Pachycephalosauria is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some fossils, the skull roof is domed and several inches thick; in others it is flat or wedge-shaped. While traditionally regarded as distinct species or even families, the flat-headed pachycephalosaurs may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.
The adaptive significance of the skull dome has been heavily debated. The popular hypothesis among the general public that the skull was used in head-butting, as sort of a dinosaurian battering ram, was first proposed by Colbert 1955. This view was popularized in the 1956 science fiction story “A Gun for Dinosaur” by L. Sprague de Camp. Many paleontologists have since argued for the head-butting hypothesis, including Galton 1970 and Sues 1978. In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goats and musk oxen.
I’m not a fan of C&P’ing from Wikipedia, but as I stated earlier, I don’t have the time to write things up for Tumblr. That’ll change shortly, but for now, enjoy this!