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Pachycephalosaurus ( , meaning "thick headed lizard," from Greek pachy-/παχυ- "thick", cephale/κεφαλη "head" and saurus/σαυρος "lizard") is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montanamarker, South Dakotamarker, and Wyomingmarker. It was an herbivorous or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs. This dinosaur is monotypic, meaning the type species, P. wyomingensis, is the only known species. Pachycephalosaurus was one of the last non-avian dinosaurs before the K–T extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus.

Like other pachycephalosaurids, Pachycephalosaurus was a bipedal omnivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest known pachycephalosaur, measuring nearly in length and up to in weight.

The thick skull domes of Pachycephalosaurus and related genera gave rise to the theory that pachycephalosaurs used their skulls in intraspecific combat. This theory has been discredited in recent years.


Size comparison of Pachycephalosaurus wyomingensis and a human.
The anatomy of Pachycephalosaurus is poorly known, as only skull remains have been described. Pachycephalosaurus is famous for having a large, bony dome atop its skull, up to 25 cm (10 in) thick, which safely cushioned its tiny brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. The spikes were probably blunt, not sharp.

The skull was short, and possessed large, rounded eye sockets that faced forward, suggesting that the animal had good vision and was capable of binocular vision. Pachycephalosaurus had a small muzzle which ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck.

Pachycephalosaurus was probably bipedal and was the largest of the pachycephalosaurid (bone-headed) dinosaurs. Using data from other pachycephalosaurids, it has been estimated that Pachycephalosaurus was approximately the length of a large car, perhaps around 4.6 meters (15 ft) long, and had a fairly short, thick neck, short fore limbs, a bulky body, long hind legs and a heavy tail, which was likely held rigid by ossified tendons.

Classification and systematics

The cladogram presented here follows an analysis by Williamson and Carr, 2002.
One genus listed above, Yaverlandia, is no longer considered a pachycephalosaurian.

Pachycephalosaurus gives its name to the Pachycephalosauria, a clade of herbivorous ornithischian ("bird hipped") dinosaurs which lived during the Late Cretaceous Period in North America and Asia. Despite their bipedal stance, they were likely more closely related to the ceratopsians than the ornithopods.

Pachycephalosaurus is the most famous member of the Pachycephalosauria (though 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 Dracorex and Stygimoloch, although these may be juvenile forms of Pachycephalosaurus.

Discovery and history

A Pachycephalosaurus skeleton, on display at the Royal Ontario Museum, Toronto.
Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandiveer Hayden, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the Missouri Rivermarker, from what is now known to be the Lance Formation in southeastern Montana. This specimen, now ANSPmarker 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal. Its actual nature was not found until Baird restudied it 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 it should be preferred. Baird successfully petitioned to have Pachycephalosaurus used instead of older 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. This may not be the end of the story; 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.

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 USNMmarker 12031, from the Lance Formation of Niobrara Countymarker, Wyomingmarker. 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 not corrected until 1945, by Charles M. Sternberg.
The holotype adult skull of Pachycephalosaurus "reinheimeri" (DMNS 469)
In 1943, Barnum Brown and Erich Maren Schlaikjer, with newer, more complete material, established the genus Pachycephalosaurus and made "T. wyomingensis" its type species.They also named two more species: Pachycephalosaurus grangeri and Pachycephalosaurus reinheimeri. P. grangeri was based on AMNHmarker 1696, a nearly complete skull from the Hell Creek Formation of Ekalakamarker, Carter Countymarker, Montana. P. reinheimeri was based on what is now DMNHmarker 469, a dome and a few associated elements from the Lance Formation of Corson Countymarker, South Dakota. These later two species have been considered synonymous with P. wyomingensis since 1983.

At the 2007 annual meeting of the Society of Vertebrate Paleontology, Jack Horner of Montana State Universitymarker presented evidence that Dracorex and Stygimoloch may be juvenile forms of Pachycephalosaurus.


Nearly all Pachycephalosaurus fossils have been recovered from the Lance Formation and Hell Creek Formation of the western United States. Pachycephalosaurus co-existed alongside fellow pachycephalosaurs Dracorex and Stygimoloch. Other dinosaurs that shared its time and place include Thescelosaurus, the hadrosaurids Edmontosaurus and Anatotitan, ceratopsids Triceratops and Torosaurus, ankylosaurid Ankylosaurus, and the theropods Ornithomimus, Dromaeosaurus, and Tyrannosaurus.

Scientists once suspected that Pachycephalosaurus and its relatives were the bipedal equivalents of bighorn sheep or musk oxen; that male individuals would ram each other headlong. It was also believed that they would make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, it is now believed that the pachycephalosaurs would not have used their domes in this way.
A Pachycephalosaurus displaying its prominent skull roof.
Foremost, the skull roof could not have adequately sustained impact associated with such ramming. Also, there is no evidence of scars or other damage on fossilized Pachycephalosaurus skulls. 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 thus unfit for direct head-butting. Lastly, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows.

It is more probable that the Pachycephalosaurus and other pachycephalosaurid genera engaged in flank-butting in 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 pachycephalosaur on its flank. This hypothesis is supported by the relatively broad width of most pachycephalosaurs, a trait that would have protected vital organs from harm. The flank-butting theory was first proposed by Sues in 1978, and expanded upon by Ken Carpenter in 1997.


Scientists do not yet know what these dinosaurs ate. Having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that pachycephalosaurs lived on a mixed diet of leaves, seeds, fruit and insects. The sharp, serrated teeth would have been very effective for shredding plants.


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