# Pachycephalosauria

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Extinct clade of dinosaurs

Pachycephalosaurs Temporal range: Aptian–Maastrichtian PreꞒ Ꞓ O S D C P T J K Pg N Montage of four pachycephalosaurs. Clockwise from top left: Stegoceras, Prenocephale, Pachycephalosaurus and Homalocephale Scientific classification Kingdom: Animalia Phylum: Chordata Class: Reptilia Clade: Dinosauria Clade: †Ornithischia Clade: †Pyrodontia Clade: †Cerapoda Clade: †Marginocephalia Clade: †Pachycephalosauria Maryańska & Osmólska, 1974 Type species †Pachycephalosaurus wyomingensis Gilmore, 1931 (conserved name) Genera †Albalophosaurus?[1] †Drinker?[2] †Micropachycephalosaurus?[1] †Wannanosaurus †Zavacephale †Pachycephalosauridae Sternberg, 1945 †Colepiocephale †Gravitholus †Hanssuesia †Stegoceras †Texacephale †Pachycephalosaurinae Sereno, 1997 †Acrotholus †Amtocephale †Brontotholus †Foraminacephale †Goyocephale †Homalocephale †Platytholus †Prenocephale †Sinocephale †Sphaerotholus †Tylocephale †Pachycephalosaurini Sullivan, 2003 †Alaskacephale?[3] †Pachycephalosaurus

**Pachycephalosauria** ([/ˌpækɪsɛfələˈsɔːriə, -ˌkɛf-/](https://en.wikipedia.org/wiki/Help:IPA/English);[4] from [Greek](/source/Ancient_Greek) παχυκεφαλόσαυρος for 'thick headed lizards') is a [clade](/source/Clade) of [ornithischian](/source/Ornithischian) [dinosaurs](/source/Dinosaur). Along with [Ceratopsia](/source/Ceratopsia), it makes up the clade [Marginocephalia](/source/Marginocephalia). Pachycephalosaurs lived exclusively in the [Northern Hemisphere](/source/Northern_Hemisphere) during the [Cretaceous](/source/Cretaceous) period, with all members being found in [North America](/source/North_America) and [Asia](/source/Asia). They were all [bipedal](/source/Bipedal), [herbivorous](/source/Herbivorous)/[omnivorous](/source/Omnivorous) animals with thick skulls. Skulls can be domed, flat, or wedge-shaped depending on the species, and are all heavily ossified. The domes were often surrounded by nodes and/or spikes. Partial skeletons have been found of several pachycephalosaur species, but to date no complete skeletons have been discovered. Often isolated skull fragments are the only bones that are found.[5]

The oldest known definitive pachycephalosaur is *[Zavacephale](/source/Zavacephale)* from the [Early Cretaceous](/source/Early_Cretaceous) ([Aptian](/source/Aptian)-[Albian](/source/Albian)) of [Mongolia](/source/Mongolia).[6] Candidates for the earlier pachycephalosaurs include [*Ferganocephale adenticulatum*](/source/Ferganocephale) from [Middle Jurassic](/source/Middle_Jurassic) [Period](/source/Period_(geology)) strata of [Kyrgyzstan](/source/Kyrgyzstan) and [*Stenopelix valdensis*](/source/Stenopelix) from [Early Cretaceous](/source/Early_Cretaceous) strata of [Germany](/source/Germany), although R.M. Sullivan has doubted that either of these species are pachycephalosaurs.[7] *[Albalophosaurus](/source/Albalophosaurus)* from the [Early Cretaceous](/source/Early_Cretaceous) strata of [Japan](/source/Japan) might also represent a basal pachycephalosaur,[1] as well as the undiagnostic remains of *[Drinker](/source/Drinker_nisti)* from the [Late Jurassic](/source/Late_Jurassic) North America.[2] In 2017, a phylogenetic analysis conducted by Han and colleagues identified *Stenopelix* as a member of the [Ceratopsia](/source/Ceratopsia).[8]

## Description

Life restoration of *[Wannanosaurus](/source/Wannanosaurus)* (primitive member)

Life restoration of *[Pachycephalosaurus](/source/Pachycephalosaurus)* (advanced member)

Pachycephalosaurs were [bipedal](/source/Bipedal) [ornithischians](/source/Ornithischia) characterized by their thickened skulls. They had a bulky torso with an expanded gut cavity and broad hips, short forelimbs, long legs, a short, thick neck, and a heavy tail. Large orbits and a large [optic nerve](/source/Optic_nerve) point to pachycephalosaurs having good vision, and uncharacteristically large [olfactory lobes](/source/Olfactory_bulb) indicate that they had a good sense of smell relative to other dinosaurs.[5] They were fairly small dinosaurs, with most falling in the range of 2–3 meters (6.6–9.8 feet) in length and the largest, *[Pachycephalosaurus wyomingensis](/source/Pachycephalosaurus),* estimated to measure 4.5 meters (14.8 feet) long and weigh 450 kilograms (990 pounds).[9][10] The characteristic skull of pachycephalosaurs is a result of the fusion and [thickening](/source/Pachyostosis) of the [frontals](/source/Frontal_bone#Dinosaurs) and [parietals](/source/Parietal_bone#In_dinosaurs), accompanied by the closing of the [supratemporal fenestra](/source/Skull#Temporal_fenestrae). In some species this takes the form of a raised dome; in others, the skull is flat or wedge-shaped. While the flat-headed pachycephalosaurs are traditionally regarded as distinct species or even families, they may represent juveniles of dome-headed adults.[7] All display highly ornamented [jugals](/source/Jugal_bone), [squamosals](/source/Squamosal_bone), and [postorbitals](/source/Postorbital_bone) in the form of blunt horns and nodes. Many species are only known from skull fragments, and a complete pachycephalosaur skeleton is yet to be found.[5]

Members of Pachycephalosauria characteristically have an unusually domed head reminiscent of the earlier [Protopyknosia](/source/Protopyknosia) in an example of [convergent evolution](/source/Convergent_evolution).[11]

