:Pachycephalosaurus

File:Pachycephalosaurus Skull AMNH.jpg

Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860, Ferdinand Vandeveer Hayden, an early fossil collector in the American West, collected a bone fragment in the vicinity of the head of the Missouri River, from what is now known to be the Lance Formation of southeastern Montana.{{cite journal |last=Baird |first=Donald |year=1979 |title=The dome-headed dinosaur Tylosteus ornatus Leidy 1872 (Reptilia: Ornithischia: Pachycephalosauridae) |journal=Notulae Naturae |volume=456 |pages=1–11}} This specimen, ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal.{{cite journal |last=Leidy |first=Joseph |author-link=Joseph Leidy |year=1872 |title=Remarks on some extinct vertebrates |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |pages=38–40}} It became known as Tylosteus. Its actual nature was not revealed until Baird studied it again over a century later and identified it as a squamosal (bone from the back of the skull) of Pachycephalosaurus, including a set of bony knobs corresponding to those found on other specimens of Pachycephalosaurus. Because the name Tylosteus predates Pachycephalosaurus, according to the International Code of Zoological Nomenclature Tylosteus would normally be preferred. In 1985, Baird successfully petitioned to have Pachycephalosaurus used instead of Tylosteus because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information.ICZN Opinion 1371, "[https://archive.org/stream/bulletinofzoolog43inte/bulletinofzoolog43inte_djvu.txt Pachycephalosaurus Brown & Schlaikjer, 1943 and Troodon wyomingensis Gilmore, 1931 (Reptilia, Dinosauria): Conserved.]" Bulletin of Zoological Nomenclature, 43 (1): April 1986.{{cite book|chapter=Pachycephalosaurus |last=Glut |first=Donald F. |author-link=Donald F. Glut |title=Dinosaurs: The Encyclopedia |url=https://archive.org/details/dinosaursencyclo04dfgl_143 |url-access=limited |year=1997 |publisher=McFarland & Co |location=Jefferson, North Carolina |pages=[https://archive.org/details/dinosaursencyclo04dfgl_143/page/n663 664]–668 |isbn=978-0-89950-917-4}} This may not be the end of the story, however. Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of Dracorex than that of Pachycephalosaurus. The issue is of uncertain importance, though, if Dracorex actually represents a juvenile Pachycephalosaurus, as has been recently proposed.{{cite journal |last=Stokstad |first=Erik |year=2007 |title=SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species? |journal=Science |pmid=18033861 |volume=318 |issue=5854 |pages=1236 |doi=10.1126/science.318.5854.1236 |s2cid=36443204 |url=http://doc.rero.ch/record/15460/files/PAL_E2847.pdf |archive-date=February 3, 2023 |access-date=January 13, 2023 |archive-url=https://web.archive.org/web/20230203172529/https://doc.rero.ch/record/15460/files/PAL_E2847.pdf |url-status=live }}

