Pachycephalosaurus Guide | Ancient Beasts

"The Pachycephalosaurus was a wonderful dinosaur with one of the hardest heads. Check out our genus guide to learn more about them here!"

Pachycephalosaurus, also known as Pachysaurus for short, was an Ornithischian dinosaur that lived during the Late Cretaceous Period. Its name means thick (pachys) headed (kephale) lizard (sauros). Its remains were mostly found in North America, a region where it resided during that time.

In this article, we’ll cover a few topics on Pachycephalosaurus. Besides a general overview, we’ll discuss when Pachysaurs were discovered, which dinosaurs lived with them, and in which games you’ll find them. Also, you’ll see the chief characteristics of this genus.

Table of contents


Overview of the Pachycephalosaurus genus

The only discovered species of this genus that scientists have studied thus far is P. wyomingensis. Out of all the characteristics that we’ll discuss later, the domed head of Pachycephalosaurus is the one that probably represents it the best. Many children and adults instantly recognize the dinosaur by this feature alone.

Having lived during the Late Cretaceous Period, it was among the last non-avian dinosaurs when the Cretaceous–Paleogene extinction event occurred. Some studies have equated the Pachysaurus with Tylosteus, Stygimoloch, or Dracorex genera. While some features are the same and some suggest they belong to the same family, many paleontologists believe they are separate entities.

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Pachycephalosaurus classification

Kingdom: Animalia

Phylum: Chordata

Clade: Dinosauria

Order: Ornithischia

Suborder: Pachycephalosauria

Family: Pachycephalosauridae

Tribe: Pachycephalosaurini

Genus: Pachycephalosaurus


Pachycephalosaurus Fact Card



When was Pachycephalosaurus discovered?

There have been quite a few discoveries of Pachycephalosaurus remains over the years, starting in 1859. However, it’s been a matter of contention between scientists and paleontologists, especially to which genera the fossils belong to. Here are some details of the finds.

The first discovery determining the skull’s nature (1859 – 1890)

Ferdinand Vandeveer Hayden was a geologist that was well-known for his Rocky Mountain expeditions in the 19th century. While collecting fossils along the Missouri River between 1859 and 1860, he found an interesting bone fragment. Today, that Montana area is known as the Lance Formation.

It wasn’t until 1872 that Joseph Leidy determined that what we call specimen ANSP 8568 was either part of an armadillo-type creature or an armored reptile. Scientists called it Tylosteus even though no one took the time to study the nature of the fossils.

That all changed in 1985 when Donald Baird, after performing a thorough investigation, identified the bone fragment as belonging to the back of a dinosaur’s skull. He also discovered bony knobs belonging to the skull. Based on his findings, he made a petition that specimen ANSP 8568 be attributed to Pachycephalosaurus instead of Tylosteus.

That wasn’t the end of the confusion, though. In 2006, Robert Sullivan had another look at specimen ANSP 8568 and suggested that perhaps it belonged to Dracorex instead of Pachycephalosaurus. However, some paleontologists believe that Dracorex may have been a juvenile form of Pachysaurus.


The Bone Wars: Stygimoloch, Stegosaurus, or Pachysaurus? (1890 – 1892)

The Great Dinosaur Rush, or the Bone Wars, was an incredible time of fossil hunting in America. While Edward Drinker Cope and Othniel Charles Marsh were racing to discover the most dinosaurs at that time, some other discoveries were being made. John Bell Hatcher was one of Marsh’s collectors that collected a skull bone fragment in 1890, officially recorded as  specimen YPM VP 335.

It was also found in the Lance Formation. While it was later defined as part of Stygimoloch spinifer, Marsh initially believed the fragment was a spike normally seen on Stegosaurus. However, after some consideration, he stated it was probably body armor that belonged to Triceratops in 1892. 

Hatcher also found other bones with a specific tooth as part of that collection, specimen YPM 4810. Marsh believed it belonged to an Ankylosaur dinosaur named Palaeoscinus latus. It wasn’t until 1990 that Coombs determined that the tooth most probably belonged to a Pachycephalosaurus. That means that the skull fragment found by Hatcher might also have belonged to this genus.