## Classification

Diagram showing *Dracorex* and *Stygimoloch* as growth stages of *Pachycephalosaurus*

Most pachycephalosaurid remains are not complete, usually consisting of portions of the frontoparietal bone that forms the distinctive dome. This can make [taxonomic](/source/Taxonomy_(biology)) identification a difficult task, as the classification of genera and species within *Pachycephalosauria* relies almost entirely on cranial characteristics. Consequently, improper species have historically been appointed to the clade. For instance, *[Majungatholus](/source/Majungatholus)*, once thought to be a pachycephalosaur, is now recognized as a specimen of the [abelisaurid](/source/Abelisauridae) [theropod](/source/Theropoda) *[Majungasaurus](/source/Majungasaurus)*,[12] and *[Yaverlandia](/source/Yaverlandia)*, another archosaur initially described as a pachycephalosaurid, has also since been reclassified as an archosaur of uncertain affinities.[13]

A 2009 paper proposed that *Dracorex* and *Stygimoloch* were just early growth stages of *Pachycephalosaurus*, rather than distinct genera.[14]

A 2020 reworking of [Cerapoda](/source/Cerapoda) by Dieudonné *et al.* recovered the animals traditionally considered '[heterodontosaurids](/source/Heterodontosauridae)' as a basal grouping within Pachycephalosauria, paraphyletic with respect to the traditional, dome-headed pachycephalosaurs.[15] The same conclusion had previously been reached by George Olshevsky in 1991, who classified heterodontosaurids as basal pachycephalosaurs on the basis of perceived [cranial kinesis](/source/Cranial_kinesis), the presence of fanglike premaxillary teeth, and the prominent [diastema](/source/Diastema) present in many genera.[16]

### Taxonomy

The Pachycephalosauria was first named as a [suborder](/source/Order_(biology)) of the order [Ornithischia](/source/Ornithischia) by [Maryańska & Osmólska (1974)](#CITEREFMaryańskaOsmólska1974). They included within it only one [family](/source/Family_(taxonomy)), the Pachycephalosauridae.[17] Later researchers, such as Michael Benton, have ranked it as an [infraorder](/source/Order_(biology)) of a suborder [Cerapoda](/source/Cerapoda), which unites the [ceratopsians](/source/Ceratopsia) and [ornithopods](/source/Ornithopod).[18] In 2006, Robert Sullivan published a re-evaluation of pachycephalosaur taxonomy. Sullivan considered attempts by Maryańska and Osmólska to restrict the definition of Pachycephalosauria redundant with their Pachycephalosauridae, since they were diagnosed by the same anatomical characters. Sullivan also rejected attempts by [Sereno (1986)](#CITEREFSereno1986), in his [phylogenetic](/source/Phylogenetic) studies,[19] to redefine Pachycephalosauridae to include only "dome-skulled" species (including *Stegoceras* and *Pachycephalosaurus*), while leaving more "basal" species outside that family in Pachycephalosauria. Therefore, Sullivan's use of Pachycephalosauridae is equivalent to Sereno and Benton's use of Pachycephalosauria.

Sullivan diagnosed the Pachycephalosauridae-based only on characters of the skull, with the defining character being a dome-shaped frontoparietal skull bone. According to Sullivan, the absence of this feature in some species assumed to be primitive led to the split in classification between domed and non-domed pachycephalosaurs; however, discovery of more advanced and possibly juvenile pachycephalosaurs with flat skulls (such as *Dracorex hogwartsia*) show this distinction to be incorrect. Sullivan also pointed out that the original diagnosis of Pachycephalosauridae centered around "flat to dome-like" skulls, so the flat-headed forms should be included in the family.[20] In a paper published in 2003, [Thomas E. Williamson](https://en.wikipedia.org/w/index.php?title=Thomas_E._Williamson&action=edit&redlink=1) and Thomas D. Carr discovered a clade of the Pachycephalosauridae that was a [sister taxa](/source/Sister_taxa) to the genus *[Stegoceras](/source/Stegoceras)*, made up of "all other dome-headed pachycephalosaurians; this was referred to as Pachycephalosaurinae[21]

### Phylogeny

Phylogenetic analyses by many authors have found Pachycephalosauria to be a group with *Stegoceras* as one of the earliest fully-domed members, with flat-headed and potentially juvenile taxa like *Homalocephale* and *Goyocephale* either just outside or just within the clade of it and more derived pachycephalosaurs. These studies began with the phylogenetic work of [Paul Sereno](/source/Paul_Sereno), which has been modified in many iterations to include newer taxa and additional characters.[22][23][24] The version of the analysis published by Woodruff and colleagues in 2023 is below.[25]

Pachycephalosauria Wannanosaurus Pachycephalosauridae Hanssuesia Colepiocephale Stegoceras validum Stegoceras novomexicanum Pachycephalosaurinae Goyocephale Homalocephale Tylocephale Foraminacephale Amtocephale Acrotholus Prenocephale Alaskacephale Pachycephalosaurini Stygimoloch Pachycephalosaurus Sphaerotholus S. goodwini S. lyonsi S. triregnum S. buchholtzae S. edmontonensis

Below is a cladogram published by Dieudonné and colleagues (2020)[26] which controversially found [heterodontosaurids](/source/Heterodontosaurid) to be [paraphyletic](/source/Paraphyletic) with respect to pachycephalosauria. This analysis was proposed as a hypothesis for the complete lack of [Jurassic](/source/Jurassic) and Early Cretaceous pachycephalosaur fossils, even though they should have existed if the modern understanding of ornithischian phylogeny is correct. However, this hypothesis has not been widely accepted by other paleontologists.[1]

Pachycephalosauria Fruitadens Lycorhinus Heterodontosaurus Abrictosaurus "Heterodontosaurids" (paraphyletic) Tianyulong Echinodon Wannanosaurus Goyocephale Prenocephale Homalocephale Stegoceras Pachycephalosaurus

