Open Paleo

Pachycephalosaurus

Brown & Schlaikjer, 1943 pak-i-SEF-a-lo-SAWR-us /ˌpækɪˌsɛfələˈsɔːrəs/

From Greek pachys (παχύς, 'thick'), kephalon (κεφαλή, 'head'), and sauros (σαῦρος, 'lizard'), meaning 'thick-headed lizard'

72.2–66.0 million years ago Maastrichtian, Late Cretaceous
Late Cretaceous stages

Pachycephalosaurus is a genus of pachycephalosaurid ornithischian dinosaur. The type species, P. wyomingensis, is the only known definitive species. The possibly synonymous taxon, Stygimoloch, might represent a distinct genus or a second species, P. spinifer. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta. Mainly known from a single skull and a few extremely thick skull roofs (at 22 cm (8.7 in) thick), Pachycephalosaurus is estimated to have reached 4.5 m (15 ft) long and weighed 370-450 kg (820-990 lb). More complete fossils would come to be found in the following years.

Diagnosis

  1. Frontoparietal area low and only moderately thickened
  2. Low parieto-squamosal shelf
  3. Frontals low and fairly broad where in contact with nasals
  4. Short distance from occipital condyle to posterior margin of skull
  5. Less vaulted and relatively broader frontoparietal area than P. grangeri

Species

Pachycephalosaurus wyomingensis Brown & Schlaikjer, 1943

Type species

Named for Wyoming, the U.S. state from which the holotype was recovered.

Holotype
USNM 12031 , National Museum of Natural History, Smithsonian Institution
Material
Fragmentary frontoparietal area with squamosals and part of occipital region.
Horizon
Lance Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
9 miles southwest of Warren P.O., Buck Creek, Wyoming, United States

Synonyms

  • Pachycephalosaurus grangeri junior subjective

    Erected by Brown and Schlaikjer (1943) on a more complete skull; now regarded as conspecific with, and a junior synonym of, the type species P. wyomingensis.

  • Stenotholus junior subjective

    S. kohleri was reclassified as juvenile Stygimoloch spinifer; with Stygimoloch itself now interpreted as a juvenile of Pachycephalosaurus wyomingensis, Stenotholus is also treated as a junior synonym.

  • Stygimoloch junior subjective

    S. spinifer is currently interpreted as a juvenile of Pachycephalosaurus wyomingensis; if future research confirms it as distinct, the species would be treated as P. spinifer.

Synonyms of the genus

Name Type Reason
Dracorex junior subjective Now generally interpreted as a juvenile individual of Pachycephalosaurus.
Tylosteus nomen rejectum Formally rejected by ICZN in favor of Pachycephalosaurus, which became a conserved name.

Notable specimens

  • BMR P2001.4.5

    Burpee Museum of Natural History pathology

    Frontal bone bears 23 lesions consistent with osteomyelitis from penetrating trauma. Cited as evidence that pachycephalosaurid domes were used in intra-specific agonistic combat rather than being taphonomic artifacts.

    Peterson et al. (2013)

References

  1. Peterson, J. E.; Dischler, C.; Longrich, N. R. (2013). Distributions of cranial pathologies provide evidence for head-butting in dome-headed dinosaurs (Pachycephalosauridae). PLOS ONE 8(7): e68620. Public Library of Science (PLoS). doi:10.1371/journal.pone.0068620 peterson2013a
  2. Brown, B.; Schlaikjer, E. M. (1943). A study of the troödont dinosaurs with the description of a new genus and four new species. Bulletin of the American Museum of Natural History 82(5): 115-150. brown1943a
  3. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the hip height estimate, and the appearance features.

    jones2026a