Pachy the Pachycephalosaurus

## Pachy

Pachy Hes a Pachycephalosaurus with body is Blue and White Belly And Red Head

Pachy as a Pachycephalosaurus is Gooding Dinosaur is My Friends Four Pachycephalosaurus

Pachy Eating Food Sush a Plant

## Information

***Pachycephalosaurus*** (/ˌpækɪˌsɛfələˈsɔːrəs/;[2] meaning "thick-headed lizard", from Greek *pachys-/παχύς-* "thick", *kephale/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/9” 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.[3][4]

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[edit]

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.[5] 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.[6] 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*.[5] 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.[7][8] 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*.[9] The issue is of uncertain importance, though, if *Dracorex* actually represents a juvenile *Pachycephalosaurus*, as has been recently proposed.[10]

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.[11][12][13] 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*.[13] The squamosal spike was even featured in Charles Knight’s painting of Cope’s ceratopsid *Agathaumas,* likely based on Marsh’s hypothesis.[12] 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.[14] The tooth was named *Palaeoscinus latus*, but in 1990, Coombs found the tooth to be from a pachycephalosaurid, possibly even *Pachycephalosaurus* itself.[15] Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.[16][17][18]

*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*.[19] 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.[20]

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.[21] 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.[22]

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 didn't have discrete faunal provinces.[23]

## Description[edit]

The anatomy of *Pachycephalosaurus* itself is poorly known, as only skull remains have been described.[9] *Pachycephalosaurus* is famous for having a large, bony dome on top of its skull, up to 25 cm (10 in) 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.[24]

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.[24] 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.[3][4]

*Pachycephalosaurus* was bipal and possibly the largest of all pachycephalosaurids.[25] It has been estimated that *Pachycephalosaurus* was about 4.5 metres (14.8 ft) long and weighed about 370–450 kilograms (820–990 lb).[26][27] 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.[28]

## Classification[edit]

*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.[29]

*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*.[10][3]

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*.[26] This idea has been regarded as a way of interpretation by Mark Witton and Thomas Holtz.[30][31] 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.[32] However, David Evans himself noted in a Twitter post that he and his colleagues would also consider *Stygimoloch* as *P. spinifer*.[33]

Below is a cladogram modified from Evans *et al.*, 2013.[34]