## Paleobiology

### Feeding

The small size of most pachycephalosaur species and lack of skeletal adaptation indicates that they were not climbers and primarily ate food close to the ground. Mallon et al. (2013) examined herbivore coexistence on the island continent of [Laramidia](/source/Laramidia), during the Late Cretaceous and concluded that pachycephalosaurids were generally restricted to feeding on vegetation at, or below, the height of 1 meter.[27] They exhibit [heterodonty](/source/Heterodont), having different tooth morphology between the [premaxillary](/source/Premaxilla) teeth and [maxillary](/source/Maxilla) teeth. Front teeth are small and peg-like with an ovular cross section and were most likely used for grabbing food. In some species, the last premaxillary tooth was enlarged and canine-like. Back teeth are small and triangular with [denticles](/source/Denticle_(tooth_feature)) on the front and back of the crown, used for mouth processing. In species in which the [dentary](/source/Mandible) has been found, mandibular teeth are similar in size and shape to those in the upper jaw. Wear patterns on the teeth vary by species, indicating a range of food preferences which could include seeds, stems, leaves, fruits, and possibly insects. A very wide rib cage and large gut cavity extending all the way to the base of the tail suggests the use of fermentation to digest food.[5]

### Head-butting behavior

Hypothetical examples of pachycephalosaur combat behavior, varying per species: *[Pachycephalosaurus](/source/Pachycephalosaurus)* (A), *[Prenocephale](/source/Prenocephale)* (B), *[Stygimoloch](/source/Stygimoloch)* (C)

The adaptive significance of the skull dome has been heavily debated. The popular hypothesis among the general public that the skull was used in head-butting, as sort of a dinosaurian battering ram, was first proposed by [Colbert (1955)](#CITEREFColbert1955), p. [*[page needed](https://en.wikipedia.org/wiki/Wikipedia:Citing_sources)*]. This view was popularized in the 1956 science fiction story "[A Gun for Dinosaur](/source/A_Gun_for_Dinosaur)" by [L. Sprague de Camp](/source/L._Sprague_de_Camp). Many paleontologists have since argued for the head-butting hypothesis, including [Galton (1970)](#CITEREFGalton1970) and [Sues (1978)](#CITEREFSues1978). In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day [bighorn sheep](/source/Bighorn_sheep) and [musk oxen](/source/Musk_oxen).

Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature.[28] It has been suggested that a pachycephalosaur could make its head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur could the head, neck, and body be oriented in such a position. Instead, the cervical and anterior dorsal vertebrae of pachycephalosaurs show that the neck was carried in an "S"- or U-shaped curve.[29]

Also, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Other possibilities include flank-butting, defense against predators, or both. The relatively wide build of pachycephalosaurs (which would protect vital internal organs from harm during flank-butting) and the squamosal horns of the *[Stygimoloch](/source/Stygimoloch)* (which would have been used to great effect during flank-butting) add credence to the flank-butting hypothesis.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

Restoration of head-butting *Pachycephalosaurus*

A histological study conducted by [Goodwin & Horner (2004)](#CITEREFGoodwinHorner2004) argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage", the spongy bone structure could not sustain the blows of combat, and the radial pattern was simply an effect of rapid growth.[30] Later biomechanical analyses by [Snively & Cox (2008)](#CITEREFSnivelyCox2008) and [Snively & Theodor (2011)](#CITEREFSnivelyTheodor2011) concluded, however, that the domes could withstand combat stresses.[28][Lehman (2010)](#CITEREFLehman2010) argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.[31]

[Goodwin & Horner (2004)](#CITEREFGoodwinHorner2004) instead argued that the dome functioned for species recognition. There is evidence that the dome had some form of external covering, and it is reasonable to consider the dome may have been brightly covered, or subject to change color seasonally.[30] Due to the nature of the fossil record, however, it cannot be observed whether or not color played a role in dome function.

[Longrich, Sankey & Tanke (2010)](#CITEREFLongrichSankeyTanke2010) argued that species recognition is an unlikely evolutionary cause for the dome, because dome forms are not notably different between species. Because of this general similarity, several genera of Pachycephalosauridae have sometimes been incorrectly lumped together. This is unlike the case in ceratopsians and [hadrosaurids](/source/Hadrosaur), which had much more distinct cranial ornamentation. Longrich *et al.* argued that instead the dome had a mechanical function, such as combat, one which was important enough to justify the resource investment.[23]

#### Dome paleopathology

Skull domes of *[Gravitholus](/source/Gravitholus)*, *[Sphaerotholus](/source/Sphaerotholus)*, *[Stegoceras](/source/Stegoceras)*, and *[Pachycephalosaurus](/source/Pachycephalosaurus)* showing injuries

[Peterson, Dischler & Longrich (2013)](#CITEREFPetersonDischlerLongrich2013) studied cranial pathologies among the Pachycephalosauridae and found that 22% of all domes examined had lesions that are consistent with [osteomyelitis](/source/Osteomyelitis), an infection of the bone resulting from penetrating trauma, or trauma to the tissue overlying the skull leading to an infection of the bone tissue. This high rate of pathology lends more support to the hypothesis that pachycephalosaurid domes were employed in intra-specific combat.[32] The frequency of trauma was comparable across the different genera in this [family](/source/Family_(taxonomy)), despite the fact that these genera vary with respect to the size and architecture of their domes, and fact that they existed during varying geologic periods.[32] These findings were in stark contrast with the results from analysis of the relatively flat-headed pachycephalosaurids, where there was an absence of pathology. This would support the hypothesis that these individuals represent either females or juveniles,[23] where intra-specific combat behavior is not expected.

