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Pale bipedal dinosaur with a thick domed skull ringed by small knobs
Reconstruction of Pachycephalosaurus wyomingensis based on the 'Sandy' specimen.Image credit: Fred Wierum, CC BY 4.0, via Wikimedia CommonsCC BY 4.0Source (opens in a new tab)

Pachycephalosaurus wyomingensis

Pronounced PAK-ee-SEF-ah-loh-SORE-us wye-oh-ming-EN-sis

Pachycephalosauria · Pachycephalosauridae

A dome-headed dinosaur from the latest Cretaceous of western North America, a neighbour of T. rex. Its skull roof was a thick dome of solid bone; injuries on many domes suggest that pachycephalosaurs butted heads.

  • Bipedal
  • Unusual anatomy
Compare
Period
Cretaceous· Maastrichtian
Diet
Herbivore
Length
4.5 m
Mass
370–450 kg
Location
Montana, Wyoming, South Dakota, North Dakota, United States

Quick answers

How big was Pachycephalosaurus wyomingensis?
Pachycephalosaurus wyomingensis was about 4.5 m long. It stood about 1.2–1.6 m tall at the hips. It weighed about 370–450 kg. (Wikipedia: Pachycephalosaurus 2026; Paul 2016)
When did Pachycephalosaurus wyomingensis live?
Pachycephalosaurus wyomingensis lived during the Late Cretaceous, about 72.1–66 million years ago. (Paleobiology Database: Pachycephalosaurus occ… 2026; Cohen et al. 2023)
Where have Pachycephalosaurus wyomingensis fossils been found?
Fossils of Pachycephalosaurus wyomingensis have been found in Hell Creek Formation (Montana), Lance Formation (Wyoming), and United States. (Gilmore 1931; Brown & Schlaikjer 1943)
What did Pachycephalosaurus wyomingensis eat?
Pachycephalosaurus wyomingensis was a herbivore. (Gilmore 1931; Brown & Schlaikjer 1943)
Who named Pachycephalosaurus wyomingensis, and when?
Pachycephalosaurus wyomingensis was described by Charles W. Gilmore in 1931. (Gilmore 1931; Paleobiology Database: Pachycephalosaurus occ… 2026)
Was Pachycephalosaurus wyomingensis in Jurassic Park?
Yes. Pachycephalosaurus wyomingensis appears in The Lost World: Jurassic Park (1997). Rams a hunters' vehicle with its domed skull in The Lost World. The film animal is about half the length of the real one. (Wikipedia: Dinosaurs in Jurassic Park 2026; Peterson et al. 2013)

On this page

Period
Cretaceous72.1–66 Ma
Diet
Herbivore
Length
4.5 m
Mass
0.4–0.5 t
Location
United States
Classification
Pachycephalosauria · Pachycephalosauridae
Described
1931, Charles W. Gilmore

Overview

Pachycephalosaurus wyomingensis lived in western North America at the end of the Cretaceous, in the Hell Creek and Lance formations. Charles Gilmore named the species in 1931 as Troodon wyomingensis, from a skull dome found in Niobrara County, Wyoming; Barnum Brown and Erich Schlaikjer created the genus Pachycephalosaurus in 1943.

It is known mostly from skulls. The skull roof formed a high dome of dense bone, ringed with bony knobs and spikes. Jack Horner and Mark Goodwin (2009) argued that the flat-headed, spiky "Dracorex" and "Stygimoloch" are younger growth stages of Pachycephalosaurus, whose dome inflated and spikes shrank as it grew.

How the dome was used is debated. A survey of 109 pachycephalosaurid skulls found healed lesions on 22% of the domes, clustered at the top, which fits head-to-head butting (Peterson et al. 2013). Flat-headed individuals had no such injuries.

Name meaning

"Thick-headed lizard", from Greek pachys ("thick"), kephalē ("head") and sauros ("lizard") (Brown & Schlaikjer 1943); wyomingensis for Wyoming (Gilmore 1931).

Biology & evidence

What scientists think about how this animal lived, and how strong the evidence for each idea is.

Growth

  • Dracorex and Stygimoloch are young Pachycephalosaurus.

    Evidence level: Moderate evidence

    Their skulls show immature features and fit a growth series in which the dome inflates and the spikes shrink. The idea is widely followed but not universally accepted.

    Evidence for / against · 1
    Evidence for

    • Comparative anatomy · Pachycephalosaurus cranial growth series

      CT and histology of "Dracorex" and "Stygimoloch" skulls show open sutures and juvenile tissue; their spikes and flat heads match early stages of Pachycephalosaurus skulls such as AMNH 1696.

