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What Was The Dinosaur With The Bump On Its Head? The Hard-Headed Dinos

Imagine you are hunting fossils, and you find a fossil skull, some rib bones, and thigh bones. As you chip away the dirt and rock from the skull, to your amazement, you find that it has a well-defined bump on the top of it as well as cone-shaped spikes and knobs surrounding it, almost in the shape of a crown. What dinosaur could this be? You check the rest of the fossil, and nothing really out of the ordinary stands out. After examining the skull closer, you find that the bump most likely is made of bone, several inches thick!

So, what was the dinosaur with the bump on its head? Dinosaurs with dome-shaped bumps on their heads are Pachycephalosaurs. It’s a group of dinosaurs that had species such as Pachycephalosaurus and Stegoceras. The pronounced bump is made of bone and sometimes has spikes and knobs surrounding it and was up to 9 inches thick. Some fossils Pachycephalosaur fossils have been found in North America, Mongolia, and Europe. They lived during the Cretaceous period.


It’s interesting to point out that the Pachycephalosaurs did not have its spikey, knobbed head just for show. Some fossil evidence points to combat with other Pachycephalosaurs or other dinosaurs, perhaps even predators. 

It might be similar to combat behavior that bighorn sheep have. Read on to find out more about this dinosaur group, their amazing skull structures, and habitats where they lived. I’ll also take a deeper look at a few of the notable species in this group. Let’s get started.


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How Thick is a Pachycephalosaurus Skull? How Tall is a Pachycephalosaurus?

Pachycephalosaurs are most noted for the large bump on their head, but as they were different species, there were also different skull shapes. A few species had small bumps and cone-shaped spikes, but the head was flat, while others just had a large dome bump on its head. The anatomy of these dome-headed dinosaurs was also a little bit different depending on the species.

Photo Credit: Dracorex skeleton restoration, The Children’s Museum of Indianapolis by Daniel Schwen (CC BY-SA 4.0)

The Skull of the Pachycephalosaurs

Fossil skulls tend to fossilize well because the bone is considerably thicker than some smaller bones. When it comes to Pachycephalosaurs: 

● The skull on the dome or bump was so thick that it often was the only part of the dinosaur that survived. 

● The thickness of the bump was different depending on the species. In the Pachycephelosaurus wyomingensis, the skull was 9 inches (22.8cm) thick. 

● In others, it was only 3-6 inches thick. Nevertheless, the bump thickness was thicker than other skulls of similar-sized dinosaurs.

● Many of the fossilized skulls of Pachycephelosaurs had cone-shaped spikes and bumps circling the bump, which made it look like a crown. 

YouTube Video Testing How Hard a Pachycephalosaurus’ Head Was

YouTube Video by BBC Earth Unplugged that shows the force and strength of a Pachycephalosaurus. Massive power shown in the tests!

● Interestingly, some of the bumps and spikes extended from the dinosaur’s snout towards the dome bump. These rows of bumps and spikes were found, for instance, on the narrow skull of Amtocephale gobiensis.

● There were also Pachycephelosaurs with quite the opposite – no spikes or bumps (let’s use the term flat). The dome was still pronounced, but it was void of all bumps and spikes seen on other species. 

● The Tylocephale had the tallest dome of these types of dinosaurs. 

One area of controversy with the Pachycephelosaurs is that some paleontologists suspect that dinosaurs with fewer bumps and spikes on the skull were juvenile or sub-adult Pachycephelosaurs and not a separate species. 

Unfortunately, there is not enough evidence to prove either side conclusively until more fossils are found, but the argument and research done in 2016 by Evans and Goodwin present a good case. (Source)

The Anatomy

Pachycephelosaurs were bipedal with short forearms. They grew to be 16ft (5m) long. The height of the Pachycepholosaurs could reach 5ft-7ft tall, based on the reconstruction of North American fossils.