In 1890, during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope, one of Marsh's collectors, John Bell Hatcher, collected a partial left squamosal (YPM VP 335) later referred to Stygimoloch spinifer near Lance Creek, Wyoming, in the Lance Formation.{{cite journal | url=https://www.tandfonline.com/doi/abs/10.1080/02724634.1998.10011064 | doi=10.1080/02724634.1998.10011064 | title=Cranial anatomy and diagnosis of Stygimoloch spinifer(Ornithischia: Pachycephalosauria) with comments on cranial display structures in agonistic behavior | year=1998 | last1=Goodwin | first1=Mark B. | last2=Buchholtz | first2=Emily A. | last3=Johnson | first3=Rolf E. | journal=Journal of Vertebrate Paleontology | volume=18 | issue=2 | pages=363–375 | bibcode=1998JVPal..18..363G | archive-date=April 16, 2022 | access-date=April 16, 2022 | archive-url=https://web.archive.org/web/20220416120234/https://www.tandfonline.com/doi/abs/10.1080/02724634.1998.10011064 | url-status=live | url-access=subscription }}Greenfield, Tyler (December 8, 2020). "[https://incertaesedisblog.wordpress.com/2020/12/08/armor-for-agathaumas/ Armor for Agathaumas]". Incertae Sedis. Retrieved March 20, 2022.Marsh, O. C. (1891). I.—The Gigantic Ceratopsidæ, or Horned Dinosaurs, of North America 1. Geological Magazine, 8(5), 193-199. Marsh described the squamosal along with the dermal armor of Denversaurus as the body armor of Triceratops in 1892, believing that the squamosal was a spike akin to the plates on Stegosaurus. The squamosal spike was even featured in Charles Knight's painting of Cope's ceratopsid Agathaumas, likely based on Marsh's hypothesis. Marsh also named a species of now-dubious ankylosaur Palaeoscincus in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from the Lance.{{Cite journal |last=Marsh |first=Othniel Charles |date=August 1, 1892 |title=Notes on Mesozoic vertebrate fossils |url=https://www.ajsonline.org/content/s3-44/260/171 |journal=American Journal of Science |language=en |volume=s3-44 |issue=260 |pages=171–176 |doi=10.2475/ajs.s3-44.260.171 |bibcode=1892AmJS...44..171M |s2cid=130167326 |issn=0002-9599 |access-date=April 16, 2022 |archive-date=June 28, 2022 |archive-url=https://web.archive.org/web/20220628122055/https://www.ajsonline.org/content/s3-44/260/171 |url-status=dead }} The tooth was named Palaeoscinus latus, but in 1990, Coombs found the tooth to be from a pachycephalosaurid, possibly even Pachycephalosaurus itself.Coombs Jr., W.P. (1990). Teeth and taxonomy in ankylosaurs. In: Carpenter, K., and Currie, P.J. (eds.). Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press:Cambridge, 269-279. {{isbn|0-521-36672-0}} Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.{{Cite web |title=Paleobiology Collections Search |url=https://collections.nmnh.si.edu/search/paleo/?ark=ark:/65665/35c9c7b1de60c410798ddad1cba709847 |access-date=April 21, 2022 |website=collections.nmnh.si.edu |archive-date=April 21, 2022 |archive-url=https://web.archive.org/web/20220421142829/https://collections.nmnh.si.edu/search/paleo/?ark=ark:/65665/35c9c7b1de60c410798ddad1cba709847 |url-status=live }}{{Cite web |title=Paleobiology Collections Search |url=https://collections.nmnh.si.edu/search/paleo/?ark=ark:/65665/3722c00335e674a75af971910fadb8d5c |access-date=April 21, 2022 |website=collections.nmnh.si.edu |archive-date=June 18, 2023 |archive-url=https://web.archive.org/web/20230618172928/https://collections.nmnh.si.edu/search/paleo/?ark=ark:/65665/3722c00335e674a75af971910fadb8d5c |url-status=live }}{{Cite web |last=Gilmore |first=Charles W. |title=Pachycephalosaurus wyomingensis; YPM VP 003272; North America; USA; Wyoming; Niobrara County |url=https://collections.peabody.yale.edu/search/Record/YPM-VP-003272 |access-date=April 21, 2022 |website=collections.peabody.yale.edu |language=en |archive-date=June 18, 2023 |archive-url=https://web.archive.org/web/20230618172928/https://collections.peabody.yale.edu/search/Record/YPM-VP-003272 |url-status=live }}

P. wyomingensis, the type and currently only valid species of Pachycephalosaurus, was named by Charles W. Gilmore in 1931. He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County, Wyoming. Gilmore assigned his new species to Troodon as T. wyomingensis.{{cite journal |last=Gilmore |first=Charles W. |year=1931 |title=A new species of troodont dinosaur from the Lance Formation of Wyoming |journal=Proceedings of the United States National Museum |volume=79 |issue=9 |pages=1–6 |doi=10.5479/si.00963801.79-2875.1 |url=http://repository.si.edu/bitstream/handle/10088/16005/1/USNMP-79_2875_1931.pdf |archive-date=May 3, 2020 |access-date=August 6, 2018 |archive-url=https://web.archive.org/web/20200503222855/https://repository.si.edu/bitstream/handle/10088/16005/1/USNMP-79_2875_1931.pdf |url-status=live }} At the time, paleontologists thought that Troodon, then known only from teeth, was the same as Stegoceras, which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae, a misconception which was not corrected until 1945 by Charles M. Sternberg.{{cite book|chapter=Troodon |last=Glut |first=Donald F. |author-link=Donald F. Glut |title=Dinosaurs: The Encyclopedia |year=1997 |publisher=McFarland & Co |location=Jefferson, North Carolina |pages=933–938 |isbn=978-0-89950-917-4}}

