What is Pachycephalosauria?

The most inclusive clade containing Pachycephalosaurus wyomingensis but not Triceratops horridus, Hypsilophodon foxii, Heterodontosaurus tucki, Ankylosaurus magniventris. Pachycephalosaurs, known colloquially as "bone heads", were small to medium-sized (2-4.5 meters) marginocephalian ornithischian dinosaurs that lived mainly during the Mid-to-Late Cretaceous in Asia and North America, and have been the cause of much head-banging between palaeontologists for decades because of one seemingly unanswerable question: "were they head-bangers or not?"

Some researchers insist that the domed skulls of pachycephalosaurs, which in some cases are six inches thick, were made from brittle bone, and those with particularly high and curved domes would require the precision of a surgeon to pull off anything more than a glancing blow. Also, despite being quite stocky and well-made, their head, neck and spine could not be aligned in such a way to transmit stress, unlike modern-day Muskox. So perhaps, instead, they participated in bouts of flank butting which would inflict pain aplenty on whatever was on the receiving end, especially when you consider the lumps and horns that adorn some domes.

The opposition researchers, well, they are all for head-banging. As well as a unique "wall" of bone on the inside of the eye socket to perhaps secure the eyeball and protect it from contact shock during ramming contests, around 22% of 100 pachycephalosaur skulls that were scrutinized as part of a 2013 study sported damage that had previously been brushed off as artefacts of preservation, but which were consistent with osteomyelitis; an infection of the bone as a direct result of penetrative trauma, or trauma to the surrrounding tissue which led to bone infection. Unless consistent external factors were at work, it seems likely that these surface wounds, which had healed as well as could be, were incurred during bouts of head-to-head combat. Flat-headed specimens, which may well be juveniles rather than distinct adult species, were damage free.

Although they were never as diverse or abundant as ornithopods or ceratopsians, pachycephalosaurs filled a unique ecological niche. They had a stocky torso with broad hips and an expanded gut, short forelimbs, robust legs, a short, thick neck, and a heavy tail. Large eye sockets and a large optic nerve suggest good vision, uncharacteristically large olfactory lobes point to a good sense of smell, and they had "heterodont" dentition—different kinds of teeth for mixed purposes in one mouth, some of which have been historically confused with those of the small theropod called Troodon—suggesting they were likely omnivorous rather than purely herbivorous. Whether pachycephalosaurs were headbangers or not, we'll never know for sure, but we're willing to bet that males with the biggest domes were more popular with females than those with smaller ones.

Click here to search Dinochecker for pachycephalosaurs. Pachycephalosauria is derived from the Greek "pakhys" (thick), "kephale" (head) and "sauros" (lizard), and the Latin "-ia" (plural-forming suffix).