*Pachycephalosaurus* life restoration with cranial injury

Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar bone[33] which contains [fibroblasts](/source/Fibroblasts) that play a critical role in wound healing, and are capable of rapidly depositing bone during remodeling.[14] Peterson *et al.* (2013) concluded that, taken together, the frequency of lesion distribution and the bone structure of frontoparietal domes lend strong support to the hypothesis that pachycephalosaurids used their unique cranial structures for [agonistic behavior](/source/Agonistic_behavior).[32]

## Paleoecology

Restoration of *[Homalocephale](/source/Homalocephale)* in its paleoenvironment

The Asian and North American species of pachycephalosaurs lived in markedly different environments. Asian specimens are normally more intact, indicating they were not transported far from their place of death before fossilization. They likely lived in a large desert region in central Asia with a hot and [arid climate](/source/Desert_climate). North American specimens are typically found in rocks that were formed by erosion from the [Rocky Mountains](/source/Rocky_Mountains). Specimens are far less intact; usually only skull caps are recovered, and those found regularly exhibit surface exfoliation and other signs that they were transported long distances by water before fossilization. It is assumed that they lived in the mountains in a [temperate climate](/source/Temperate_climate) and were carried by erosion after death to their final resting place.[34]

### Distribution

Pachycephalosaurs lived exclusively in [Laurasia](/source/Laurasia), being found in western [North America](/source/North_America) and central Asia. Pachycephalosaurs originated in Asia and had two major dispersal events, resulting in the two separate waves of pachycephalosaur evolution observed in Asia. The first, occurring before the late [Santonian](/source/Santonian) or early [Campanian](/source/Campanian), involved a migration from Asia to North America, most likely by way of the [Bering Land Bridge](/source/Beringia). This migration was by a common ancestor of *[Stygimoloch](/source/Stygimoloch)*, *[Stegoceras](/source/Stegoceras)*, *[Tylocephale](/source/Tylocephale)*, *[Prenocephale](/source/Prenocephale),* and *[Pachycephalosaurus](/source/Pachycephalosaurus)*. The second event occurred before the middle [Campanian](/source/Campanian), and involved a migration back into Asia from North America by a common ancestor of *[Prenocephale](/source/Prenocephale)* and *[Tylocephale](/source/Tylocephale)*. Two species originally reported to be pachycephalosaurs discovered outside this range, *[Yaverlandia bitholus](/source/Yaverlandia)* of [England](/source/England) and *[Majungatholus atopus](/source/Majungasaurus)* of [Madagascar](/source/Madagascar), have recently been shown to actually be [theropods](/source/Theropoda).[5][35]

## See also

- [Dinosaurs portal](https://en.wikipedia.org/wiki/Portal:Dinosaurs)

- [Timeline of pachycephalosaur research](/source/Timeline_of_pachycephalosaur_research)

- [Triopticus](/source/Triopticus) Convergent evolution

## Notes

1. ^ [***a***](#cite_ref-Fonseca2024_1-0) [***b***](#cite_ref-Fonseca2024_1-1) [***c***](#cite_ref-Fonseca2024_1-2) [***d***](#cite_ref-Fonseca2024_1-3) Fonseca, A. O.; Reid, I. J.; Venner, A.; Duncan, R. J.; Garcia, M. S.; Müller, R. T. (2024). "A comprehensive phylogenetic analysis on early ornithischian evolution". *Journal of Systematic Palaeontology*. **22** (1). 2346577. [Bibcode](/source/Bibcode_(identifier)):[2024JSPal..2246577F](https://ui.adsabs.harvard.edu/abs/2024JSPal..2246577F). [doi](/source/Doi_(identifier)):[10.1080/14772019.2024.2346577](https://doi.org/10.1080%2F14772019.2024.2346577).

1. ^ [***a***](#cite_ref-Morrison_2-0) [***b***](#cite_ref-Morrison_2-1) Barrett, Paul M.; Maidment, Susannah C.R. (2025-04-25). ["A Review of Nanosaurus agilis Marsh and Other Small-Bodied Morrison Formation "Ornithopods""](https://bioone.org/journals/bulletin-of-the-peabody-museum-of-natural-history/volume-66/issue-1/014.066.0102/A-Review-of-Nanosaurus-agilis-Marsh-and-Other-Small-Bodied/10.3374/014.066.0102.full). *Bulletin of the Peabody Museum of Natural History*. **66** (1). [doi](/source/Doi_(identifier)):[10.3374/014.066.0102](https://doi.org/10.3374%2F014.066.0102). [ISSN](/source/ISSN_(identifier)) [0079-032X](https://search.worldcat.org/issn/0079-032X).

1. **[^](#cite_ref-3)** Dyer, Aaron D.; Powers, Mark J.; [Currie, Philip J.](/source/Philip_J._Currie) (2023). ["Problematic putative pachycephalosaurids: Synchrotron µCT imaging shines new light on the anatomy and taxonomic validity of *Gravitholus albertae* from the Belly River Group (Campanian) of Alberta, Canada"](https://journals.library.ualberta.ca/vamp/index.php/VAMP/article/view/29388). *Vertebrate Anatomy Morphology Palaeontology*. **10** (1): 65–110. [doi](/source/Doi_(identifier)):[10.18435/vamp29388](https://doi.org/10.18435%2Fvamp29388). [ISSN](/source/ISSN_(identifier)) [2292-1389](https://search.worldcat.org/issn/2292-1389).

1. **[^](#cite_ref-4)** ["Pachycephalosauria"](http://www.lexico.com/definition/Pachycephalosauria). *[Oxford Dictionaries](/source/Lexico) UK English Dictionary*. [Oxford University Press](/source/Oxford_University_Press).[*[dead link](https://en.wikipedia.org/wiki/Wikipedia:Link_rot)*]

1. ^ [***a***](#cite_ref-UC_5-0) [***b***](#cite_ref-UC_5-1) [***c***](#cite_ref-UC_5-2) [***d***](#cite_ref-UC_5-3) [***e***](#cite_ref-UC_5-4) Maryanska, T.; Chapman, R.E.; Weishampel, D.B. (2004). "Pachycephalosauria". *The Dinosauria* (2nd ed.). Berkeley: University of California Press. pp. 464–477. [ISBN](/source/ISBN_(identifier)) [978-0-520-24209-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-520-24209-8). [OCLC](/source/OCLC_(identifier)) [493366196](https://search.worldcat.org/oclc/493366196).

