Pachycephalosaurus

Pachycephalosaurus wyomingensis is the largest member of its clade. It possessed a frontoparietal skull dome measuring up to 25 cm thick, the greatest of any known dinosaur. A halo of nodes and short spikes surrounded this central dome, with additional spikes located along the postorbital and squamosal areas. The dome grew significantly as the animal matured. Juveniles started with flat skulls and longer spikes, a morphology previously used to define the genera Stygimoloch and Dracorex. These are now generally understood to be younger stages of Pachycephalosaurus.

The dinosaur moved on two legs and had relatively short forelimbs, sturdy hindlimbs, and a stiff tail. Its mouth contained small, leaf-shaped cheek teeth and sharp, canine-like front teeth. This dental arrangement suggests a varied diet that likely included plants, insects, and perhaps small vertebrates. Complete skeletons are rare because the dense bone of the skull dome preserves far more effectively than the rest of the body.

The purpose of the thick skull dome is the subject of three primary theories:

1. Head-butting: This theory proposes frontal-impact combat similar to modern bighorn sheep. It is supported by the dome's thickness and internal shock-absorbing structure (Snively & Cox 2008).
2. Flank-butting: This hypothesis suggests that individuals used their heads for sideways shoves. Biomechanical studies of the neck and spine indicate that the braincase was not perfectly aligned for direct head-on impacts.
3. Display: Some researchers argue the dome functioned primarily for sexual display or species recognition, citing a lack of definitive combat injuries in many fossils.

However, a 2013 study by Peterson et al. identified healed cranial injuries on several skulls that are consistent with intraspecific impacts. Most paleontologists now accept that the dome was used for some form of combat, though it undoubtedly played a role in display and recognition as well.

• UCMP 130051 — P. wyomingensis holotype, University of California Museum of Paleontology.
• MPM VP6135 ("Sandy"-type specimen) — Milwaukee Public Museum.
• MPM VP7585 ("Stygimoloch"-stage juvenile) — now considered juvenile P. wyomingensis.

Stygimoloch and Dracorex synonymy — Horner & Goodwin (2009) argue both are juvenile Pachycephalosaurus; widely (but not universally) accepted. Combat behaviour — head-butting vs flank-butting vs display only; current consensus: combat plus display, with flank-butting probably more common than direct head-on impact. Dome growth pattern — well-resolved as ontogenetic thickening.

Pachycephalosaurus is a fixture of dinosaur toy lines, almost always shown head-butting. Jurassic Park: The Lost World (1997) features one breaking out of its enclosure. The animal is one of the canonical Hell Creek species alongside T. rex, Triceratops, Edmontosaurus, and Ankylosaurus.

• pachycephalosauria
• stygimoloch
• ornithischia

• 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.
• Peterson, J. E., Dischler, C., & Longrich, N. R. (2013). Distributions of cranial pathologies provide evidence for head-butting in dome-headed dinosaurs. PLOS ONE, 8, e68620.