Pachycephalosaurus

Pachycephalosaurus is a genus of pachycephalosaurid ornithischian dinosaur. Its type species, P. wyomingensis, is the only species known for certain. It was one of the last kinds of non-avian dinosaurs on Earth, living just 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 found in Montana, South Dakota, Wyoming, and Alberta. Most of what scientists know comes from one skull and a few very thick skull roofs. These roofs were about 22 cm (8.7 in) thick. Pachycephalosaurus is estimated to have been about 4.5 m (15 ft) long and to have 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 fairly small arms with five-fingered hands. It is the largest known pachycephalosaur. It is known for its very thick skull roof, which was slightly rounded like a dome. The shape of the skull suggests a "battering ram" use, which may have been for defense or for fights between members of the same species. This is similar to today's bighorn sheep or muskoxen, where males often charge and head-butt each other to decide who is in charge. This idea has been questioned 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 questioned in recent research because the rock layers where its fossils and those of Pachycephalosaurus were found are from different times. Dracorex has been considered a match for 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. He based the name on a bone fragment that he thought was armor from a reptile. Ferdinand Vandeveer Hayden collected the bone from the "head of the Missouri River." The next year, it was given a Cretaceous age. Oliver Perry Hay later called it a dinosaur in 1902, but he was not sure. Charles Whitney Gilmore called it a dinosaur more firmly in 1928. In 1979, Donald Baird and John R. Horner examined the holotype ANSP 8568 at the Academy of Natural Sciences of Philadelphia. They decided it was part of a skull of Pachycephalosaurus. This genus is 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 trip. In 1986, the International Commission on Zoological Nomenclature set aside the name Tylosteus so that Pachycephalosaurus could be kept.

In 1890, 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 found a partial skull in Niobrara County, Wyoming. In 1931, Gilmore described it as a new species, Troodon wyomingensis. It was later named P. wyomingensis, which is 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. In 1945, Charles M. Sternberg corrected this mistake.

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

Description

Scientists know very little about the body of Pachycephalosaurus. They have only found its skull. It is famous for a thick, bony dome on top of its head. The dome could be up to 25 cm (10 in) thick, and it kept the brain safe. Bony knobs lined the back of the dome. Short bony spikes stuck up from the snout, but they were probably blunt rather than sharp. The skull was short, with large, round eye sockets that faced forward. This suggests the animal could see with binocular vision. It had a small snout with a pointed beak and tiny teeth. Its neck was shaped like an S or a U. Young animals may have had flatter skulls and bigger horns at the back of the head. As they grew, the horns became smaller and rounder as the dome grew.

Pachycephalosaurus walked on two legs (bipedal). It may have been the largest of all the pachycephalosaurids. It is estimated to have been about 4.5 metres (14.8 ft) long. It probably weighed about 370–450 kilograms (820–990 lb). Animals related to it suggest it had a short, thick neck, short arms, a bulky body, and long legs. Its heavy tail was likely held stiff by ossified tendons.

Classification and species

Pachycephalosaurus gives its name to Pachycephalosauria, a clade of herbivorous ornithischian dinosaurs. They lived during the Late Cretaceous period in North America and Asia. These dinosaurs belonged to 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. This would mean they should be treated as the same animal under a different name. Scientists have continued to debate these names. In 2010, 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. They thought 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 kinds of dinosaurs because of their unusual head decorations. In 2007, scientists suggested that they might be young or female forms of Pachycephalosaurus. At that year's meeting of the Society of Vertebrate Paleontology, Jack Horner of Montana State University shared evidence for this idea. In 2009, Horner and M.B. Goodwin concluded that all three were probably the same dinosaur. The flat-headed forms were young animals that lost their spikes and grew domes as they got older. A 2010 study by Nick Longrich and colleagues also supported this idea. Later, baby skulls described in 2016 were also found to match this pattern of growth.

Scientists have long thought that these dinosaurs may have used their domes like bighorn sheep or musk oxen, bumping heads with each other. Other ideas have also been suggested, and some critics think the top of the skull could not have handled such hits. Another idea, first proposed in 1978 and later expanded by Ken Carpenter, is flank-butting. In this idea, one animal struck a rival on its side during a contest. Evidence from skull damage supports the idea that domes were used in fights, since the damage may come from agonistic behavior between rivals. Some damage that was once blamed on taphonomic changes may actually have come from these fights. Researchers also noticed that many domes show signs of a bone infection called osteomyelitis. The frontal bones of some skulls also had many small damaged spots.

Scientists still do not know exactly what these dinosaurs ate. Their small, ridged teeth could not chew tough, fibrous plants or flowering shrubs as well as other dinosaurs could. 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

Almost all Pachycephalosaurus fossils have been found in the Lance Formation and Hell Creek Formation in the northwestern United States. It may have lived alongside another pachycephalosaur, Sphaerotholus. Other dinosaurs from its time and place include Thescelosaurus, the hadrosaurid Edmontosaurus, and possibly Parasaurolophus. There were also ceratopsians like Triceratops, Torosaurus, Nedoceratops, Tatankaceratops, and Leptoceratops.

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