1. **[^](#cite_ref-6)** Chinzorig, Tsogtbaatar; Takasaki, Ryuji; Yoshida, Junki; Tucker, Ryan T.; Buyantegsh, Batsaikhan; Mainbayar, Buuvei; Tsogtbaatar, Khishigjav; Zanno, Lindsay E. (September 17, 2025). "A domed pachycephalosaur from the early Cretaceous of Mongolia". *[Nature](/source/Nature_(journal))*: 1–8. [doi](/source/Doi_(identifier)):[10.1038/s41586-025-09213-6](https://doi.org/10.1038%2Fs41586-025-09213-6). [ISSN](/source/ISSN_(identifier)) [1476-4687](https://search.worldcat.org/issn/1476-4687).

1. ^ [***a***](#cite_ref-S6_7-0) [***b***](#cite_ref-S6_7-1) Sullivan, Robert M. (2006). ["A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia)"](https://web.archive.org/web/20070927214216/http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF). *New Mexico Museum of Natural History and Science Bulletin*. **35** (47): 347–365. Archived from [the original](http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF) on 2007-09-27. Retrieved 2006-07-09.

1. **[^](#cite_ref-8)** Han, F.-L.; Forster, C.A.; Clark, J.M.; Xu, X. (2016). "Cranial anatomy of *Yinlong downsi* (Ornithischia: Ceratopsia) from the Upper Jurassic Shishugou Formation of Xinjiang, China". *Journal of Vertebrate Paleontology*. **36** (1) e1029579. [Bibcode](/source/Bibcode_(identifier)):[2016JVPal..36E9579H](https://ui.adsabs.harvard.edu/abs/2016JVPal..36E9579H). [doi](/source/Doi_(identifier)):[10.1080/02724634.2015.1029579](https://doi.org/10.1080%2F02724634.2015.1029579). [S2CID](/source/S2CID_(identifier)) [130791053](https://api.semanticscholar.org/CorpusID:130791053).

1. **[^](#cite_ref-GL_9-0)** Holtz, Thomas R. Jr. (2007) [Last updated Jan 28, 2011]. ["Genus List for Holtz (2007) Dinosaurs"](http://www.geol.umd.edu/~tholtz/dinoappendix/HoltzappendixWinter2010.pdf) (PDF).

1. **[^](#cite_ref-10)** [Paul, Gregory S.](/source/Gregory_S._Paul) (2010). [*The Princeton field guide to dinosaurs*](https://archive.org/details/princetonfieldgu0000paul). Princeton University Press. pp. 241–244. [ISBN](/source/ISBN_(identifier)) [978-0-691-13720-9](https://en.wikipedia.org/wiki/Special:BookSources/978-0-691-13720-9). [OCLC](/source/OCLC_(identifier)) [930852339](https://search.worldcat.org/oclc/930852339).

1. **[^](#cite_ref-11)** Stocker, M.R.; Nesbitt, S.J.; Criswell, K.E.; Parker, W.G.; Witmer, L.M.; Rowe, T.B.; Ridgely, R.; Brown, M.A. (2016). ["A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives"](https://doi.org/10.1016%2Fj.cub.2016.07.066). *Current Biology*. **26** (19): 2674–2680. [Bibcode](/source/Bibcode_(identifier)):[2016CBio...26.2674S](https://ui.adsabs.harvard.edu/abs/2016CBio...26.2674S). [doi](/source/Doi_(identifier)):[10.1016/j.cub.2016.07.066](https://doi.org/10.1016%2Fj.cub.2016.07.066). [PMID](/source/PMID_(identifier)) [27666971](https://pubmed.ncbi.nlm.nih.gov/27666971).

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1. **[^](#cite_ref-13)** Naish, Darren; Sweetman, Steven C. (2026-03-16). "A second specimen of the enigmatic Wealden reptile *Yaverlandia*". *[Proceedings of the Geologists' Association](/source/Proceedings_of_the_Geologists'_Association)* 101179. [doi](/source/Doi_(identifier)):[10.1016/j.pgeola.2026.101179](https://doi.org/10.1016%2Fj.pgeola.2026.101179).

1. ^ [***a***](#cite_ref-HornerGoodwin2009_14-0) [***b***](#cite_ref-HornerGoodwin2009_14-1) [Horner, J.R.](/source/Jack_Horner_(paleontologist)); Goodwin, M.B. (2009). ["Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur *Pachycephalosaurus*"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762616). *PLOS ONE*. **4** (10) e7626. [Bibcode](/source/Bibcode_(identifier)):[2009PLoSO...4.7626H](https://ui.adsabs.harvard.edu/abs/2009PLoSO...4.7626H). [doi](/source/Doi_(identifier)):[10.1371/journal.pone.0007626](https://doi.org/10.1371%2Fjournal.pone.0007626). [PMC](/source/PMC_(identifier)) [2762616](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762616). [PMID](/source/PMID_(identifier)) [19859556](https://pubmed.ncbi.nlm.nih.gov/19859556).

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1. **[^](#cite_ref-16)** Olshevsky, George (1991). ["A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia"](http://www.miketaylor.org.uk/tmp/Olshevsky_1991_A_revision_of_the_parainfraclass_Archosauria_Cope_1869_excluding_the_advanced_Crocodylia.pdf) (PDF). *Earth Sciences Library*: 104.

1. **[^](#cite_ref-17)** [Maryańska T](/source/Teresa_Marya%C5%84ska), [Osmólska H](/source/Halszka_Osm%C3%B3lska) (1974). "Pachycephalosauria, a new suborder of ornithischian dinosaurs". *Palaeontologica Polonica*. **30**: 45–102.

1. **[^](#cite_ref-18)** [Benton, Michael J.](/source/Michael_Benton) (2004). *Vertebrate Palaeontology* (3rd ed.). Blackwell Publishing. p. 472. [ISBN](/source/ISBN_(identifier)) [978-0-632-05637-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-632-05637-8).

