Pachycephalosaurus wyomingensis

Pachycephalosaurus wyomingensis was a bipedal pachycephalosaurid from the Hell Creek and Lance formations near the end of the Cretaceous. Its adult skull roof expanded into an exceptionally thick dome built chiefly from fused frontal and parietal bones, surrounded by knobs and short spikes. The skull is much better represented than the rest of the skeleton, so full-body restorations remain composite. Histology and CT evidence led to an influential hypothesis that the flat-headed Dracorex and spike-fringed Stygimoloch are juvenile and subadult stages of Pachycephalosaurus. That model explains dramatic skull change during growth, but not every researcher treats the proposed synonymy as settled.

Its fossils occur between approximately 68 and 66 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The adult frontals and parietals fused into a tall, solid cranial dome, while the rear and sides of the skull carried clusters of rounded nodes and shortened horn-like projections. Large eye sockets faced partly sideways, and a narrow beak ended the snout. Small leaf-shaped cheek teeth suit plant processing, but sharper, serrated teeth toward the front of the mouth complicate a strictly herbivorous reconstruction. The animal walked on long hind limbs and had a stiffening tail, short forelimbs and a relatively deep torso. Because no single universally accepted specimen preserves the whole body, many postcranial proportions are inferred from partial remains and other pachycephalosaurids.

• Massively thickened dome formed chiefly by fused frontal and parietal bones
• Clusters of blunt nodes and reduced horn-like projections around the rear skull
• Narrow toothless beak at the front of the jaws
• Small leaf-shaped cheek teeth suited to cutting plant material
• Sharper serrated teeth near the front of the mouth
• Large rounded eye sockets and a relatively short braincase
• Long hind limbs supporting habitual bipedal locomotion
• Short forelimbs and a long tail reconstructed from incomplete postcrania

George F. Sternberg collected the partial domed skull USNM 12031 from the Lance Formation of Niobrara County, Wyoming, on 25 August 1930. Charles W. Gilmore named it Troodon wyomingensis in 1931, when dome-headed dinosaurs and the tooth-based Troodon were mistakenly grouped together. Barnum Brown and Erich Schlaikjer established Pachycephalosaurus in 1943 from the nearly complete adult skull AMNH 1696, naming it P. grangeri and transferring Gilmore’s older species into the genus. Later revision treated P. grangeri and P. reinheimeri as younger names for P. wyomingensis, leaving one generally recognized species.

Discovery credit: George F. Sternberg.

Naming authors: Charles W. Gilmore.

Pachycephalosaurus lived on the latest Maastrichtian coastal plain represented by the Hell Creek and Lance formations. Meandering rivers crossed wooded floodplains with ponds, wetlands and seasonally drier ground. Flowering plants, conifers, ferns and low vegetation supported a diverse herbivore fauna including Triceratops, Edmontosaurus, Thescelosaurus and Ankylosaurus. Tyrannosaurus occupied the role of largest predator. The known localities span parts of present-day Montana and Wyoming rather than one narrowly defined habitat patch.

Plants were probably the main food, based on the beak and cheek teeth, but the pointed front teeth leave occasional animal matter or a broader omnivorous diet possible; no gut contents decide the issue. The dome’s function remains one of dinosaur palaeobiology’s best-known debates. Finite-element models show that a dome could dissipate substantial low-speed impact forces, and healed cranial lesions cluster on the tops of many pachycephalosaurid domes. Histological work and the rounded impact surface have instead been used to question repeated high-speed head-on ramming. Display, pushing, flank strikes and lower-energy head contact are not mutually exclusive possibilities.

• The original 1931 species was placed in Troodon because palaeontologists then confused dome-headed dinosaur teeth with the theropod tooth on which Troodon was based.
• AMNH 1696 was named Pachycephalosaurus grangeri in 1943 but is now the best-known adult skull of P. wyomingensis.
• In the proposed growth series, long juvenile spikes are resorbed and remodelled into blunt adult nodes while the dome inflates dramatically.
• A 2013 survey found dome lesions in about 22 percent of the sampled pachycephalosaurid specimens, supporting—but not proving—combat behaviour.
• The famous thick head is known far better than the body; a complete mounted silhouette should not be mistaken for one complete skeleton.

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1. 01
   The Paleobiology Database

   Open source
2. 02
   A new species of troödont dinosaur from the Lance Formation of Wyoming

   Charles W. Gilmore · Proceedings of the United States National Museum · 1931

   Open source
3. 03
   A study of the troödont dinosaurs with the description of a new genus and four new species

   Barnum Brown, Erich M. Schlaikjer · Bulletin of the American Museum of Natural History · 1943

   Open source
4. 04
   Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus

   John R. Horner, Mark B. Goodwin · PLOS ONE · 2009

   Open source
5. 05
   Structural mechanics of pachycephalosaur crania permitted head-butting behavior

   Eric Snively, Andrew Cox · Palaeontologia Electronica · 2008

   Open source
6. 06
   Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae)

   Joseph E. Peterson, Collin Dischler, Nicholas R. Longrich · PLOS ONE · 2013

   Open source
7. 07
   Pachycephalosaurus wyomingensis Gilmore, 1931, USNM 12031

   Smithsonian National Museum of Natural History

   Open source
8. 08
   Accessibility: tactile objects in the fossil halls

   American Museum of Natural History

   Open source