      Specimen: AMNH 1696

Behavior

  • Pachycephalosaurus used its dome to butt heads.

    Evidence level: Moderate evidence

    Healed injuries on about a fifth of domes, concentrated at the top, fit head-to-head impacts as in some living hoofed mammals. The dome may also have served for display.

    Evidence for / against · 1
    Evidence for

    • Specimen · Pachycephalosaurid dome lesion survey

      Of 109 pachycephalosaurid domes, 24 (22%) from at least 9 species have lesions consistent with healed trauma and infection, clustered near the top of the dome; flat-headed specimens have none.

Other

  • The Pachycephalosaurus dome was made of dense bone.

    Evidence level: Direct evidence

    Thin sections of an adult dome show solid, dense bone tissue.

    Evidence for / against · 1
    Evidence for

    • Histology · Pachycephalosaurus dome histology

      Sections of an adult dome show dense metaplastic bone with fibrous, acellular tissue.

Three skulls at the Museum of the Rockies illustrating the proposed growth series of Pachycephalosaurus, including the 'Dracorex' skull interpreted as a juvenile.Image credit: Tim Evanson, CC BY-SA 2.0, via Wikimedia Commons (unmodified except resizing), Museum of the RockiesCC BY-SA 2.0 (unmodified)Source (opens in a new tab)

Size

Length
4.5 m

(14.8 ft)

Height
1.4 mpublished range 1.2–1.6 m

(4.6 ft)

Mass
370–450 kg
Pachycephalosaurus wyomingensis was about 4.5 m (14.8 ft) long, roughly 2.5 times the height of a 1.8 m adult human.

Timeline

Lived from about 72.1 to 66 million years ago (Ma), within the Mesozoic era (252–66 Ma).

Periods: Maastrichtian (72.1–66 Ma), Late Cretaceous (100.5–66 Ma)

Maastrichtian bounds of the last-appearance interval in PBDB. PBDB also lists a single Campanian occurrence from Alberta (first appearance 83.6 Ma), left out here as doubtful.

Location

Formations

Map data: Natural Earth (public domain). Modern positions, not where the continents were then.

These are the places where fossils have been found, not the full area where the animal lived.

Ecosystem

Hell Creek Formation scene with Dakotaraptor, Edmontosaurus, Pachycephalosaurus and Tyrannosaurus.Image credit: ABelov2014, CC BY 3.0, via Wikimedia CommonsCC BY 3.0Source (opens in a new tab)

Lived in: Hell Creek

68–66 million years ago · floodplain, river system, forest, coastal plain

Warm, humid coastal lowlands and river floodplains west of the retreating Western Interior Seaway at the very end of the Cretaceous, with forests of conifers and flowering plants. The ecosystem ends at the Cretaceous-Paleogene extinction.

Same ecosystem

Tyrannosaurus rex

Cretaceous· MaastrichtianCarnivore

Same ecosystem

Triceratops horridus

Cretaceous· MaastrichtianHerbivore

Same ecosystem

Ankylosaurus magniventris

Cretaceous· MaastrichtianHerbivore

Same ecosystem

Edmontosaurus annectens

Cretaceous· MaastrichtianHerbivore

Classification

  1. cladeDinosauria
  2. cladeOrnithischia
  3. cladePachycephalosauria
  4. familyPachycephalosauridae
  5. genusPachycephalosaurus
  6. speciesPachycephalosaurus wyomingensis

Taxonomic status: Valid

Fossil record

Pachycephalosaurus wyomingensis skull AMNH 1696, collected in 1940 near Ekalaka, Montana, at the American Museum of Natural History.Image credit: Skye McDavid, CC BY-SA 4.0, via Wikimedia Commons (unmodified except resizing), American Museum of Natural HistoryCC BY-SA 4.0 (unmodified)Source (opens in a new tab)
Specimens
Catalog numberStatusInstitutionFound
USNM 12031HolotypeNational Museum of Natural History, Smithsonian Institution (USNM)—
AMNH 1696Referred specimenAmerican Museum of Natural History (AMNH)—