1. **[^](#cite_ref-19)** [Sereno, P.C.](/source/Paul_Sereno) (1986). "Phylogeny of the bird-hipped dinosaurs (Order Ornithischia)". *National Geographic Research*. **2**: 234–256.

1. **[^](#cite_ref-20)** Sullivan RM (2006). ["A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia)"](https://web.archive.org/web/20070927214216/http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF). *New Mexico Museum of Natural History and Science Bulletin*. **35**: 347–365. Archived from [the original](http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF) on 2007-09-27.

1. **[^](#cite_ref-21)** Williamson, Thomas E.; Carr, Thomas D. (2003-01-14). ["A new genus of derived pachycephalosaurian from western North America"](https://www.tandfonline.com/doi/abs/10.1671/0272-4634%282002%29022%5B0779%3AANGODP%5D2.0.CO%3B2). *Journal of Vertebrate Paleontology*. **22** (4): 779–801. [doi](/source/Doi_(identifier)):[10.1671/0272-4634(2002)022\[0779:ANGODP\]2.0.CO;2](https://doi.org/10.1671%2F0272-4634%282002%29022%5B0779%3AANGODP%5D2.0.CO%3B2). [ISSN](/source/ISSN_(identifier)) [0272-4634](https://search.worldcat.org/issn/0272-4634). [S2CID](/source/S2CID_(identifier)) [86112901](https://api.semanticscholar.org/CorpusID:86112901).

1. **[^](#cite_ref-22)** Williamson TE, Carr TD (2002). "A new genus of derived pachycephalosaurian from western North America". *Journal of Vertebrate Paleontology*. **22** (4): 779–801. [doi](/source/Doi_(identifier)):[10.1671/0272-4634(2002)022\[0779:ANGODP\]2.0.CO;2](https://doi.org/10.1671%2F0272-4634%282002%29022%5B0779%3AANGODP%5D2.0.CO%3B2). [ISSN](/source/ISSN_(identifier)) [0272-4634](https://search.worldcat.org/issn/0272-4634). [S2CID](/source/S2CID_(identifier)) [86112901](https://api.semanticscholar.org/CorpusID:86112901).

1. ^ [***a***](#cite_ref-longrichetal2010_23-0) [***b***](#cite_ref-longrichetal2010_23-1) [***c***](#cite_ref-longrichetal2010_23-2) Longrich, N.R.; Sankey, J.; [Tanke, D.](/source/Darren_Tanke) (2010). "Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA". *Cretaceous Research*. **31** (2): 274–284. [Bibcode](/source/Bibcode_(identifier)):[2010CrRes..31..274L](https://ui.adsabs.harvard.edu/abs/2010CrRes..31..274L). [doi](/source/Doi_(identifier)):[10.1016/j.cretres.2009.12.002](https://doi.org/10.1016%2Fj.cretres.2009.12.002).

1. **[^](#cite_ref-24)** Evans DC, Schott RK, Larson DW, Brown CM, Ryan MJ (2013). ["The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs"](https://doi.org/10.1038%2Fncomms2749). *Nature Communications*. **4**. 1828. [Bibcode](/source/Bibcode_(identifier)):[2013NatCo...4.1828E](https://ui.adsabs.harvard.edu/abs/2013NatCo...4.1828E). [doi](/source/Doi_(identifier)):[10.1038/ncomms2749](https://doi.org/10.1038%2Fncomms2749). [PMID](/source/PMID_(identifier)) [23652016](https://pubmed.ncbi.nlm.nih.gov/23652016).

1. **[^](#cite_ref-25)** Woodruff, D. Cary; Schott, Ryan K.; Evans, David C. (2023-11-15). "Two new species of small-bodied pachycephalosaurine (Dinosauria, Marginocephalia) from the uppermost Cretaceous of North America suggest hidden diversity in well-sampled formations". *Papers in Palaeontology*. **9** (6). e1535. [Bibcode](/source/Bibcode_(identifier)):[2023PPal....9E1535W](https://ui.adsabs.harvard.edu/abs/2023PPal....9E1535W). [doi](/source/Doi_(identifier)):[10.1002/spp2.1535](https://doi.org/10.1002%2Fspp2.1535). [ISSN](/source/ISSN_(identifier)) [2056-2799](https://search.worldcat.org/issn/2056-2799).

1. **[^](#cite_ref-26)** Dieudonné, P. -E.; Cruzado-Caballero, P.; Godefroit, P.; Tortosa, T. (2021). ["A new phylogeny of cerapodan dinosaurs"](http://rid.unrn.edu.ar/handle/20.500.12049/5441). *Historical Biology*. **33** (10): 2335–2355. [Bibcode](/source/Bibcode_(identifier)):[2021HBio...33.2335D](https://ui.adsabs.harvard.edu/abs/2021HBio...33.2335D). [doi](/source/Doi_(identifier)):[10.1080/08912963.2020.1793979](https://doi.org/10.1080%2F08912963.2020.1793979). [hdl](/source/Hdl_(identifier)):[11336/147083](https://hdl.handle.net/11336%2F147083). [S2CID](/source/S2CID_(identifier)) [221854017](https://api.semanticscholar.org/CorpusID:221854017).

1. **[^](#cite_ref-27)** Mallon, Jordan C.; Evans, David C.; Ryan, Michael J.; Anderson, Jason S. (2013). ["Feeding height stratification among the herbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3637170). *BMC Ecology*. **13** (1): 14. [Bibcode](/source/Bibcode_(identifier)):[2013BMCE...13...14M](https://ui.adsabs.harvard.edu/abs/2013BMCE...13...14M). [doi](/source/Doi_(identifier)):[10.1186/1472-6785-13-14](https://doi.org/10.1186%2F1472-6785-13-14). [PMC](/source/PMC_(identifier)) [3637170](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3637170). [PMID](/source/PMID_(identifier)) [23557203](https://pubmed.ncbi.nlm.nih.gov/23557203).

