Stygimoloch

''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thickness", ''kephalon/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, ''P. wyomingensis'', is the only known definitive species. The possibly synonymous taxon, ''Stygimoloch'', might represent a distinct genus or a second species, ''P. spinifer''. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America. Remains have been excavated in Montana, South Dakota, Wyoming, and Alberta. Mainly known from a single skull and a few extremely thick skull roofs (at 22 cm or 9 in thick), ''Pachycephalosaurus'' is estimated to have reached long and weighed . More complete fossils would come to be found in the following years.

''Pachycephalosaurus'' was among the last species of non-avian dinosaurs on Earth before the Cretaceous–Paleogene extinction event. The genus ''Tylosteus'' has been synonymized with ''Pachycephalosaurus'', as have the genera ''Stygimoloch'' and ''Dracorex'', in recent studies.

Like other pachycephalosaurids, ''Pachycephalosaurus'' was a bipedal herbivore, possessing long, strong legs and somewhat small arms with five-fingered hands. ''Pachycephalosaurus'' is the largest-known pachycephalosaur, known for having an extremely thick, slightly domed skull roof; visually, the structure of the skull suggests a "battering ram" function in life, evolved for use as a defensive mechanism or intra-species combat, similar to what is seen with today's bighorn sheep or muskoxen (with male animals routinely charging and head-butting each other for dominance). This hypothesis has actually been highly disputed in recent years.

History of discovery

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. 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. 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,
''Pachycephalosaurus'' Brown & Schlaikjer, 1943 and ''Troodon wyomingensis'' Gilmore, 1931 (Reptilia, Dinosauria): Conserved.
''Bulletin of Zoological Nomenclature'', 43 (1): April 1986. 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.

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.Greenfield, Tyler (December 8, 2020).
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. The tooth was named ''Palaeoscinus latus'', but in 1990, Coombs found the tooth to be from a pachycephalosaurid, possibly even ''Pachycephalosaurus'' itself. Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.

''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''. 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.

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. 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.

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. 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.

Description

The anatomy of ''Pachycephalosaurus'' itself is poorly known, as only skull remains have been described. ''Pachycephalosaurus'' is famous for having a large, bony dome on top of its skull, up to 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.

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.

''Pachycephalosaurus'' was bipedal and possibly the largest of all pachycephalosaurids. It has been estimated that ''Pachycephalosaurus'' was about long and weighed about . Previous estimations calculated the initial size of ''Pachycephalosaurus'' at around 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.

Classification

''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.

''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''.
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. 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'').

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''. The version of the analysis published by Woodruff and colleagues in 2023 is below.

Paleobiology

Growth

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, 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
Online full text
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''.The 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. This has also led to suggestions that ''Stygimoloch'' might represent its own species, ''P. spinifer'', or a distinct genus.

Dome function

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). 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.

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.

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. ''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, where intra-specific combat behavior is not expected.

Histological examination reveals that pachycephalosaurid domes are composed of a unique form of fibrolamellar bone that contains fibroblasts, which play a critical role in wound healing and are capable of rapidly depositing bone during remodeling. 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. Micro-CT scans of the pachycephalosaurid specimen, identified as cf. '' Foraminacephale brevis'', also support that pachycephalosaurids likely engaged in head-butting.

Diet

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. The sharp, serrated teeth would have been very effective for shredding plants. 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.

Paleoecology

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. .

See also

* Timeline of pachycephalosaur research

References

External links

''Pachycephalosaurus'' in the Dinodictionary


from ''National Geographic'' Online

TEDx talk by Jack Horner
on shape-shifting dinosaur skulls and dinosaur misclassification.

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Pachycephalosauria
Dinosaur genera
Maastrichtian dinosaurs
Lance Formation
Hell Creek Formation
Scollard Formation
Taxa named by Barnum Brown
Taxa named by Erich Maren Schlaikjer
Fossil taxa described in 1943
Dinosaurs of Canada
Dinosaurs of the United States