File:Pachycephalosaurus skull.png

In 1943, Barnum Brown and Erich Maren Schlaikjer, with newer, more complete material, established the genus Pachycephalosaurus. They named two species: Pachycephalosaurus grangeri, the type species of their new genus, and Pachycephalosaurus reinheimeri. P. grangeri was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka, Carter County, Montana. P. reinheimeri was based on what is now DMNS 469, a dome and a few associated elements from the Lance Formation of Corson County, South Dakota.{{cite journal |last=Brown |first=Barnum |author-link=Barnum Brown |author2=Schlaikjer, Erich M. |year=1943 |title=A study of the troödont dinosaurs with the description of a new genus and four new species |journal=Bulletin of the American Museum of Natural History |volume=82 |issue=5 |pages=115–150 |url=http://digitallibrary.amnh.org/dspace/bitstream/2246/387/1/B082a05.pdf |archive-date=February 21, 2007 |access-date=December 29, 2007 |archive-url=https://web.archive.org/web/20070221111126/http://digitallibrary.amnh.org/dspace/bitstream/2246/387/1/B082a05.pdf |url-status=live }} They also referred the older species "Troodon" wyomingensis to their new genus. Their two newer species have been considered synonymous with P. wyomingensis since 1983.{{cite journal |last=Galton |first=Peter M. |author-link=Peter Galton |author2=Sues, Hans-Dieter |year=1983 |title=New data on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America |journal=Canadian Journal of Earth Sciences |volume=20 |issue=3 |pages=462–472 | doi = 10.1139/e83-043 |bibcode=1983CaJES..20..462G }}

In 2015, some pachycephalosaurid material and a domed parietal attributable to Pachycephalosaurus were discovered in the Scollard Formation of Alberta, implying that the dinosaurs of this era were cosmopolitan and did not have discrete faunal provinces.{{cite journal|last1=Evans|first1=D. C.|last2=Vavrek|first2=M. J.|last3=Larsson|first3=H. C. E.|title=Pachycephalosaurid (Dinosauria: Ornithischia) cranial remains from the latest Cretaceous (Maastrichtian) Scollard Formation of Alberta, Canada|journal=Palaeobiodiversity and Palaeoenvironments|date=2015|volume=95|issue=4|pages=579–585|doi=10.1007/s12549-015-0188-x|bibcode=2015PdPe...95..579E |s2cid=129253322}} In 2025, Wroblewski described a partial squamosal (UW 26525) and two teeth (UW 26611 and UW 26526) from the Ferris Formation as Stygimoloch spinifer, which would be its southernmost record.

The anatomy of Pachycephalosaurus itself is poorly known, as only skull remains have been described.{{cite journal| last =Sullivan| first =Robert M.| title =A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)| journal =Late Cretaceous Vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin| volume =35| pages =347–366| year =2006| url =http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf| access-date =November 10, 2010| archive-date =September 27, 2007| archive-url =https://web.archive.org/web/20070927214216/http://www.robertmsullivanphd.com/uploads/130_Sullivan__2006__-Pachycephalosauridae.pdf| url-status =live}} Pachycephalosaurus is famous for having a large, bony dome on top of its skull, up to {{Cvt|25|cm|0}} thick, which safely cushioned its brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. However, the spikes were probably blunted, not sharp.{{cite journal |last=Carpenter |first=Kenneth |author-link=Kenneth Carpenter |date=December 1, 1997 |url=http://rmg.geoscienceworld.org/cgi/content/abstract/32/1/19 |title=Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior |journal=Contributions to Geology |volume=32 |issue=1 |pages=19–25 |format=pdf |archive-date=October 2, 2011 |access-date=November 14, 2007 |archive-url=https://web.archive.org/web/20111002113817/http://rmg.geoscienceworld.org/cgi/content/abstract/32/1/19 |url-status=live }}

The skull was short and possessed large, rounded eye sockets that faced forward, suggesting that the animal had binocular vision. Pachycephalosaurus had a small muzzle that ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an S- or U-shaped neck. Younger individuals of Pachycephalosaurus might have had flatter skulls and larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out as the dome grew.

