Pachycephalosaurus

The history and taxonomy of Pachycephalosaurus is storied and convoluted, as it involved confusion with other genera, dubious species, and a lack of complete remains. While Leidy first identified the bone as dermal armor of a reptile, it was doubtfully identified as a dinosaur instead by Oliver Perry Hay in 1902, and then more definitively as a dinosaur by Charles Whitney Gilmore in 1928. Tylosteus remained ignored until, in 1979, American paleontologist Donald Baird attempted to reidentify the original place of discovery and the taxonomic identity of the taxon. Baird and John R. Horner independently came across the Tylosteus holotype ANSP 8568 in the collections of the Academy of Natural Sciences of Philadelphia and concluded that it was the from a skull of Pachycephalosaurus, a genus known only from the Lance and Hell Creek Formations of Montana, Wyoming, and South Dakota. A manual was also assigned to Tylosteus by Leidy (1873), however it is now believed to belong to Edmontosaurus annectens. In 1892, Marsh interpreted the squamosal and the dermal armor of Denversaurus as being the body armor of Triceratops, hypothesizing that the squamosal was a spike akin to the plates on Stegosaurus. In 1990, American paleontologist W. P. Coombs stated the tooth came 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. Troodon wyomingensis In 1930, American paleontologist George Fryer Sternberg discovered a partial skull within the Lance Formation of Niobrara County, Wyoming, around southwest of Warren Post Office along Buck Creek. This specimen, USNM 12031, was then described by Gilmore in 1931 as the holotype of a new species Troödon wyomingensis, and suggested more definitively that the genus Troodon was present in the Lance Formation. Previously, isolated teeth from the region had been suggested to be from Troodon. Troodon itself had first been described by Leidy in 1856 as a lizard from the Judith River Formation of Montana, but with the 1924 description of a nearly complete skull and partial skeleton from the Belly River Group in Alberta Gilmore found Troodon to be the same as the existing ornithischian genus Stegoceras. T. wyomingensis could not be compared with the type species of Troodon, T. formosus, as the latter only preserved teeth, but significant differences in size, the , and age could separate T. wyomingensis and T. validus (previously Stegoceras), potentially even to the point of requiring a new genus name for T. wyomingensis. As well as USNM 12031, Gilmore referred the specimens USNM 7806 and 8795 to 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. 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. ==Classification and species==

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 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. 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 five 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. shown in relation to the skull 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. 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. ==See also==