Pachycephalosauria

Pachycephalosauria, meaning "thick-headed lizards," is a dinosaur clade distinguished by exceptionally dense skull roofs. Because these domes preserve much better than other bones, many species are known only from skull fragments, while complete skeletons remain rare. This clade belongs to the Marginocephalia along with Ceratopsia. Both groups share a common ancestor and the characteristic bony shelf at the back of the skull.

The hallmark of the group is the frontoparietal dome. Depending on the species, this structure could be rounded, flat, or wedge-shaped and was often surrounded by bony nodes and spikes. Their jaws contained small leaf-shaped cheek teeth and sharp, canine-like teeth at the front, which may indicate an omnivorous diet. Articulated specimens like Stegoceras and juvenile Pachycephalosaurus show that these dinosaurs were bipedal with short forelimbs and stiff tails.

The group includes genera such as Pachycephalosaurus, Stegoceras, Prenocephale, Homalocephale, and Wannanosaurus. Two other named genera, Stygimoloch and Dracorex, are widely considered to be growth stages of Pachycephalosaurus. Asian members of the clade are typically found in the Nemegt and Bayan Mandahu formations, while North American species occur in units like the Hell Creek and Dinosaur Park formations.

The thick skull dome has long been associated with head-butting combat, similar to modern bighorn sheep. Research using CT scans and bone histology (Snively & Cox 2008; Goodwin & Horner 2009) has provided a more complex picture. While the dome could absorb high-impact forces, the braincase beneath it was less protected. Furthermore, many adult skulls show injuries consistent with impact. Most paleontologists now believe that both head-butting and flank-butting occurred, with sideways shoving perhaps being more common than frontal collisions. The domes also likely served for display and species identification.

While their diet is still debated, the presence of sharp front teeth suggests they may have supplemented their plant-based diet with insects or other small sources of animal protein.

• Pachycephalosaurus wyomingensis holotype (UCMP 130051) — University of California Museum of Paleontology.
• "Sandy" (PCMP V-31) — recently named Sphaerotholus triregnum skull; well-preserved dome.
• Homalocephale calathocercos holotype (MPC-D 100/51) — Mongolian Academy of Sciences.
• Stegoceras validum specimens — multiple Royal Tyrrell and AMNH mounts.

Genus splits: Stygimoloch and Dracorex are now widely considered ontogenetic stages of Pachycephalosaurus (Horner & Goodwin 2009), reducing pachycephalosaur diversity. Combat behaviour — head-butting vs flank-butting vs both vs display only; current consensus: combat plus display. Dome growth — pachycephalosaur domes thickened with age, complicating species identification from juvenile specimens.

Pachycephalosaurus is a fixture of dinosaur toy lines and children's books, almost always shown head-butting. Jurassic Park: The Lost World (1997) features a pachycephalosaur using its dome to break out of a holding pen.

• pachycephalosaurus
• stygimoloch
• ceratopsia
• ornithischia

• Sullivan, R. M. (2003). Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae). Journal of Vertebrate Paleontology, 23, 181–207.
• Horner, J. R., & Goodwin, M. B. (2009). Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLOS ONE, 4, e7626.
• Snively, E., & Cox, A. (2008). Structural mechanics of pachycephalosaur crania. Acta Palaeontologica Polonica, 53, 581–599.
• Schott, R. K., et al. (2011). Cranial ontogeny in Stegoceras validum. PLOS ONE, 6, e21092.