File:Pachycephalosaurus Reconstruction.jpg

File: Pachycephalosaurus wyomingensis size chart.png

Pachycephalosaurus was bipedal and possibly the largest of all pachycephalosaurids.{{cite web | last=Black | first=Riley | title="Bone-Headed" Dinosaurs Reshaped Their Skulls | website=Smithsonian Magazine | date=October 28, 2009 | url=https://www.smithsonianmag.com/science-nature/bone-headed-dinosaurs-reshaped-their-skulls-56078928/ | access-date=February 11, 2023 | archive-date=June 10, 2023 | archive-url=https://web.archive.org/web/20230610020459/https://www.smithsonianmag.com/science-nature/bone-headed-dinosaurs-reshaped-their-skulls-56078928/ | url-status=live }} It has been estimated that Pachycephalosaurus was about {{convert|4.5|m|1}} long and weighed about {{convert|370-450|kg}}.{{cite book |last=Paul |first=Gregory S. |year=2010 |title=The Princeton Field Guide to Dinosaurs |url=https://archive.org/details/princetonfieldgu0000paul |url-access=registration |publisher=Princeton University Press |location=Princeton, NJ |page=[https://archive.org/details/princetonfieldgu0000paul/page/244 244] |isbn=978-0-691-13720-9}}{{Cite journal |last1=Benson |first1=Roger B. J. |last2=Campione |first2=Nicolás E. |last3=Carrano |first3=Matthew T. |last4=Mannion |first4=Philip D. |last5=Sullivan |first5=Corwin |last6=Upchurch |first6=Paul |last7=Evans |first7=David C. |date=May 6, 2014 |title=Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage |journal=PLOS Biology |language=en |volume=12 |issue=5 |pages=e1001853 |doi=10.1371/journal.pbio.1001853 |issn=1545-7885 |pmc=4011683 |pmid=24802911 |doi-access=free }} Previous estimations calculated the initial size of Pachycephalosaurus at around {{convert|8|m|1}} in length. Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short arms, a bulky body, long legs, and a heavy tail that was likely held rigid by ossified tendons.{{cite journal| last = Organ| first = Christopher O.| author2 = Adams, Jason| title = The histology of ossified tendon in dinosaurs| journal = Journal of Vertebrate Paleontology| volume = 25| issue = 3| pages = 602–613| year = 2005| url = http://www.oeb.harvard.edu/faculty/edwards/people/postdocs/documents/JVP_25_3_602-613.pdf| doi = 10.1671/0272-4634(2005)025[0602:THOOTI]2.0.CO;2| s2cid = 85983799| access-date = June 10, 2008| url-status = dead| archive-url = https://web.archive.org/web/20080829120536/http://www.oeb.harvard.edu/faculty/edwards/people/postdocs/documents/JVP_25_3_602-613.pdf| archive-date = August 29, 2008}}

File:Hell Creek pachycephalosaurs.jpg]]

Pachycephalosaurus gives its name to Pachycephalosauria, a clade of herbivorous ornithischian dinosaurs that lived during the Late Cretaceous period in North America and Asia. Pachycephalosaurs were a part of Marginocephalia, thus being likely more closely related to the ceratopsians than the ornithopods.{{cite journal| last = Pisani| first = Davide |author2=Yates, Adam M. |author3=Langer, Max C. |author4=Benton, Michael J.| title = A genus-level supertree of the Dinosauria| journal = Proceedings of the Royal Society B| volume = 269| issue = 1494| pages = 915–921| year = 2002| doi =10.1098/rspb.2001.1942| pmid = 12028774| pmc = 1690971}}

Pachycephalosaurus is the most famous member of Pachycephalosauria, even if it is not the best-preserved member. The clade also includes Stenopelix, Wannanosaurus, Goyocephale, Stegoceras, Homalocephale, Tylocephale, Sphaerotholus, and Prenocephale. Within the tribe Pachycephalosaurini, Pachycephalosaurus is most closely related to Alaskacephale. Dracorex and Stygimoloch have also been synonymized with Pachycephalosaurus.{{cite journal |last=Horner |first=J. R. |author2=Goodwin, M. B. |year=2009 |title=Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus |journal=PLOS ONE |pmid=19859556 |volume=4 |pmc=2762616 |issue=10 |doi=10.1371/journal.pone.0007626 |pages=e7626 |bibcode=2009PLoSO...4.7626H |editor1-last=Sereno |editor1-first=Paul|doi-access=free }}