1. ^ [***a***](#cite_ref-snively_28-0) [***b***](#cite_ref-snively_28-1) Snively E, Cox A (March 2008). ["Structural mechanics of pachycephalosaur crania permitted head-butting behavior"](http://palaeo-electronica.org/2008_1/140/index.html). *Palaeontologia Electronica*. **11** (1). [ISSN](/source/ISSN_(identifier)) [1094-8074](https://search.worldcat.org/issn/1094-8074).

1. **[^](#cite_ref-29)** [Carpenter, K.](/source/Kenneth_Carpenter) (1997). ["Agonistic behavior in pachycephalosaurs (Ornithischia:Dinosauria): a new look at head-butting behavior"](https://web.archive.org/web/20160303221633/http://www.le-monde-des-dinosaures.net/pachycephalosaures.pdf) (PDF). *Contributions to Geology*. **32** (1): 19–25. Archived from [the original](http://www.le-monde-des-dinosaures.net/pachycephalosaures.pdf) (PDF) on 2016-03-03. Retrieved 2011-07-16.

1. ^ [***a***](#cite_ref-goodwin_30-0) [***b***](#cite_ref-goodwin_30-1) Goodwin MB, Horner JR (June 2004). ["Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior"](http://doc.rero.ch/record/14876/files/PAL_E2014.pdf) (PDF). *Paleobiology*. **30** (2): 253–267. [Bibcode](/source/Bibcode_(identifier)):[2004Pbio...30..253G](https://ui.adsabs.harvard.edu/abs/2004Pbio...30..253G). [doi](/source/Doi_(identifier)):[10.1666/0094-8373(2004)030<0253:CHOPOM>2.0.CO;2](https://doi.org/10.1666%2F0094-8373%282004%29030%3C0253%3ACHOPOM%3E2.0.CO%3B2). [S2CID](/source/S2CID_(identifier)) [84961066](https://api.semanticscholar.org/CorpusID:84961066).

1. **[^](#cite_ref-31)** Lehman TM (2010). "Pachycephalosauridae from the San Carlos and Aguja Formations (Upper Cretaceous) of west Texas, and observations of the frontoparietal dome". *Journal of Vertebrate Paleontology*. **30** (3): 786–798. [Bibcode](/source/Bibcode_(identifier)):[2010JVPal..30..786L](https://ui.adsabs.harvard.edu/abs/2010JVPal..30..786L). [doi](/source/Doi_(identifier)):[10.1080/02724631003763532](https://doi.org/10.1080%2F02724631003763532). [ISSN](/source/ISSN_(identifier)) [0272-4634](https://search.worldcat.org/issn/0272-4634). [S2CID](/source/S2CID_(identifier)) [129592428](https://api.semanticscholar.org/CorpusID:129592428).

1. ^ [***a***](#cite_ref-Peterson13_32-0) [***b***](#cite_ref-Peterson13_32-1) [***c***](#cite_ref-Peterson13_32-2) Peterson JE, Dischler C, Longrich NR (2013). ["Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae)"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712952). *PLOS ONE*. **8** (7) e68620. [Bibcode](/source/Bibcode_(identifier)):[2013PLoSO...868620P](https://ui.adsabs.harvard.edu/abs/2013PLoSO...868620P). [doi](/source/Doi_(identifier)):[10.1371/journal.pone.0068620](https://doi.org/10.1371%2Fjournal.pone.0068620). [PMC](/source/PMC_(identifier)) [3712952](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712952). [PMID](/source/PMID_(identifier)) [23874691](https://pubmed.ncbi.nlm.nih.gov/23874691).

1. **[^](#cite_ref-33)** Reid, R.E.H. (1997). "Histology of bones and teeth". In Currie, P.J.; Padian, K. (eds.). *Encyclopedia of Dinosaurs*. San Diego, CA: Academic Press. pp. 329–339.

1. **[^](#cite_ref-34)** Fastovsky, David E.; [Weishampel, David B.](/source/David_B._Weishampel) (2009). *Dinosaurs: a concise natural history*. Cambridge University Press. pp. 110–117. [ISBN](/source/ISBN_(identifier)) [978-0-521-71902-5](https://en.wikipedia.org/wiki/Special:BookSources/978-0-521-71902-5). [OCLC](/source/OCLC_(identifier)) [476234422](https://search.worldcat.org/oclc/476234422).

1. **[^](#cite_ref-35)** [Naish, D.](/source/Darren_Naish); Martill, D.M. (2008). "Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: Ornithischia". *Journal of the Geological Society*. **165** (3): 613–623. [Bibcode](/source/Bibcode_(identifier)):[2008JGSoc.165..613N](https://ui.adsabs.harvard.edu/abs/2008JGSoc.165..613N). [doi](/source/Doi_(identifier)):[10.1144/0016-76492007-154](https://doi.org/10.1144%2F0016-76492007-154). [S2CID](/source/S2CID_(identifier)) [129624992](https://api.semanticscholar.org/CorpusID:129624992).

## References

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- [Carpenter, Kenneth](/source/Kenneth_Carpenter) (1997). ["Agonistic behavior in pachycephalosaurs (Ornithischia:Dinosauria): a new look at head-butting behavior"](https://web.archive.org/web/20160303221633/http://www.le-monde-des-dinosaures.net/pachycephalosaures.pdf) (PDF). *Contributions to Geology*. **32** (1): 19–25. Archived from [the original](http://www.le-monde-des-dinosaures.net/pachycephalosaures.pdf) (PDF) on 2016-03-03. Retrieved 2011-07-16.

- [Colbert, Edwin](/source/Edwin_Harris_Colbert) (1955). *Evolution of the Vertebrates*. New York: John Wiley.

