulls that have been found. There are very few complete skeletons. The whole skull is about as large as a medium-sized suitcase, with a tiny brain at the center. There is a series of rounded, bony projections under the dome. The teeth are small, and come in several different varieties. Their compressed, curvy shape and serrated edges make it clear that this animal ate only plant material.
Something that is especially strange about these creatures is that, although the skull is very solid, the rest of the body has no armor. Protecting only the head would have been of little use against the jaws of Tyrannosaurus Rex or other large, predatory dinosaurs.
Some scientists suggest that the function of the bony dome was to present a large, formidable display to rivals, as moose do in contests for mates. Others propose that its purpose had nothing to do with fighting at all, but that it was used to identify individual members of the species. Still others believe that this dome may have been a pointy horn when the animal was young, which wore down over time.
Although fossilized skulls are very common; with few skeletons it has been very difficult for paleontologists to reconstruct the lifestyle of this mysterious and misunderstood branch of the dinosaur family.
There are a lot of great coffee table books about dinosaurs, which make a great gift. Some of my favorites, which I rely on when I write these posts, are National Geographic Dinosaurs, by Paul Barrett; The Illustrated Encyclopedia of Dinosaurs, by Dougal Dixon; and Dinosaurus, by Steve Parker.
For my previous posts on dinosaurs, click here.