File:Pachycephalosaurus ontogeny skulls - Museum of the Rockies - 2013-07-08.jpg]] In 2010, Gregory S. Paul proposed that, while Stygimoloch and Dracorex possibly represent different growth stages of Pachycephalosaurus, Stygimoloch might represent a different species, P. spinifer. In his supplementary material of a 2017 paper, Fowler noted that Stygimoloch is only known from younger rock layers than Pachycephalosaurus, so he tentatively classified both to be separate, though Dracorex was included as a synonym of either taxa. A 2021 phylogenetic analysis by Evans and colleagues accepted the validity of the genus Stygimoloch based on the same reason as Fowler (2017), but agreed with the consensus that Dracorex represents an ontogimorph of either Stygimoloch or Pachycephalosaurus instead of a distinct taxon.{{cite journal | url=https://cdnsciencepub.com/doi/full/10.1139/cjes-2020-0190 | title=Description and revised diagnosis of Asia's first recorded pachycephalosaurid, Sinocephale bexelli gen. nov., from the Upper Cretaceous of Inner Mongolia, China | last1=Evans | first1=David | last2=Brown | first2=Caleb M. | last3=You | first3=Hailu | last4=Campione | first4=Nicolás E. | journal=Canadian Journal of Earth Sciences | year=2021 | volume=58 | issue=10 | pages=981–992 | doi=10.1139/cjes-2020-0190 | bibcode=2021CaJES..58..981E | s2cid=244227050 | url-access=subscription }} In 2025, Wroblewski treated Stygimoloch as a separate genus to which he attributed three specimens from the Ferris Formation, providing further evidence that this taxon was restricted to the uppermost Maastrichtian, but agreed with the consensus that Dracorex is morphologically identical to Stygimoloch (or Pachycephalosaurus).{{Cite journal |last=Wroblewski |first=A. F.-J. |title=Southernmost record of the pachycephalosaurine Stygimoloch spinifer and palaeobiogeography of latest Cretaceous North American dinosaurs |year=2025 |journal=Lethaia |volume=57 |issue=4 |pages=1–10 |doi=10.18261/let.57.4.7 |doi-access=free }}

Phylogenetic analyses by Evans and colleagues have been used to resolve the relationships within Pachycephalosauridae, consistently finding Pachycephalosaurus as one of the most derived taxa closer to Prenocephale and Sphaerotholus than Stegoceras.{{Cite journal | last1 = Evans | first1 = D. C. | last2 = Schott | first2 = R. K. | last3 = Larson | first3 = D. W. | last4 = Brown | first4 = C. M. | last5 = Ryan | first5 = M. J. | title = The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs | doi = 10.1038/ncomms2749 | journal = Nature Communications | volume = 4 | pages = 1828 | year = 2013 | pmid = 23652016| bibcode = 2013NatCo...4.1828E | doi-access = free }} The version of the analysis published by Woodruff and colleagues in 2023 is below.{{Cite journal |last1=Woodruff |first1=D. Cary |last2=Schott |first2=Ryan K. |last3=Evans |first3=David C. |date=November 15, 2023 |title=Two new species of small-bodied pachycephalosaurine (Dinosauria, Marginocephalia) from the uppermost Cretaceous of North America suggest hidden diversity in well-sampled formations |journal=Papers in Palaeontology |language=en |volume=9 |issue=6 |at=e1535 |doi=10.1002/spp2.1535 |bibcode=2023PPal....9E1535W |issn=2056-2799}}

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File:Pachycephalosaurus ontogeny.png