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- Lehman, TM (2010). "Pachycephalosauridae from the San Carlos and Aguja Formations (Upper Cretaceous) of west Texas, and observations of the frontoparietal dome". *Journal of Vertebrate Paleontology*. **30** (3): 786–798. [Bibcode](/source/Bibcode_(identifier)):[2010JVPal..30..786L](https://ui.adsabs.harvard.edu/abs/2010JVPal..30..786L). [doi](/source/Doi_(identifier)):[10.1080/02724631003763532](https://doi.org/10.1080%2F02724631003763532). [ISSN](/source/ISSN_(identifier)) [0272-4634](https://search.worldcat.org/issn/0272-4634). [S2CID](/source/S2CID_(identifier)) [129592428](https://api.semanticscholar.org/CorpusID:129592428).

- Longrich, NR; Sankey, J; Tanke, D (2010). "*Texacephale langstoni*, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA". *Cretaceous Research*. **31** (2): 274–284. [Bibcode](/source/Bibcode_(identifier)):[2010CrRes..31..274L](https://ui.adsabs.harvard.edu/abs/2010CrRes..31..274L). [doi](/source/Doi_(identifier)):[10.1016/j.cretres.2009.12.002](https://doi.org/10.1016%2Fj.cretres.2009.12.002).

- [Maryańska, T](/source/Teresa_Marya%C5%84ska); [Osmólska, H](/source/Halszka_Osm%C3%B3lska) (1974). "Pachycephalosauria, a new suborder of ornithischian dinosaurs". *Palaeontologica Polonica* (30): 45–102.

- [Sereno, PC](/source/Paul_Sereno) (1986). "Phylogeny of the bird-hipped dinosaurs (Order Ornithischia)". *National Geographic Research*. **2**: 234–256.

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- Snively, E; Theodor, JM (2011). Carpenter, Kenneth (ed.). ["Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3125168). *PLOS ONE*. **6** (6) e21422. [Bibcode](/source/Bibcode_(identifier)):[2011PLoSO...621422S](https://ui.adsabs.harvard.edu/abs/2011PLoSO...621422S). [doi](/source/Doi_(identifier)):[10.1371/journal.pone.0021422](https://doi.org/10.1371%2Fjournal.pone.0021422). [PMC](/source/PMC_(identifier)) [3125168](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3125168). [PMID](/source/PMID_(identifier)) [21738658](https://pubmed.ncbi.nlm.nih.gov/21738658).

- Stokstad, Erik (November 2007). ["Society of Vertebrate Paleontology Meeting: Did Horny Young Dinosaurs Cause Illusion of Separate Species?"](http://www.sciencemag.org/cgi/content/full/318/5854/1236). *Science*. **318** (5854): 1236. [doi](/source/Doi_(identifier)):[10.1126/science.318.5854.1236](https://doi.org/10.1126%2Fscience.318.5854.1236). [PMID](/source/PMID_(identifier)) [18033861](https://pubmed.ncbi.nlm.nih.gov/18033861). [S2CID](/source/S2CID_(identifier)) [36443204](https://api.semanticscholar.org/CorpusID:36443204).

- Sues, H-D (1978). "Functional morphology of the dome in pachycephalosaurid dinosaurs". *Neues Jahrbuch für Geologie und Paläontologie*: 459–472.

- Sullivan, RM (2006). ["A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia)"](https://web.archive.org/web/20070927214216/http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF). *New Mexico Museum of Natural History and Science Bulletin* (35): 347–365. Archived from [the original](http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf) (PDF) on 2007-09-27.

- Watabe, Mahito; Tsogtbaatar, Khishigjaw; Sullivan, Robert M. (2011). ["A new pachycephalosaurid from the Baynshire Formation (Cenomanian-late Santonian), Gobi Desert, Mongolia"](https://web.archive.org/web/20110929031439/http://www.robertmsullivanphd.com/uploads/174.Watabe_et_al__Mongolian_pachy_.pdf) (PDF). *Fossil Record 3. New Mexico Museum of Natural History and Science, Bulletin*. **53**: 489–497. Archived from [the original](http://www.robertmsullivanphd.com/uploads/174.Watabe_et_al__Mongolian_pachy_.pdf) (PDF) on 2011-09-29.

- Williamson, TE; Carr, TD (2002). "A new genus of derived pachycephalosaurian from western North America". *Journal of Vertebrate Paleontology*. **22** (4): 779–801. [doi](/source/Doi_(identifier)):[10.1671/0272-4634(2002)022\[0779:ANGODP\]2.0.CO;2](https://doi.org/10.1671%2F0272-4634%282002%29022%5B0779%3AANGODP%5D2.0.CO%3B2). [ISSN](/source/ISSN_(identifier)) [0272-4634](https://search.worldcat.org/issn/0272-4634). [S2CID](/source/S2CID_(identifier)) [86112901](https://api.semanticscholar.org/CorpusID:86112901).

## External links

- [TEDx talk](https://www.youtube.com/watch?v=xYbMXzBwpIo) on [YouTube](/source/YouTube_video_(identifier)) by [Jack Horner](/source/John_R._Horner) on shape-shifting dinosaur skulls and dinosaur misclassification

v t e Pachycephalosauria Kingdom: Animalia Phylum: Chordata Clade: Dinosauria Clade: Ornithischia Clade: Marginocephalia Avemetatarsalia see Avemetatarsalia Ornithischia see Ornithischia Pachycephalosauria see below↓ Pachycephalosauria Pachycephalosauria Ferganocephale? Wannanosaurus Zavacephale Pachycephalosauridae Colepiocephale Gravitholus Hanssuesia Stegoceras Texacephale Pachycephalosaurinae Acrotholus Alaskacephale Amtocephale Brontotholus Foraminacephale Goyocephale Homalocephale Pachycephalosaurus Platytholus Prenocephale Sinocephale Sphaerotholus Tylocephale See also: Timeline Category

Taxon identifiers Pachycephalosauria Wikidata: Q131376 Wikispecies: Pachycephalosauria EoL: 42418466 Paleobiology Database: 38782 Paleobiology Database: 147917 Taxonomicon: 646659

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Adapted from the Wikipedia article [Pachycephalosauria](https://en.wikipedia.org/wiki/Pachycephalosauria) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pachycephalosauria?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.