April 4, 2022 Scott Kraus

Pachycephalosaurus is a Rare Ornithiscian Herbivore

An ornithischian dinosaur, Pachycephalosaurus lived during the Late Cretaceous Period about 65 million years ago. It was one of the last non-avian dinosaurs living on earth before the Cretaceous-Paleogene extinction event. Known as the bone-headed, or dome-headed, dinosaur this is the largest dinosaur of the Pachycephalosaria family to date and is the most famous, even though very few fossils have been discovered for it.

Pachycephalosaurus (pronounced  PACK-ee-seff-ah-low-SORE-uss ) translates as “thick headed lizard” from the Greek language and has given its name to an entire breed of “bone head dinosaurs” known as Pachycephalosauria which includes Stegoceras and Wannanosaurus as well as Dracorex and Stygimoloch.

Quick Facts

What did Pachycephalosaurus Look Like?

It’s difficult for palaeontologists to know for sure what Pachycephalosaurus looked like as no skeletal remains have been found apart from skull pieces, so they have had to base all of their estimates on these few pieces. It is known that Pachycephalosaurus had a short skull with a large distinctive dome-shaped head and large eyes that faced forward suggesting good eyesight with binocular vision.

It is thought that Pachycephalosaurus was a medium sized dinosaur with a bulky body and that it walked on two long hind legs, measuring roughly 18 feet long and was just over 10 feet tall with a weight of around 4,000 pounds.

Pachycephalosaurus had a narrow face with a small muzzle which ended in a pointed beak. It also had a dome-shaped head which was very thick, measuring more than 9 inches thick which, along with the snout, was covered by bumps and wart-like knobs with a fringe of dull spikes measuring 5 inches tall.

It is thought that Pachycephalosaurus would have been similar to other ornithopods having short forelimbs with five-fingered hands and three-toed feet. Additionally it would likely have had a long heavy and rigid tail and a fairly short yet thick neck which it would have held in an “S” or “U” shaped curve.

Physical Attributes of Pachycephalosaurus

Though it had a large head Pachycephalosaurus had a small brain giving it rather low intelligence compared with other dinosaurs from the same time, though it is thought it had a good sense of smell.

Palaeontologists once thought that male Pachycephalosaur rammed each other with the top of their thick skulls, in battles over dominance as modern day rams and goats do with their horns. However it is now thought that ramming each other in this way would have resulted in a broken neck killing the dinosaur as its neck wouldn’t have been strong enough to survive such collisions. Additionally, the thick skull that was once thought to be rigid and solid has now been discovered to be porous and fragile when put under extreme pressure.

Instead of head-butting it is now thought that these herding dinosaur’s used their thick skull to show who was the dominant male and also perhaps to flank-butt a rival in intrasectoral combat.  In this theory a Pachycephalosaurus may have stood parallel to or faced its rival in an attempt to intimidate it. If this failed, Pachycephalosaurus would have bent its head downwards and charged, striking the rival on its flank.

What did Pachycephalosaurus Eat?

This dinosaur was most likely a herbivore that lived off of a diet of low-growing berries, seeds, leaves and other soft plant vegetation as well as perhaps insects. Because of its very small yet sharp serrated teeth it is thought Pachycephalosaurus would have been able to shred plant matter effectively, yet wouldn’t have been able to easily chew tough fibrous plants. It is unlikely that this dinosaur was a meat-eater although it’s possible that Pachycephalosaurus could have been an omnivore, eating both plants and meat. Until more evidence of this species is found however, it is assumed that this dinosaur was most likely herbivorous.

How did Pachycephalosaurus Move?

It is thought that Pachycephalosaurus would have been a rather slow dinosaur but because of the lack of skeletal remains it’s difficult to know exactly how Pachycephalosaurus would have walked and run. Palaeontologists believe that this was a bipedal dinosaur, meaning it walked on its two back legs, but instead of walking upright it likely held its back level to the ground.

Where did Pachycephalosaurus Live?

Thought to have lived in small groups, Pachycephalosaurus would have lived in coastal and upland regions in what is now the Western U.S.A around the states of modern-day Montana, South Dakota and Wyoming. All of the fossils of this dinosaur to date have been recovered from the Lance Formation and Hell Creek Formation.

Pachycephalosaurus co-existed alongside what were perhaps juvenile Pachycephalosaurus; Dracorex and Stygimoloch. Other dinosaurs around at this time and in the same location would have included Triceratops, Ankylosaurus and Thescelosaurus as well as Tyrannosaurus Rex and Albertosaurus which would have preyed on Pachycephalosaurus.

The Discovery of Pachycephalosaurus

A skull and several thick skull roof fragments have been discovered so far for Pachycephalosaurus with remains excavated in the Western U.S states of Montana, South Dakota and Wyoming. No skeletons have been discovered so the anatomy of this dinosaur is relatively unknown and palaeontologists have to rely on estimates based on the skull remains and similar dinosaurs of the same time period.

P. wyomingenis is the type species name of Pachycephalosaurus and is the only known and valid species today. The previous species of P. grangeri and P. reinheimeri have been considered specimens of P. wyomingensis since 1983.

The species was named in 1931 by Charles Gilmore though was originally assigned to the family Troodontidae (T. wyomingensis). It wasn’t until 1943 that it was assigned to the Pachycephalosaurids family after newer and more complete material was established.

Fossils attributable to Pachycephalosaurus (originally thought to have been, and called, Tylosteus) were found during the late 1850’s when Ferdinand Vandiveer Haydon collected a bone fragment near the head of the Missouri River, an area which today is known as the Lance Formation in South-East Montana. In 1938 William Winkley also discovered Pachycephalosaurus remains on his family ranch in Montana.

The first remains found of Pachycephalosaurus in the 1800’s were described by Joseph Leidy over 20 years later in 1872 as “belonging to the dermal armor of a reptile or an armadillo-like animal.” It’s actual nature was not found until a century later when Donald Baird restudied the piece and identified it as a bone known as a squamosal, belonging to Pachycephalosaurus,  from the back of the skull.

However in 2006 Robert Sullivan suggested that the squamosal is more similar to the bone of Dracorex than Pachycephalosaurus. It has since been proposed that Dracorex represents a juvenile Pachycephalosaurus.

Pachycephalosaurus remains have so far only been found in the Western United States though other Pachycephalosaur remains have been found as far afield as Mongolia, Madagascar and the Isle of Wight in the UK as well as in Canada. This suggests that this family of dinosaurs roamed far and wide during the Cretaceous period.

 The Importance of Pachycephalosaurus

 Pachycephalosaurus is important because it was one of the last non-avian dinosaurs to have existed. Though little is known for sure about the species of P. wyomingensis due to a lack of skeletal remains, Palaeontologists have been able to see how this sub-family of dinosaur evolved. Additionally through study of similar remains being found from the U.S.A and Canada to as far away as Mongolia, researchers can estimate the range of unique species similar to Pachycephalosaurus.