Aside from Pachycephalosaurus itself, two other pachycephalosaurs were described from the latest Cretaceous of the northwestern United States: Stygimoloch spinifer ("thorny Moloch of the Styx") and Dracorex hogwartsia ("dragon king of Hogwarts"). The former is only known from a juvenile skull with a reduced dome and large spikes, while the latter, also known from only a juvenile skull, had a seemingly flat head with short horns. Due to their unique head ornamentation, they were seen as separate species for a number of years. However, in 2007, they were proposed to be juvenile or female morphologies of Pachycephalosaurus. At that year's meeting of the Society of Vertebrate Paleontology,Erik Stokstad,"SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?", Science Vol. 18, November 23, 2007, p. 1236; http://www.sciencemag.org/cgi/content/full/318/5854/1236 {{Webarchive|url=https://web.archive.org/web/20090924102022/http://www.sciencemag.org/cgi/content/full/318/5854/1236 |date=September 24, 2009 }} Jack Horner of Montana State University presented evidence, from analysis of the skull of the Dracorex specimen, that it may be a juvenile form of Stygimoloch. In addition to this, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus. Horner and M.B. Goodwin published their findings in 2009, showing that the spike and skull dome bones of all three "species" exhibit extreme plasticity and that both Dracorex and Stygimoloch are known only from juvenile specimens, while Pachycephalosaurus is known only from adult specimens. These observations, in addition to the fact that all three forms lived in the same time and place, led them to conclude that Dracorex and Stygimoloch were simply juvenile Pachycephalosaurus, which lost spikes and grew domes as they aged.Horner J.R. and Goodwin, M.B. (2009). "Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus." PLoS ONE, 4(10): e7626. [http://www.plosone.org/article/info:doi/10.1371/journal.pone.0007626 Online full text] {{Webarchive|url=https://web.archive.org/web/20110501071517/http://www.plosone.org/article/info:doi/10.1371/journal.pone.0007626 |date=May 1, 2011 }} A 2010 study by Nick Longrich and colleagues also supported the hypothesis that all flat-skulled pachycephalosaur species were juveniles of the dome-headed adults, such as Goyocephale and Homalocephale.{{cite journal | last1 = Longrich | first1 = N.R. | last2 = Sankey | first2 = J. | last3 = Tanke | first3 = D. | year = 2010 | title = Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA | journal = Cretaceous Research | volume = 31| issue = 2| pages = 274–284| doi = 10.1016/j.cretres.2009.12.002 | bibcode = 2010CrRes..31..274L }}File:Pachycephalosaurus scale.pngThe discovery of baby skulls assigned to Pachycephalosaurus that were described in 2016 from two different bone beds in the Hell Creek Formation has been presented as further evidence for this hypothesis. The fossils, as described by David Evans and Mark Goodwin et al are identical to all three supposed genera in the placement of the rugose knobs on their skulls, and the unique features of Stygimoloch and Dracorex are thus instead morphologically consistent features on a Pachycephalosaurus growth curve.

It has been noted that morphological differences between Stygimoloch and Pachycephalosaurus may also partly be due to slight stratigraphic differences. The few Stygimoloch specimens that have reliable stratigraphic data were all collected from the upper part of the Hell Creek Formation, whereas Pachycephalosaurus morphs were all collected from the lower part.{{cite journal | last1 = Fowler | first1 = D.W. | year = 2017 | title = 'Revised geochronology, correlation, and dinosaur stratigraphic ranges of the Santonian-Maastrichtian (Late Cretaceous) formations of the Western Interior of North America | journal = PLOS ONE | volume = 12| issue = 11| pages = 1–20| doi = 10.1371/journal.pone.0188426 | pmid = 29166406 | pmc = 5699823 | bibcode = 2017PLoSO..1288426F | doi-access = free }} This has also led to suggestions that Stygimoloch might represent its own species, P. spinifer, or a distinct genus.

File:Pachycephalosaurus head butting.png

It has been widely hypothesized for decades that Pachycephalosaurus and its relatives were the ancient, bipedal equivalents of bighorn sheep or musk oxen, where male individuals would ram each other headlong and that they would horizontally straighten their head, neck, and body in order to transmit stress during ramming. However, there have also been alternative suggestions that the pachycephalosaurs could not have used their domes in this way.

The primary argument that has been raised against head-butting is that the skull roof may not have adequately sustained impact associated with ramming, as well as a lack of definitive evidence of scars or other damage on fossilized Pachycephalosaurus skulls. However, more recent analyses have uncovered such damage (see below).{{cite journal |last=Goodwin |first=Mark |author2=Horner, John R. |year=2004 |title=Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior |journal=Paleobiology |volume=30 |issue=2 |pages=253–267 |doi=10.1666/0094-8373(2004)030<0253:CHOPOM>2.0.CO;2 |bibcode=2004Pbio...30..253G |s2cid=84961066 |url=http://doc.rero.ch/record/14876/files/PAL_E2014.pdf |archive-date=February 3, 2023 |access-date=January 13, 2023 |archive-url=https://web.archive.org/web/20230203231334/https://doc.rero.ch/record/14876/files/PAL_E2014.pdf |url-status=live }}{{Cite journal | last1 = Peterson | first1 = J. E. | last2 = Vittore | first2 = C. P. | editor1-last = Farke | editor1-first = Andrew A | doi = 10.1371/journal.pone.0036227 | title = Cranial Pathologies in a Specimen of Pachycephalosaurus | journal = PLOS ONE | volume = 7 | issue = 4 | pages = e36227 | year = 2012 | pmid = 22558394| pmc =3340332 | bibcode = 2012PLoSO...736227P | doi-access = free }} Furthermore, the cervical and anterior dorsal vertebrae show that the neck was carried in an S- or U-shaped curve, rather than a straight orientation and that it might have been unfit for transmitting stress from direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.

