Genus
Valid Extinct

Pachycephalosaurus

Brown and Schlaikjer 1943
Etymology Reptile à tête épaisse

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, 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.

Temporal range
Triassic
Jurassic
Cretaceous
Paleogene
Neogene
252 201 145 66 0 Ma
PBDB occurrences
34
Group
Dinosaures
Herbivore Ground dwelling, gregarious Terrestrial
Pachycephalosaurus
click to enlarge
Pachycephalosaurus wyomingensis "Sandy" specimen, Royal Ontario Museum © IJReid · CC BY 4.0 · Wikimedia
PBDB ↗ Wikipedia ↗
Classification
Dinosauria Unranked clade
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Ornithischia Unranked clade
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Neornithischia Unranked clade
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Pyrodontia Unranked clade
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Cerapoda Unranked clade
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Marginocephalia Unranked clade
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Pachycephalosauria Suborder
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Eupachycephalosauria Unranked clade
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Pachycephalosaurus Genus
Measurements
Length
3 m ⓘ
Mass
450 kg ⓘ
Fossil sites 34 geolocated sites
Distribution
Top countries
🇺🇸 United States
32
🇨🇦 Canada
2
Geological formations
Hell Creek
19
Lance
9
Ferris
3
Dinosaur Park
1
Denver
1
Scollard
1
Temporal distribution
Maastrichtian (72.2–66 Ma)
33
Campanian (83.6–72.2 Ma)
1
Species (7)
Dracorex hogwartsia subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Pachycephalosaurus grangeri subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Pachycephalosaurus reinheimeri subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Pachycephalosaurus wyomingensis 84 Ma
Stenotholus kohleri subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Stygimoloch spinifer subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Tylosteus ornatus subjective synonym of Pachycephalosaurus wyomingensis 84 Ma
Synonyms (4)
Dracorex subjective synonym of Pachycephalosaurus
Stenotholus subjective synonym of Pachycephalosaurus
Stygimoloch subjective synonym of Pachycephalosaurus
Tylosteus subjective synonym of Pachycephalosaurus
Bibliography
Original description
B. Brown and E. M. Schlaikjer. 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
Bibliography (19)
A. F. Wroblewski. 2024. Southernmost record of the pachycephalosaurine Stygimoloch spinifer and palaeobiogeography of latest Cretaceous North American dinosaurs. Lethaia 57(4):1–10 DOI ↗
W. W. Stein. 2021. The paleontology, geology and taphonomy of the Tooth Draw Deposit; Hell Creek Formation (Maastrictian), Butte County, South Dakota. The Journal of Paleontological Sciences JPS.C.21:0001:1-108
K. Snyder, M. McLain, and J. Wood, A. V. Chadwick. 2020. Over 13,000 elements from a single bonebed help elucidate disarticulation and transport of an Edmontosaurus thanatocoenosis. PLoS One 15(5):e0233182:1-31 DOI ↗
T. S. Kelly. 2014. Preliminary report on the mammals form Lane's Little Jaw Site Quarry: a latest Cretaceous (earliest Puercan?) local fauna, Hell Creek Formation, southeastern Montana. Paludicola 10(1):50-91
T. E. Williamson, T. D. Carr, and S. A. Williams, K. Tremaine. 2009. Early ontogeny of pachycephalosaurine squamosals as revealed by juvenile specimens from the Hell Creek Formation, eastern Montana. Journal of Vertebrate Paleontology 29(1):291-294 DOI ↗
R. T. Bakker, R. M. Sullivan, and V. Porter, P. Larson, S. J. Saulsbury. 2006. Dracorex hogwartsia, n. gen., n. sp., a spiked, flat-headed pachycephalosaurid dinosaur from the Upper Cretaceous Hell Creek Formation of South Dakota. Late Cretaceous Vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35:331-345
M. T. Carrano. 2005. Fossil Vertebrate Collections, University of California Museum of Paleontology
R. M. Sullivan. 2003. Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae). Journal of Vertebrate Paleontology 23(1):181-207 DOI ↗
J. A. Lillegraven and J. J. Eberle. 1999. Vertebrate faunal changes through Lancian and Puercan time in southern Wyoming. Journal of Paleontology 73(4):691-710 DOI ↗
M. B. Goodwin, E. A. Buccholtz, and R. E. Johnson. 1998. Cranial anatomy and diagnosis of Stygimoloch spinifer (Ornithischia: Pachycephalosauria) with comments on cranial display structures in agonistic behavior. Journal of Vertebrate Paleontology 18(2):363-375 DOI ↗
M. B. Goodwin and R. E. Johnson. 1995. A new skull of the pachycephalosaur Stygimoloch casts doubt on head butting behavior. Journal of Vertebrate Paleontology 15(3, suppl.):32A
E. B. Giffin. 1989. Notes on pachycephalosaurs (Ornithischia). Journal of Paleontology 63(4):525-529 DOI ↗
M. B. Goodwin. 1989. New occurrences of pachycephalosaurid dinosaurs from the Hell Creek Formation, Garfield County, Montana. Journal of Vertebrate Paleontology 9(3, suppl.):23A
E. B. Giffin, D. L. Gabriel, and R. E. Johnson. 1988. A new pachycephalosaurid skull (Ornithischia) from the Cretaceous Hell Creek Formation of Montana. Journal of Vertebrate Paleontology 7(4):398-407 DOI ↗
P. M. Galton and H.-D. Sues. 1983. New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America. Canadian Journal of Earth Sciences 20(3):462-472 DOI ↗
W. P. Wall and P. M. Galton. 1979. Notes on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America, with comments on their status as ornithopods. Canadian Journal of Earth Sciences 16:1176-1186 DOI ↗
B. Brown and E. M. Schlaikjer. 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
C. W. Gilmore. 1931. A new species of troödont dinosaur from the Lance Formation of Wyoming. Proceedings of the United States National Museum 79(9):1-6 DOI ↗
J. Leidy. 1872. Remarks on some extinct vertebrates. Proceedings of the Academy of Natural Sciences of Philadelphia 24(1):38-40