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Genus of pachycephalosaurid dinosaurs

Pachycephalosaurus (,/p//æ//k//ᵻ/,/s//ɛ//f//əl//ə/'/s//ɔː//r//ə//s
/; meaning "thick headed lizard", from Greek pachys /παχύς "thickness", kephalon/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, P. wyomingensis, is the only known species, but some researchers argue that the genus Stygimoloch might be a second species, P. spinifer or a juvenile specimen of P. wyomingensis. 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. The species is known mainly from a single skull, plus a few extremely thick skull roofs (at 22 cm or 9 in thick). 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.

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. 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. 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. 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 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.

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. It has been estimated that Pachycephalosaurus was about long and weighed about 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.

Pachycephalosaurus gives its name to Pachycephalosauria, a clade of herbivorous ornithischian dinosaurs that lived during the Late Cretaceous period in North America and Asia. Despite their bipedal stance, they 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. A phylogenetic analysis from 2021 by Evans and colleagues accepted the validity of the genus Stygimoloch on the basis of it being found in later rock layers than Pachycephalosaurus, but agreed with the consensus that Dracorex represents an ontogimorph instead of a distinct taxon. However, David Evans himself noted in a Twitter post that he and his colleagues would also consider Stygimoloch as P. spinifer. Below is a cladogram modified from Evans et al., 2013. Below is a cladogram from Evans et al., 2021. 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. 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. This has also led to suggestions that Stygimoloch might represent its own species, P. spinifer. 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. 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. 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.

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.

Nearly all Pachycephalosaurus fossils have been recovered from the Lance Formation and Hell Creek Formation of the northwestern United States.