Pachycephalosaurus

Pachycephalosaurus is a genus of pachycephalosaurid ornithischian dinosaur. Its type species, P. wyomingensis, is the only known definitive species. It was among the last species of non-avian dinosaurs on Earth before the Cretaceous–Paleogene extinction event. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America.

Fossils have been dug up in Montana, South Dakota, Wyoming, and Alberta. Most of what scientists know comes from a single skull and a few very thick skull roofs, which were 22 cm (8.7 in) thick. Pachycephalosaurus is estimated to have reached 4.5 m (15 ft) long and weighed 370–450 kg (820–990 lb). More complete fossils were found in the following years.

Like other pachycephalosaurids, Pachycephalosaurus walked on two legs and ate plants. It had long, strong legs and somewhat small arms with five-fingered hands. It is the largest known pachycephalosaur, and it is known for its extremely thick, slightly domed skull roof. The shape of the skull suggests a "battering ram" function, which may have been used for defense or for fights between members of the same species. This is similar to what is seen in today's bighorn sheep or muskoxen, where males often charge and head-butt each other to decide who is dominant. This idea has been disputed in recent years.

The genus Tylosteus is synonymous with Pachycephalosaurus, meaning the two names refer to the same animal. Another proposed match, Stygimoloch, has been disputed in recent research because the rock layers where its fossils and those of Pachycephalosaurus were found are from different times. The genus Dracorex has been considered synonymous with either of the two genera.

History of discovery

The history of Pachycephalosaurus is long and tangled. Scientists sometimes confused it with other dubious species, and many of its remains were incomplete. Seventy-one years before the dinosaur was named, American paleontologist Joseph Leidy named Tylosteus ornatus from a bone fragment he thought was the armor of a reptile. Ferdinand Vandeveer Hayden collected the bone from the "head of the Missouri River," and it was given a Cretaceous age the next year. Oliver Perry Hay later doubtfully called it a dinosaur in 1902, and Charles Whitney Gilmore more firmly called it one in 1928. In 1979, Donald Baird and John R. Horner examined the holotype ANSP 8568 at the Academy of Natural Sciences of Philadelphia. They concluded it was part of a skull of Pachycephalosaurus, a genus known only from the Lance and Hell Creek Formations of Montana, Wyoming, and South Dakota. Baird suggested the bone more likely came from the head of the Little Missouri River, a stop on Hayden's 1859–1860 expedition. In 1986, the International Commission on Zoological Nomenclature set aside the name Tylosteus so that Pachycephalosaurus could be kept.

In (/wiki/1890_in_paleontology), during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope, one of Marsh's collectors, John Bell Hatcher, found a partial left squamosal (YPM VP 335) near Lance Creek, Wyoming. In 1930, American paleontologist George Fryer Sternberg discovered a partial skull in Niobrara County, Wyoming. Gilmore described it in 1931 as a new species, Troodon wyomingensis, and it was later named P. wyomingensis, the only currently valid species of Pachycephalosaurus. At the time, paleontologists thought Troodon was the same as Stegoceras, so these dinosaurs were placed in the family Troodontidae. This mistake was corrected in (/wiki/1945_in_paleontology) by Charles M. Sternberg.

In (/wiki/1943_in_paleontology), Barnum Brown and Erich Maren Schlaikjer, using newer and more complete material, established the genus Pachycephalosaurus. They named two species, with P. grangeri as the type species of the new genus, based on a nearly complete skull from the Hell Creek Formation of Ekalaka, Carter County, Montana. Their other species, P. reinheimeri, was based on a dome and some associated bones from Corson County, South Dakota. Both were later considered the same as P. wyomingensis. In (/wiki/2015_in_paleontology), pachycephalosaurid remains were found in the Scollard Formation of Alberta. In 2025, Wroblewski described material from the Ferris Formation as Stygimoloch spinifer, which is its southernmost record. A 2026 study mentioned an associated skeleton of Stygimoloch, now kept at the National Science Museum in Tokyo.

Description

The body of Pachycephalosaurus is poorly known, because only skull remains have been described. It is famous for a large, bony dome on top of its skull, up to 25 cm (10 in) thick, which safely cushioned its brain. Bony knobs edged the back of the dome, and short bony spikes pointed upwards from the snout, though the spikes were probably blunted rather than sharp. The skull was short, with large, rounded eye sockets that faced forward, suggesting the animal had binocular vision. It had a small muzzle ending in a pointed beak, tiny teeth with leaf-shaped crowns, and an S- or U-shaped neck. Young individuals might have had flatter skulls and larger horns at the back of the head. As the animal grew, the horns shrank and rounded out as the dome grew.