The neck of the dinosaur was short and thick. Some scientists believe it was this way because it lessened injury for the dinosaur. It did not have a long tail, but was rather short and enabled better balance when on its hind legs. 

Photo Credit: Stegoceras skeletons, Royal Tyrrell Museum, Alberta, Canada by Sebastian Bergmann (CC BY-SA 2.0)

Stegoceras was a dome-headed dinosaur that roamed in North America. It’s fossils were found in Alberta, Canada. The skull did not have spikes or bumps and it was a smaller species compared to Pachycephelosaurus that was found in Wyoming.

These dinosaurs are suspected to be herbivorous (plant-eating), based on its habitat. However, due to the shape of its teeth, it might have been omnivorous (eats plants and meat). More on this later when I cover habitat.

Table of Pachycephelosaurs

In the table below, I list out several Pachycephelosaurs starting with the name of the dinosaur in the first column, and a brief description of about it in the second column. In the third column, the skull type is described and I indicate if it was flat or domed, as well as if there were bumps or spikes on the fossils skulls found. Lastly, I indicate where the fossils of the Pachycephalosaurs are found.

Table 1 - List of Pachycephalosaurs and Fossil Information

Dinosaur NameDescriptionSkull TypeWhere Fossils Found
Wannanosaurus yansiensisConsidered a basal or primitive PachycephalosaurFlat, NO bumps or spikesAnhui, China
TexacephaleSkull found with evidence of stress from headbuttingDome, NO bumps or spikesTexas, USA
PachycephalosaurusSome scientists consider Dracorex, Tylosteus, and Stygimoloch to be juveniles Dome WITH bumps and spikesMontana and South Dakota, USA; Alberta, Canada
Colepiocephale lambeiFound in 1945, its name means "knucklehead"Dome, WITH bumps, NO spikesAlberta, Canada
Stegoceras validumSkull and skeleton found nearly complete, considered to be basal or primitiveDome, NO bumps or spikesAlberta, Canada
Acrotholus audetiPredates the flat headed Pachycephalosaurs in AsiaDome, NO bumps or spikesAlberta, Canada
Prenocephale prenesSimilar to Stegoceras. Considered to possibly be omnivorous, eating mostly fruit and leavesDome WITH small bumps and spikesMongolia
Alaskacephale gangloffiDinosaur was half the size of P. wyomingensisDome WITH small spikes surrounding the domeAlaska, USA
Amtocephale gobiensisOne of the oldest PachycephalosaursDome, NO bumps or spikesMongolia
Foraminacephale brevisThere were grooves between the middle lobe and side lobesDome, NO bumps or spikesAlberta, Canada
Dinosaur NameDescriptionSkull TypeWhere Fossils Found

Is the Dracorex a Real Dinosaur? Classification of Pachycephalosauria and Habitat

Paleontologists have classified more than 15 types of Pachycephalosaurs, which has given much insight into how these dinosaurs lived and survived in different habitats. 

The Dracorex is a real dinosaur and lived in what is now South Dakota in the US. Continue reading to find out more how it was classified and the habitat it lived in.

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Classification and Clades of Pachycephalosaurs

The classification of Pachycephalosaurs plus modern clades can give us insight into this type of dinosaur. Pachycephelosaurs is from the branch of Ornithischia. In the last 10-15 years, there has been some restructuring of the clades and specifically where this branch belongs, and so I’ll use the cladogram from Evans (2013)

From the clade Cerapoda (species of ceratopsids) it contains the sub-clades of Orinthopoda and Marginocephalia. Pachycephelosaurs are grouped under Marginocephalia.

Figure 1 below shows a simplified (and slightly re-arranged) cladogram by Evans (2013) that lists out the species of Pachycephalosauria.

Figure 1 – Cladogram of Pachycephelosauria from Evans (2013) – AdventureDinosaurs

Three Species of Pachycephalosaurs – Or Is It Just One?

