The Dinosaur With a Bump on Its Head – 12 Pachycephalosaurus Facts

The Pachycephalosaurus (pronounced Pack – ee – sef – a-la – sore – us) may not be as famous as the Pterodactyl or the T Rex, but this dinosaur has been a staple of pop culture for decades and has plenty that makes it stand out – most notably its unique dome shaped skull.

This hard-headed creature lived in the late Cretaceous period about 76-65 million years ago. They mostly roamed around North America, and are known by many simply as the dinosaur with the bump on its head. Want to know more about this head-butting, bipedal beast? Read on!

1. Pachycephalosaurus means ‘thick headed lizard’.

The first Pachycephalosaurus fossil was found in 1859 by a man called Donald Baird who collected bone fragments around the Missouri River in Montana, America. However, it was nearly a century later when it was given its name in 1942. Looking at its skull, it’s easy to see how it got its name, but we actually haven’t found many Pachycephalosaurus fossils at all and many are just fragments of bones, so the ones we do have are very special!

2. The Pachycephalosaurus was bipedal, which means it walked on two legs.

Like humans, the Pachycephalosaurus walked on its hind legs. Whilst these back legs were very strong and muscular, its upper forelimbs were a lot smaller, shorter and less developed.

3. The Pachycephalosaurus was a herbivore – we think.

It’s long been believed that the Pachycephalosaurus was a herbivore and it has been depicted as such in pop culture for many decades, but scientists are now wondering whether it might have actually eaten both plants and meat.

Whilst their skulls show broad, leaf-shaped teeth toward the back of their mouth which are most suitable for eating plants, the most complete Pachycephalosaurus jaw ever found also has sharp, blade-like teeth towards the front, much more common in carnivorous animals. We may never know for sure, but they might have actually been omnivores!

4. The Pachycephalosaurus had a dome-shaped skull.

The Pachycephalosaurus is probably most famous for the unique shape of its hard, thick skull, shaped almost like kneecaps. In fact, before we knew it was a skull, plenty of people actually mistook them for the kneecap of other dinosaurs!

The skull of a Pachycephalosaurus. Credit to – user Ballista from the English wikipedia., CC BY-SA 3.0, via Wikimedia Commons

5. Scientists can’t agree on what they used their skull for.

There are many possible reasons for the shape, size and thickness of the Pachycephalosaurus skull, the main hypothesis being that they used their skull for head-butting each other over status and territory. However, some scientists think that the shape and design of their skulls suggest it was more for display and recognition. Maybe the Pachycephalosaurus just liked to look good to attract a potential mate!

6. The bone protecting their brain was incredibly thick.

Backing up the hypothesis that they used their skulls for fighting, the bump on their heads were around 9 inches (23cm) thick. For context, that’s 20 times thicker than a regular dinosaur skull! However, whilst the domed head may be the most famous part of their skull, they also had horns on their snouts and round the back of their head. That’s a lot of bumps on one skull!

7. It was the largest of the dome-headed dinosaurs.

It’s always possible we could find more species, but of all the known dome-headed dinosaurs, the Pachycephalosaurus is the largest. They’d grow to around 5 metres long (16 feet) and just under 2 metres high (around 6 feet), weighing around 800-1000 pounds. They may not have been the biggest animal around, but they’d certainly cause a stir walking around the local supermarket!

The estimated size of a Pachycephalosaurus. Credit: Megaraptor-The-Allo, CC0, via Wikimedia Commons

8. The Pachycephalosaurus had a third eyelid which they could still see through.

Many animals have third eyelids, but not many have clear ones. The Pachycephalosaurus had a third eyelid called the nictitating membrane which not only protected the dinosaur’s eyes, but because it was clear it still allowed them to see. Sharks, who have remained almost unchanged for millions of years, also have a nictitating membrane!

9. They share ancestors with the triceratops.

Like the triceratops, the Pachycephalosaurus belongs to a group of dino’s called the Marginocephalia (fringed headed), a group of different species that likely shared a common ancestor, resulting in a group of dinosaurs with a shelf at the back of their skull. Whilst the triceratops had a huge one, the Pachycephalosaurus’ skull is less pronounced.

10. Their main predator was the famous T-Rex.

They might have been big compared to other dinosaurs with a domed head, but a Pachycephalosaurus would have been less than half the size of a Tyrannosaurus rex! Both these dinosaurs dwelled in North America in the same period, so they will have been potential pray. But it wasn’t just the T-Rex. Allosaurus, Carnotaurus, and Velociraptors were all predators around that time too.

11. The Stygimoloch dinosaur might actually be the Pachycephalosaurus.

The Stygimoloch dinosaur was always considered as separate to the Pachycephalosaurus, but Dr. John R. Horner (who was the dino consultant on the Jurassic Park trilogy!) believes that actually, the Stygimoloch was actually the juvenile form of the Pachycephalosaurus. We all change with age, but it’s pretty rare that a juvenile skeleton might be so different to its adult self that scientists thought it was a whole new species!

12. They went extinct during the famous asteroid impact – ouch!

Living in the late Cretaceous period, the Pachycephalosaurus was part of the last group of dinosaurs ever to roam the Earth. 66 million years ago, an enormous six-mile wide asteroid called Chicxulub hit the waters of what we now call Mexico and wiped out 75% of all species! Whilst the Pachycephalosaurus was an unfortunate victim of this mass extinction, this does mean that the fossils we have are some of the most modern dinosaur remains!

The Pachycephalosaurus may be best known as the dome-headed dinosaur with a think skull, but it’s so much more than that! Not only did it live alongside some of the most famous dino’s in pop culture, much of its lifestyle and history remains a mystery today. But whilst that makes it difficult for scientists to firmly say too much about the Pachycephalosaurus, it also means there is potentially so much more to discover in the future!

Pachycephalosaurus FAQs

How thick is Pachycephalosaurus skull?

The bump on the heads of the Pachycephalosaurus were around 9 inches (23cm) thick. For context, that’s 20 times thicker than a regular dinosaur skull!

Did Pachycephalosaurus eat meat?

It was believed that the Pachycephalosaurus was a herbivore, but scientists are now wondering whether it might have actually eaten both plants and meat. Their skulls show broad, leaf-shaped teeth toward the back of their mouth which are most suitable for eating plants, but they also have sharp, blade-like teeth towards the front, much more common in carnivorous animals.

Did Pachycephalosaurus live with the T-rex?

Yes, the Pachycephalosaurus existed at the same time as predators such as the T-Rex, Velociraptor and Allosaurus, as well as herbivores such as the Triceratops.

Which dinosaur had the thickest skull?

The Pachycephalosaurus had the thickest skull, which was 9 inches (23cm) thick.

What dinosaur hits with his head?

The Pachycephalosaurus is known as the dinosaur that hits with its head, or the dinosaur with a bump on its head. How much headbutting the Pachycephalosaurus is up for debate, as some scientists believe that they would only be able to head butt once before experiencing significant damage.