Establishing Pachycephalosaurus as an official genus (1931 – 1945)

Charles W. Gilmore gained fame as a paleontologist who examined fossils for the United States National Museum. In 1931, Gilmore studied specimen USNM 12031, a skull that he believed belonged to Troodon wyomingensis. There was contention at the time with the similarities in teeth between Troodon and Stegoceras specimens, as the teeth looked very similar. 

Erich Maren Schlaikjer and Barnum Brown appeared in 1943 with newer specimens from the Hell Creek Formation of Ekalaka in Montana. They determined that the skull and remained formed two species, Pachycephalosaurus grangeri and Pachycephalosaurus reinheimeri, creating the new Pachysaur genus. 

It wouldn’t be until 1945 when Charles M. Sternberg corrected Troodon wyomingensis to Pachycephalosaurus wyomingensis as part of the genus. Since then, many paleontologists believe that the previous two species discovered in 1943 were actually part of this singular species, which is why there is only one officially recorded species for the Pachysaur genus.


Which characteristics define Pachycephalosaurus?

With some basic information about the Pachysaurus out of the way, it’s time to go into detail about the chief characteristics. We know you all want to learn more about the dome, but there are other traits you may want to know about. Here are some of the traits that paleontologists have studied and discovered thus far. We’ll update these should more information come to light.

Growth and development

The Lance and Hell Creek Formations are two locations where Pachycephalosaurus fossils were predominantly. There were mostly skulls in what was considered female and juvenile Pachysaurus dinosaurs due to the size and shape of the remains. However, there’s also a debate as to whether these belonged to Dracorex or the more probable Stygimoloch. 

From the knobs and tiny horns on the snout and along the dome, it seemed like juvenile Pachycephalosaurus dinos developed them from a young age. They were mostly small stumps. As they grew older, these ornaments became stronger and more pronounced. 

The mighty dome

One of the most popular theories surrounding the Pachycephalosaurus dome was that it was used for combat. Namely, they ran and rammed their heads together in a contest over a female or food, or perhaps even to protect their young. While this sounds like a modern ram, there are newer discussions about what they used their domed craniums for.

Let’s first look at the argument against head-butting. While they could have run horizontally to transmit the stress of the blow, recent discoveries and studies allude to the fact that they had U-shaped necks. It would have been challenging to transmit the stress in that way.

Another issue is that only a recent analysis shows cracks in the skull as evidence of damage possibly done by ramming. However, most discovered skulls are intact with no signs of cracks or injuries that would have been visible if head-butting was a frequent event.

Some paleontologists have issue with the curve of the dome. If Pachycephalosaurus did slam their heads together, they would have glanced off one another instead of having a direct impact. 

One theory presented in 1978 and expanded in 1997 was a concept called flank-butting. Besides using the domed crown as intimidation, it might have run alongside a rival and butted its flank instead. The U-shape of the neck supports this theory, as the Pachycephalosaurus would have bent its head down to strike at the flank.

A further study in 2012 proposed that there were some cases where the dome was used for what was called agonistic combat or behavior. What this means is that the Pachycephalosaurus only fought with its head in specific situations. Some of these included protecting families, feeling threatened, or putting on a display of power. Due to bone infections discovered within skull remains, it appears it was less evidence in females and juveniles.

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Diet and teeth

Pachycephalosaurs had small, blade-like teeth with serrated edges. Due to the size of these teeth, paleontologists are confused as to what they actually ate. They weren’t strong enough to tear and chew shrubs with flowers or plants made from tough fiber. Instead, scientists believe they may have opted for soft leaves, fruit like berries, or seeds. 

There’s also a theory that it may have eaten meat on some occasions. The shredding nature of the teeth was similar to some carnivore theropods. If this was the case, it could be that they ate small insects or animals running around during the Late Cretaceous Period. However, the popular belief is that they were herbivores instead of carnivores or omnivores.

