Tell me about those pachycephalosaurs
You have come to exactly the right place, my friend. I have a lot of weird and wild theories about pachycephalosaurs, and I’m excited to share them!
Pachycephalosaurus is a fairly well-known dinosaur - a bipedal herbivore with a thick dome-like cranium. Since the initial discovery of pachycephalosaurian dinosaurs in the early 20th century, fossils have been found throughout Late Cretaceous rocks in North America and Asia. But what were the pachycephalosaurs, exactly?
Pachycephalosaurs belong to a group of dinosaurs called Marginocephalia, which also includes the ceratopsians. The currently accepted theory, as pictured above, is that early marginocephalians evolved from bipedal herbivorous ornithopods, which developed bony headgear to protect themselves. This theory is supported by the anatomical similarities between primitive semi-bipedal ceratopsians, such as Psittacosaurus, and the pachycephalosaurs.
Pachycephalosaurs tend to come in two varieties: “flat-headed” species, with broad, wedge-shaped skulls; and “dome-headed” species, with rounded bone protuberances on their heads. However, some paleontologists have cast doubt on whether these different forms represent different species at all, and have put forward the idea that the differing head shapes were a form of sexual dimorphism. According to this theory, mature males were dome-headed, while females and juveniles were flat-headed.
This theory of sexual dimorphism grew out of a preexisting theory about Pachycephalosaurs: that their dome-like heads were used in head-butting competitions. According ton this theory, male pachycephalosaurs head-butted each other in displays of dominance and in competitions over mates, similar to certain modern animals, like sheep and goats. Support for this theory includes the presence of strong neck muscles in pachycephalosaurs; reinforcements to the spine that may have been an adaptation to withstand traumatic force to the head from situations like this; and injuries in numerous domed specimens resulting from trauma-induced bone infections.
However, there is also plenty of evidence against this theory. Some paleontologists have put forward that the bone of the pachycephalosaur dome was not actually strong enough to withstand being used as a ramming weapon. In addition, the neck and spine of the animals were not mechanically able to adopt a horizontal posture, preventing a pose like the one in the image above from actually taking place.
The bony dome of the male pachycephalosaur supported a keratinous horn, like that of the modern-day rhinoceros. The neck muscles and spine reinforcements were an adaptation to carry the weight of such a horn, which males used to fight for dominance (leading to the traumatic injuries present in numerous specimens). These horns would not have been present in the flat-headed females, and - due to the highly biodegradable nature of keratin - have not fossilized.
In the illustration above, the artist (MALvit on DeviantArt) also reconstructs Pachycephalosaurus with a covering of fur-like filaments. This is also not proven, but not impossible; primitive ceratopsians are known to have possessed porcupine-like quills, and such structures may be ancestral to Marginocephalia. In addition, Pachycephalosaurus was native to the American West during the Late Cretaceous, a region that may have had cold winters; perhaps it would have required such a covering for insulation purposes.
It’s likely that some of these questions will never be answered, due in part to the rarity of pachycephalosaurs compared to other types of dinosaurs. Very few species of pachycephalosaur are known, especially in light of the possibility that some known “species” may have to be combined. In addition, pachycephalosaurs were around for a fairly short time, less than 25 million years. It seems likely that pachycephalosaurs were an emerging clade of dinosaurs, not yet as omnipresent or diverse as their ceratopsian relatives. Perhaps if the Cretaceous extinction had not occurred, they would have diversified further, and become a much more widespread animal group, taking on forms that we can only imagine in the millions of years following.