What has been found

Holotype: USNM 12031

  • Preserved
  • Partial
  • Reconstructed
  • Unknown / not recorded

Holotype: USNM 12031

Region by region (table)
Skeleton regions of Holotype: USNM 12031
RegionState
SkullPartial · Frontoparietal dome.
MandibleUnknown / not recorded
TeethUnknown / not recorded
Cervical vertebraeUnknown / not recorded
Dorsal vertebraeUnknown / not recorded
SacrumUnknown / not recorded
Caudal vertebraeUnknown / not recorded
RibsUnknown / not recorded
GastraliaUnknown / not recorded
Scapula / shoulder girdleUnknown / not recorded
HumerusUnknown / not recorded
Forearm (radius / ulna)Unknown / not recorded
Hand (manus)Unknown / not recorded
PelvisUnknown / not recorded
FemurUnknown / not recorded
Tibia / fibulaUnknown / not recorded
Foot (pes)Unknown / not recorded

Referred specimen: AMNH 1696

Region by region (table)
Skeleton regions of Referred specimen: AMNH 1696
RegionState
SkullPreserved
MandibleUnknown / not recorded
TeethUnknown / not recorded
Cervical vertebraeUnknown / not recorded
Dorsal vertebraeUnknown / not recorded
SacrumUnknown / not recorded
Caudal vertebraeUnknown / not recorded
RibsUnknown / not recorded
GastraliaUnknown / not recorded
Scapula / shoulder girdleUnknown / not recorded
HumerusUnknown / not recorded
Forearm (radius / ulna)Unknown / not recorded
Hand (manus)Unknown / not recorded
PelvisUnknown / not recorded
FemurUnknown / not recorded
Tibia / fibulaUnknown / not recorded
Foot (pes)Unknown / not recorded

All specimens

Discovery

Year described
1931
Described by
Charles W. Gilmore
Type specimen
USNM 12031

Discovery timeline

  1. 1931 · Formally described

    Gilmore names Troodon wyomingensis

    Gilmore describes a thick skull dome from the Lance Formation of Wyoming as a new species of Troodon.

    People: Charles W. Gilmore

  2. 1943 · Renamed

    Brown and Schlaikjer name Pachycephalosaurus

    A study of dome-headed dinosaurs creates the genus Pachycephalosaurus and moves wyomingensis into it.

    People: Barnum Brown, Erich M. Schlaikjer

  3. 2009 · Major study

    Horner and Goodwin link Dracorex and Stygimoloch to Pachycephalosaurus

    Histology and CT scans are used to build a growth series.

    People: John R. Horner, Mark B. Goodwin

  4. 2013 · Major study

    Survey of dome injuries in pachycephalosaurs

    Peterson, Dischler and Longrich count lesions on 109 domes.

    People: Joseph E. Peterson

In Jurassic Park & Jurassic World

Films

  • The Lost World: Jurassic Park (1997)

On screen

Rams a hunters' vehicle with its domed skull in The Lost World.

Film vs fossils

  • The film animal is about half the length of the real one.
  • Whether the dome was used for ramming is debated; injuries on domes support some head-to-head combat.

Film titles are used for identification only. DinoDex is not affiliated with or endorsed by the Jurassic Park / Jurassic World franchise. All dinosaurs seen in the films

Sources

  1. (2026). Paleobiology Database: Pachycephalosaurus occurrences. The Paleobiology Database (paleobiodb.org), data service 1.2, accessed 2026-09-23. https://paleobiodb.org/data1.2/occs/list.txt?base_name=Pachycephalosaurus&show=coords,paleoloc,strat,loc,ref&pgm=gplates (opens in a new tab)
  2. (2026). Wikipedia: Pachycephalosaurus. Wikipedia (English), accessed 2026-09-23. https://en.wikipedia.org/wiki/Pachycephalosaurus (opens in a new tab)
  3. (2026). Wikipedia: Dinosaurs in Jurassic Park. Wikipedia. https://en.wikipedia.org/wiki/Dinosaurs_in_Jurassic_Park (opens in a new tab)
  4. 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.
  5. Cohen, K. M., Harper, D. A. T., Gibbard, P. L. & Car, N. (2023). International Chronostratigraphic Chart (v2023/09). International Commission on Stratigraphy. https://stratigraphy.org/ICSchart/ChronostratChart2023-09.pdf (opens in a new tab)
  6. Gilmore, C. W. (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:10.5479/si.00963801.79-2875.1 (opens in a new tab)
  7. Horner, J. R. & Goodwin, M. B. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE 4: e7626. doi:10.1371/journal.pone.0007626 (opens in a new tab)
  8. Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs (2nd edition). Princeton University Press.
  9. 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: e68620. doi:10.1371/journal.pone.0068620 (opens in a new tab)