Category: Dinosaur

Pachycephalosaurus (Pack-ee-seff-ah-low-oh-sore-us) lived during the Late Cretaceous Period, right up until the extinction event that killed almost all of the dinosaurs. It lived in what is now North America. The main distinguishing feature of this two-legged plant-eater was its thick, domed skull, which may have been used in combat between individuals to impress a potential mate.



Genera and Species

Classification: Ornithischia. Pachycephalosauridae.

Genus: Pachycephalosaurus (“Thick-headed Lizard”)

Species: P. wyomingensis


Pachycephalosaurus is poorly known beyond its skull, and much of what it’s believed to look like comes from related dinosaurs. It is thought to be a bipedal (two-legged) dinosaur with a barrel-chested body. It likeyly had short forelimbs, a thick neck, and a thick tail.

Its most prominent feature was its domed skull, which could be up to 10 inches thick. The domed area was surrounded by knobs and spikes, which surrounded the rear of the head and protruded from the nose.


LENGTH: Estimated at 4.5 meters (14.8 feet)

WEIGHT: Estimated 450 kg (990 lbs)


It has been suggested that the thick domes of pachycephalosaurs were used in combat with one another, butting their heads like bighorn sheep or muskoxen. However, it has more recently been proposed that their skulls and necks may not have been able to absorb a direct head-to-head impact as well as previously thought. Alternately, they may have used their heads to “flank butt” by striking each other on their sides. Some skulls were found to have legions or other types of damage that could indicate such head-butting behavior.

Though believed to be herbivorous (plant-eaters), the exact diet of Pachycephalosaurus is not known. Due to the small size of its teeth, it could not eat tough plants, and may have

History of Discovery

Bone fragments that would later be attributed to Pachycephalosaurus have been discovered as early as the 1850s, but the dinosaur was not formally named until 1931 based off more complete skull material. For a long time only the skull was known from this dinosaur, but recently a more complete juvenile specimen, dubbed “Sandy”, shed more light on what this dinosaur may have looked like. Sandy featured about 50% of the total skeleton, including a partial skull, neck, some ribs and vertebrae, and much of the legs.

It is not known yet whether Sandy represents Pachycephalosaurus or some closely related species. Additionally, two more recently named pachycephalosaurs, Stygimoloch (1983) and Dracorex (2006) are now thought by some to represent different growth stages of Pachycephalosaurus. This theory posits that the dome remains flatter in the earlier stages of the dinosaur’s life, growing more pronounced as they grow older


Pachycephalosaurus remains have been found in the Lance and Hell Creek Formations, in the western U.S. states of Montana, Wyoming and South Dakota. It lived alongside popular dinosaurs including Ankylosaurus and Triceratops, as well as Edmontosaurus and Anzu. Tyrannosaurus also shared its environment, and was likely its chief predator.

While the environment is largely a desert now, during the Late Cretaceous it was likely a forest floodplain with a subtropical climate. There was a shallow sea that divided the two halves of what would one day become North America, and this sea was bordered by marshes and coastal plains



2. Paul, G. (2016). The Princeton Field Guide to Dinosaurs, 2nd Edition. Princeton, New Jersey: University Press Princeton.