As mentioned previously, they now suspect the Dracorex and Stygimoloch to be juvenile or sub-adult versions of Pachycephalosaurus. In recent years there has been controversy and a bit of pushback with this idea. Let’s take a look at specific fossils, as I think each of these fossils is fascinating.

Pachycephalosaurus wyomingensis

Discovered in 1931 in Wyoming, this Pachycephalosaurus species had a series of misinterpretations from previous fossil discoveries dating back to the 1850s. It is the biggest fossil specimen of Pachycephalosaurs.

Estimated to be 14.8 feet (4.5 m), it has the recognizable dome-shaped skull with cone-shaped spikes and bumps on its snout and along the outer edges of its dome. This dinosaur is considered to be a full adult fossil skeleton and recent discoveries have added to the knowledge of Pachycephalosaurs.

Dracorex hogwartsia

The Dracorex has such a cool name, and there is a story behind this. It’s fossil remains were donated to a children’s museum in Indianapolis, and the kids were given a chance to name it. They named it after the Harry Potter movies.

Interestingly, the Dracorex is said to look very similar to the mythical dragon without wings. Check out my article on this blog about the difference between dinosaurs and dragons if you are interested to know more: What’s the Difference Between Dinosaurs and Dragons

As mentioned, the Dracorex is considered by some paleontologists to be a juvenile of the Pachycephalosaurus wyomingensis and visually you can see the resemblance and even the growth path as outlined by the research studies.

Photo Credit: Pachycephalosaurus -01 by Kabacchi (CC BY 2.0)

Stygimoloch spinifer

The Stygimoloch fossils were found in Montana, USA, and lived during the late Cretaceous period. Its name references its appearance of being a demon, from the river Styx in Greek mythology.

The skull of the Stygimoloch was very distinctive of a Pachycephalosaur having the dome bump on his head, the cone-shaped spikes along its snout and bump, and to add to this, it had spikes at the back of his head. The Stygimoloch lived during the Cretaceous period.

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Where Were Pachycephalosauria Fossils Found?

Pachycephalosauria fossils have been found in North America, Europe, and Asia. Most of these fossils date from the late Cretaceous period.

Fossil deposits in North America have been found in Wyoming, Montana, South Dakota, and Canada (Alberta). The fossils in North America are unique because a few of them, such as the ones found in Montana and Wyoming, have been comparatively complete, and this gives paleontologists a better picture of the dinosaur and the species in general.

In Europe, fragments of a young Pachycephalosaur was found in the Isle of Wight. (Source)

Skull fragments of a Pachycephalosaur were found in Madagascar; however, there seems to be some debate because the skull fragments cannot be fully confirmed. (Source

Mongolia is another place where fossil skeletons of Pachycephalosaur have been found. The skulls of these dinosaurs are more narrow and have less bony spikes and bumps.

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Habitat Where Pachycephalosauria Lived

Based on the fossils of where Pachycephalosauria were found, we can try to get a glimpse of where these dinosaurs lived and the kind of habitat they had.

Pachycephalosaurs lived in different habitats, from mixed forests to arid deserts and floodplains – AdventureDinosaurs

In North America during the late Cretaceous period, and what would be current day Wyoming, South Dakota, Montana in the US, and Alberta, Canada – this geographic area would have seas and floodplains, waterways, and arid deserts. In the late Cretaceous period, there were also mixed forests in present-day North America.

In Mongolia, the habitat was much different in the late Cretaceous period than it is today. Today it contains the Gobi desert and hot arid deserts as well as grasslands. Back then, it would have been woodland forests and a combination of deserts and mountainous regions.

Pachycephalosaurs are mostly considered to be herbivores, plant eaters that would eat vegetation. It makes sense that they would live in these areas, and scientists suggest that these dinosaurs were also migratory. (Source)

Paleontologists suggest Pachycephalosaurs bumped heads and fought during mating season – AdventureDinosaurs

Behavior and Combat

There is a lot of speculation about the behavior of Pachycephalosaurs, given that most of the species have a large dome-shaped bump on their head. Did they headbutt each other like modern-day rams and other animals that have been seen in the wild? What was the purpose of the thick-domed skull, if not? Let’s take a closer look at headbutting and the possible behaviors Pachecephalosaurs might have had.