Pachycephalosaurus was bipedal and possibly the largest of all pachycephalosaurids. It is estimated to have been about 4.5 metres (14.8 ft) long and to have weighed about 370–450 kilograms (820–990 lb). Based on related animals, 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. These dinosaurs were part of Marginocephalia, so they were probably more closely related to the ceratopsians than to the ornithopods. Pachycephalosaurus is the most famous member of this group, even though it is not the best-preserved one. Other members include Stenopelix, Wannanosaurus, Goyocephale, Stegoceras, Homalocephale, Tylocephale, Sphaerotholus, and Prenocephale.

In 2009, Horenr and Goodwin proposed that Stygimoloch and Dracorex are younger growth stages of Pachycephalosaurus, so they should be treated as junior synonyms. Scientists have continued to debate these names. In (/wiki/2010_in_paleontology), Gregory S. Paul suggested that Stygimoloch might be a different species, P. spinifer. In 2017, Fowler kept the two as separate names, noting that Stygimoloch is only known from younger rock layers. A 2021 study by Evans and colleagues agreed that Stygimoloch is a valid genus but that Dracorex is a growth stage of either Stygimoloch or Pachycephalosaurus. In 2025, Wroblewski also treated Stygimoloch as its own genus.

Studies by Evans and colleagues have helped scientists understand how pachycephalosaurs are related to one another. These studies usually find Pachycephalosaurus closer to Prenocephale and Sphaerotholus than to Stegoceras.

Paleobiology

Two other pachycephalosaurs, Stygimoloch spinifer ("thorny Moloch of the Styx") and Dracorex hogwartsia ("dragon king of Hogwarts"), were once thought to be separate species because of their unusual head decorations. In (/wiki/2007_in_paleontology), they were proposed to be juvenile or female forms of Pachycephalosaurus. At that year's meeting of the Society of Vertebrate Paleontology, Jack Horner of Montana State University presented evidence for this idea. In (/wiki/2009_in_paleontology), Horner and M.B. Goodwin concluded that all three were likely the same dinosaur, with the flat-headed forms being young animals that lost their spikes and grew domes as they aged. A 2010 study by Nick Longrich and colleagues also supported this idea. Later, baby skulls described in (/wiki/2016_in_paleontology) were also found to match this growth pattern.

Scientists have long thought that these dinosaurs may have used their domes like bighorn sheep or musk oxen, ramming each other head-first, though other ideas have also been suggested. Some critics argue that the skull roof could not have handled such hits. Another idea, first proposed in 1978 and later expanded by Ken Carpenter, is flank-butting, where one animal struck a rival on its side during a contest. Evidence from skull damage, which may come from agonistic behavior between rivals, supports the idea that domes were used in fights. Some damage that was once blamed on taphonomic changes may actually have come from such fights. Researchers also noted that many domes show signs of a bone infection called osteomyelitis and that the frontal bones of some skulls had many lesions. Scientists still do not know exactly what these dinosaurs ate. Their small, ridged teeth could not have chewed tough, fibrous plants or flowering shrubs as well as other dinosaurs did, so they likely ate a mix of leaves, seeds, and fruits. Some scientists think they may also have eaten meat, because their front teeth look like those of meat-eating dinosaurs.

Paleoecology

Nearly all Pachycephalosaurus fossils have been found in the Lance Formation and Hell Creek Formation of the northwestern United States. It possibly lived alongside other pachycephalosaurs of the genus Sphaerotholus. Other dinosaurs from its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus and possibly Parasaurolophus, and ceratopsians like Triceratops, Torosaurus, Nedoceratops, Tatankaceratops, and Leptoceratops.

The area was also home to the ankylosaurid Ankylosaurus, the nodosaurids Denversaurus and Edmontonia, and many theropods, including Acheroraptor, Dakotaraptor, Ornithomimus, Struthiomimus, Anzu, Leptorhynchos, Pectinodon, Paronychodon, Richardoestesia, Nanotyrannus, and Tyrannosaurus.