Paleoecology: Which dinosaurs lived with Pachysaurus?

Living in the Late Cretaceous Period, there’s a good chance it lived with other pachycephalosaurids at the time. One such genus was Sphaerotholus. If Dracorex and Stygimoloch weren’t simply female or juvenile Pachycephalosaurus dinosaurs, then they probably dwelled among them too.

Looking outside this family, here’s an idea of the types of dinosaurs you might have seen living in the same areas at the same time with Pachycephalosaurus:

  • Hadrosaur: Edmontosaurus, Parasaurolophus
  • Ceratopsian: Nedoceratops, Triceratops, Leptoceratops, Torosaurus, Tatankaceratops
  • Ankylosaur: Ankylosaurus
  • Nodosaur: Denversaurus, Edmontonia
  • Theropod: Acheroraptor, Ornithomimus, Dakotaraptor, Struthiomimus, Pectinodon, Anzu, Leptorhynchos, Richardoestesia, Paronychodon, Tyrannosaurus

Of course, Pachycephalosaurus may have seen many of these as threats, especially the theropods. It depends on how hungry the carnivores were and if they considered these smaller dinosaurs worthy of their appetites. With the remains found together at the two formations, it may have been that the Pachys lived together to protect their tribes from these ferocious meat eaters.


Games and Movies that feature Pachycephalosaurus

The Pachycephalosaurus dinosaur is incredibly loved by movie and game fans worldwide. There are many games where the genus is included. In some of them, you can tame and ride them or change their skins with breeding. They presented enjoyable experiences with these dinos.

Here are the top games and movies that feature Pachycephalosaurus:

  • Almost every Jurassic Park or World game or movie: There’s no doubt that the Pachy was popular in the Jurassic game and movie franchise. Some titles where you could see it included Jurassic World Evolution, LEGO Jurassic World, Operation Genesis, Warpath, and many many more. 
  • ARK Survival Evolved: Pachycephalosaurus is probably one of the cutest dinosaurs in this game. You find many of them running around, sometimes butting predatory enemies. You can even tame them, pop on a saddle, and ride across the wilderness.
  • Cell to Singularity: Here’s a mobile game not many know about. While it mainly looks at the evolution of Earth since the Big Bang, there’s a Mesozoic path that deals with dinosaurs. One of them you unlock at a later stage is the Pachy.
  • Carnivores: Dinosaur Hunter: In this game, you need to find or make weapons and hunt dinosaurs around you. While this is sad in some way, one of the dinos you’ll find is the Pachysaurus.
  • Dinosaurs Decoded: This television documentary evaluated many myths about dinosaurs that we believed, especially about which species really existed. During one of the later shows, it looks at Pachycephalosaurus and the possible skin colors it had.
  • Saurian: This video game let you live a simulated live of a dinosaur. You awoke from an egg in the Hell Creek area, learning how to survive and become strong. One of the last dinos added late in the game was Pachycephalosaurus, which you could play as one of the bonus levels.

One of the seven major dinosaur groups

Did you know that Pachycephalosaurus is actually one of the seven major dinosaur groups? We’ll cover these more in a future article. For now, we just want to highlight that it’s one of the five ornithischian groups listed as significant in the dinosaur kingdom.

The seven major dinosaur groups:


Top questions about Pachycephalosaurus

Before you leave us, we want to make sure you know all there is to know about the Pachysaurs. We’ve collected the top questions people always ask about Pachycephalosaurus and did our best to provide quality answers. If you want to know anything else, be sure to ask us so we can answer your questions in our Ancient Beasts Academy.

What dinosaur has the hardest head?

While there are many larger dinosaurs, Pachycephalosaurus certainly had one of the hardest heads relative to the rest of their bodies. The impact was quite hard, dealing damage to opponents at flat-out speed. However, there’s still quite a debate about what the hard crown was used for, other than fighting.