File:Pachycephalosaurus cranial lesion.png

Alternatively, Pachycephalosaurus and other pachycephalosaurids may have engaged in flank-butting during intraspecific combat. In this scenario, an individual may have stood roughly parallel or faced a rival directly, using intimidation displays to cow its rival. If intimidation failed, the Pachycephalosaurus would bend its head downward and to the side, striking the rival on its flank. This hypothesis is supported by the relatively broad torso of most pachycephalosaurs, which would have protected vital organs from trauma. The flank-butting theory was first proposed by Sues in 1978 and expanded upon by Ken Carpenter in 1997.

File:Pachycephalosaurus.png

In 2012, a study showed that cranial pathologies in a P. wyomingensis specimen were likely due to agonistic behavior. It was also proposed that similar damage in other pachycephalosaur specimens (previously explained as taphonomic artifacts and bone absorptions) may instead have been due to such behavior. Peterson et al. (2013) studied cranial pathologies among Pachycephalosauridae and found that 22% of all domes examined had lesions that are consistent with osteomyelitis, an infection of the bone resulting from penetrating trauma or trauma to the tissue overlying the skull that lead 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.{{cite journal | last1 = Peterson | first1 = JE | last2 = Dischler | first2 = C | last3 = Longrich | first3 = NR | year = 2013 | title = Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae) | journal = PLOS ONE | volume = 8 | issue = 7| page = e68620 | doi = 10.1371/journal.pone.0068620 | pmid = 23874691 | pmc = 3712952 | bibcode = 2013PLoSO...868620P | doi-access = free }} Pachycephalosaurus wyomingensis specimen BMR P2001.4.5 was observed to have 23 lesions in its frontal bone and P. wyomingensis specimen DMNS 469 was observed to have 5 lesions. The frequency of trauma was comparable across the different genera in the pachycephalosaurid family, despite the fact that these genera vary with respect to the size and architecture of their domes and the fact that they existed during varying geologic periods. 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,{{cite journal | last1 = Longrich | first1 = NR | last2 = Sankey | first2 = J | last3 = Tanke | first3 = D | year = 2010 | title = Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA | journal = Cretaceous Research | volume = 31 | issue = 2| pages = 274–284 | doi = 10.1016/j.cretres.2009.12.002 | bibcode = 2010CrRes..31..274L }} where intra-specific combat behavior is not expected.

Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar boneReid R.E.H. (1997) Histology of bones and teeth. In: Currie, PJ and Padian, K, editors. Encyclopedia of Dinosaurs. Academic Press, San Diego, CA. 329–339. that contains fibroblasts, which play a critical role in wound healing and are capable of rapidly depositing bone during remodeling.Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus PLoS ONE 4(10): e7626. Available: http://www.plosone.org/article/inf o%3Adoi%2F10.1371%2Fjournal.pone. 0007626. Retrieved December 4, 2012. Peterson et al. (2013) concluded that, taken together, the frequency of lesion distribution and the bone structure of frontoparietal domes lends strong support to the hypothesis that pachycephalosaurids used their unique cranial structures for agonistic behavior. CT scan comparisons of the skulls of Stegoceras validum, Prenocephale prenes, and several head-striking artiodactyls have also supported pachycephalosaurids as being well-equipped for head-butting.{{cite journal | last1 = Snively | first1 = E | last2 = Theodor | first2 = JM | year = 2011 | title = Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls | journal = PLOS ONE | volume = 6 | issue = 6| page = e21422 | doi = 10.1371/journal.pone.0021422 | pmc = 3125168 | pmid = 21738658 | bibcode = 2011PLoSO...621422S | doi-access = free }} Micro-CT scans of the pachycephalosaurid specimen, identified as cf. Foraminacephale brevis, also support that pachycephalosaurids likely engaged in head-butting.{{cite journal|author1=Dyer, Aaron D.|author2=LeBlanc, Aaron R.H.|author3=Doschak, Michael R.|author4=Currie, Philip J.|year=2021|title=Taking a crack at the dome: histopathology of a pachycephalosaurid (Dinosauria: Ornithischia) frontoparietal dome|journal=Biosis: Biological Systems|volume=2|issue=2|pages=248–270|doi=10.37819/biosis.002.02.0101|doi-access=free}}

Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth, they could not have chewed tough, fibrous plants like flowering shrubs as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, and fruits.{{Cite journal |last1=MARYANSKA |first1=TERESA |last2=OSMOLSKA |first2=HALSZKA |date=1974 |title=Pachycephalosauria, a new suborder of ornithischian dinosaurs |url=http://www.palaeontologia.pan.pl/Archive/1974_30_45-102_22-31.pdf |journal=Palaeontologia Polonica |volume=30 |pages=45–102 |via=Google Scholar |archive-date=August 31, 2021 |access-date=June 27, 2024 |archive-url=https://web.archive.org/web/20210831103028/http://www.palaeontologia.pan.pl/Archive/1974_30_45-102_22-31.pdf |url-status=live }} The sharp, serrated teeth would have been very effective for shredding plants.{{cite book |last=Maryańska |first=Teresa |author-link1 = Teresa Maryańska |author2=Chapman, Ralph E. |author3= Weishampel, David B. |editor=Weishampel, David B. |editor2=Dodson, Peter |editor3=Osmólska, Halszka |title=The Dinosauria |url=https://archive.org/details/dinosauriandedit00weis |url-access=limited |edition=2nd |year=2004 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-24209-8 |chapter=Pachycephalosauria |pages=[https://archive.org/details/dinosauriandedit00weis/page/n482 464]–477}}{{Cite web|date=February 25, 2020|title=The Real Pachycephalosaurus|url=https://dinomuseum.ca/2020/02/25/the-real-pachycephalosaurus/|access-date=August 18, 2021|website=Philip J. Currie Dinosaur Museum|language=en-CA|archive-date=August 18, 2021|archive-url=https://web.archive.org/web/20210818140404/https://dinomuseum.ca/2020/02/25/the-real-pachycephalosaurus/|url-status=live}} It has also been suspected to a degree that it may have included meat in its diet. The most complete fossil jaw shows that it had serrated blade-like front teeth, reminiscent of those of carnivorous theropods.{{Cite web|url=https://www.nationalgeographic.com/science/2018/10/news-vegetarian-dinosaur-ate-meat-pachycephalosaurus-paleontology/|archive-url=https://web.archive.org/web/20181108224457/https://www.nationalgeographic.com/science/2018/10/news-vegetarian-dinosaur-ate-meat-pachycephalosaurus-paleontology/|url-status=dead|archive-date=November 8, 2018|title=Vegetarian dinosaur may have actually eaten meat, skull suggests|date=October 24, 2018|website=Science & Innovation|access-date=May 7, 2019}}

File:Hell Creek dinosaurs and pterosaurs by durbed.jpg

Nearly all Pachycephalosaurus fossils have been recovered from the Lance Formation and Hell Creek Formation of the northwestern United States. Pachycephalosaurus possibly coexisted alongside additional pachycephalosaur species of the genera Sphaerotholus, as well as Dracorex and Stygimoloch, though these last two genera may represent different growth stages of Pachycephalosaurus itself. Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus and possibly Parasaurolophus, ceratopsians like Triceratops, Torosaurus, Nedoceratops, Tatankaceratops, and Leptoceratops, the ankylosaurid Ankylosaurus, the nodosaurids Denversaurus and Edmontonia, and the theropods Acheroraptor, Dakotaraptor, Ornithomimus, Struthiomimus, Anzu, Leptorhynchos, Pectinodon, Paronychodon, Richardoestesia, and Tyrannosaurus.Weishampel, David B.; Barrett, Paul M.; Coria, Rodolfo A.; Le Loeuff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth, M.P.; and Noto, Christopher R. (2004). "Dinosaur Distribution". In: D.B. Weishampel, P. Dodson, and H. Osmólska (eds.) The Dinosauria (2nd edition). 517–606. {{ISBN|0-520-24209-2}}.

• Timeline of pachycephalosaur research

{{Reflist}}

• [http://www.dinodictionary.com/dinos_p.asp#PACHYCEPHALOSAURUS Pachycephalosaurus in the Dinodictionary] {{Webarchive|url=https://web.archive.org/web/20120510131841/http://www.dinodictionary.com/dinos_p.asp#PACHYCEPHALOSAURUS |date=May 10, 2012 }}
• [https://web.archive.org/web/20071130185726/http://animals.nationalgeographic.com/animals/prehistoric/pachycephalosaurus-wyomingensis.html Pachycephalosaurus wyomingensis] from National Geographic Online
• [https://www.youtube.com/watch?v=xYbMXzBwpIo TEDx talk by Jack Horner] on shape-shifting dinosaur skulls and dinosaur misclassification.

• {{Wikispecies-inline|Pachycephalosaurus}}
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