[4][5] 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).

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.

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.

[14] The squamosal spike was even featured in Charles Knight's painting of Cope's ceratopsid Agathaumas, likely based on Marsh's hypothesis.

[13] 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.

P. grangeri was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka, Carter County, Montana.

[23] 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.

[10] 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.

[26] The skull was short and possessed large, rounded eye sockets that faced forward, suggesting that the animal had binocular vision.

[30] Pachycephalosaurus gives its name to Pachycephalosauria, a clade of herbivorous ornithischian dinosaurs that lived during the Late Cretaceous period in North America and Asia.

[28] 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.

[25] 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.

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").

At that year's meeting of the Society of Vertebrate Paleontology,[37] 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.

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.

[38] 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.

[39]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.

[40][41] 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.

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.

[42] 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.

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.

[38] Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus and a possible species of 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.