It would have been interesting to see if Tyrannosaurus rex could have bitten into that hard skull with its massive bite force. Despite how hard the Pachycephalosaurus head was, some believe it was for decoration or to attract mates with the knobs and small horns.  

Did Pachycephalosaurus live with T Rex?

Since Pachycephalosaurus lived during the same time as Tyrannosaurus Rex, namely the Late Cretaceous Period, they certainly lived together. How much they saw each other is another question. Most of the Pachycephalosaurus remains found what looked like female and juvenile skulls together as if they lived in herds.

Another indication is that T Rex and Pachycephalosaurus remains were found in North America. With the time and location being the same, chances are that they probably did cross paths at some point or another. If the trees and shrubs were are big as we believe, the Pachys could probably have hidden quite well in forests from the tyrannical king of dinosaurs.

How big was Pachycephalosaurus?

Based on the Pachycephalosaurus fossils discovered, they were about 2 meters high. Despite being bipedal ornithischians, they leaned forward horizontally instead of walking completely upright. That means they stretched for about 4 meters, double their height.

If we take these measurements into consideration, they were only slightly taller than the average human. You would have looked up at them unless you’re quite tall yourself. When it came to the juveniles, they would have grown from cat to dog size before reaching adulthood and stretching above you.

Did Pachycephalosaurus actually headbutt?

Scientists are still at odds as to whether Pachycephalosaurus actually collided and rammed their heads in combat. Some fossils show cracks and holes where it was possible they did fight by butting heads. Other theories suggest they did flank-butting instead to save the stress on their skulls, neck, and spine.

The difficulty comes in with erosions over the years. Depending on what killed the dinosaurs, the impact could have caused the cracks found in fossils. For now, the top suggestion is that there were agonistic battles when Pachycephalosaurus felt threatened or wanted to protect the family.

How fast can a Pachycephalosaurus run?

When studying the remains, paleontologists have stated that Pachycephalosaurus can reach speeds up to 15 miles per hour. Some have it on record that they could have gone at 35 miles per hour. This would have been incredibly fast for such a small creature, so we’re going for somewhere in the middle with maybe 20 miles per hour as a top speed.

Tyrannosaurus Rex could reach speeds between 10 and 25 miles per hour. Depending on how hungry the carnivore was, he could probably chase Pachycephalosaurus fast enough to catch one. However, it would have had to contend with that hard crown and ornamental horns on the face. 

Was Pachycephalosaurus the cutest dinosaur?

If you look hard enough online, you’ll find polls, surveys, and lists of the cutest dinosaurs ever found. We’re sure Pachycephalosaurus will be on one of these lists due to how sketch artists have portrayed them. However, the Mei dinosaur is commonly called the cutest dino due to the only fossil found being one that was curled into a ball.

Not many would consider Pachycephalosaurus cute, though. With small horns and knobs over the face and the hard skull, it was more intimidating than pretty. Still, it was smaller than most dinosaurs at the time, and running at full speed would have caught anyone’s attention.

What other dinosaurs did Pachycephalosaurus live with?

Tyrannosaurus Rex wasn’t the only dinosaur that Pachycephalosaurus kept company with in the Late Cretaceous Period. There were several ceratopsians, such as Triceratops and Torosaurus. You would have also found Ankylosaurus around at that time with plenty of carnivorous theropods around.

Perhaps the Ceratopsians actually got along with Pachycephalosaurus. It’s nice to imagine the herbivores living in harmony, like in Jurassic World or ARK: Survival Evolved. We see no reason that Triceratops would butt heads with the Pachys, especially since they probably ate from different food sources.


Final Thoughts on the Pachycephalosaurus

That’s our detailed article on Pachycephalosaurus! I had fun writing it and presenting the facts to you while adding some thoughts of my own. My family loves this stunning dinosaur, specifically the dome head, and watching them headbutt each other in Jurassic Park Evolution. While scientists continue to argue about its classification and existence, we’re content just to admire it.

Picture of Shaun M Jooste

Shaun M Jooste


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