Combat with Other Pachycephalosaurs

If we look at modern-day animals that fight each other and often headbutt, here are some of the main animal species that have this behavior:

● Bighorn sheep
● Rams
● Goats and mountain goats
● Bison
● Duiker (an African deer)
● Deer and Reindeer
● Moose and Elk

YouTube Video – Watch Bighorn Sheep Headbutting

YouTube video by WildlifeOnVideo which shows how bighorn sheep battle. It’s not too difficult to imagine Pachycephalosaurs battling each other in similar ways

In many cases with these animals, they fight and headbutt each other during mating and selection of a mate, mark territory, and establish or challenge the pecking order. The fighting is not just head-on headbutting. It also includes head shoves at close range and headbutts and shoves to the neck and flank.

Interestingly, the Duiker, a small African deer, has a head shaped similarly like a flat Pachycephalosaur. Although it does not headbutt head-on as frequently as seen in bighorn sheep, rams, this African deer engages in fighting in other ways using its head.

Scientists have suggested that Pachycephalosaurs could have fought each other in other ways, such as a head nudge with the side of the head (where the spikes and bumps are) and direct headbutts. They also suggest that adults may have fought differently than juvenile or sub-adult Pachycephalosaurs.

In one study, scientists examined 102 dinosaur skulls and found 23 lesions (injuries that may have healed), indicating that they did fight and headbutt each other. 

Behavior of Headbutting in modern-day animals and Comparison with Pachycephalosaurs

How do modern-day animals headbutt? Let’s briefly compare those animals to get an idea of what a Pachycephalosaur might do and the behavior it could have had. 

According to a study held on bighorn sheep, mountain goats, and bison (Source), these animals fought and charged each other differently. It was observed that bighorn sheep headbutted each other head-on while mountain goats target the flank, and bison use their horns to fight.

Battle of two Pachycephalosaurs - AdventureDinosaurs
Shoulder and side-bumping is thought to be one way Pachycephalosaurs fought with one another. Direct headbutting may also be a behavior too – AdventureDinosaurs

The study then compared Pachycephelosaurs and their skulls, looking for injuries. It found similarities in mountain goat and bison injuries. It may indicate that Pachecephelosaurs engaged in side-bumping and wrestling like goats and bison. 

Was the Pachycephalosaurus an Omnivore?

Pachycephalosaurs (including Pachycephalosaurus) are mostly considered to be herbivores, plant eaters that would eat vegetation. It makes sense that they would live in these areas, and scientists suggest that these dinosaurs were also migratory. (Source)

However, some paleontologists believe that the Pachycephalosaurs were omnivores (eating plants and meat). The reasoning behind this is based on the shape of the teeth in the jaw. There is a combination of flat teeth for grinding vegetation and sharper serrated incisors for tearing flesh. If they were omnivores, they would eat lizards, frogs, smaller dinosaurs, and a mix of vegetation growing in the habitat. (Source)

Final Thoughts

If a dinosaur walked around with a high dome-shaped bump on its head, it must be used for something. Add to that some horns and cone-shaped bumps around the dome, and that is one peculiar-looking dinosaur. 

Through our discussion of Pachecephelosaurs, we can see that there was more to these dinosaurs than just a peculiar look. 

It’s amazing that as we continue to find more and more fossils of these dinosaurs, our knowledge grows and adjusts. The verdict is still out as to whether a Dracorex or a Stygimoloch or a flat-headed Pachecephelosaur is a young version of a fully high-domed crown with spikes and bumps. 

Perhaps with the next fossil discoveries, we will get a better understanding. It sure is exciting to look at these skull fossils and compare them.

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