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

Key Takeaways

  • Pachycephalosaurs, known for their domed heads, were related to horned ceratopsians and lived about 100 million years ago.
  • The thick, domed skull of Pachycephalosaurs protected their brain and helped them resist impacts, aiding in hunting large prey.
  • With a short neck, a rounded skull, large eye sockets, and tiny teeth, Pachycephalosaurs’ physical features greatly impacted their lifestyle.
  • More than 15 types of Pachycephalosaurs have been classified, shedding light on how these dinosaurs adapted to different habitats.
  • Pachycephalosaurs’ headbutting behavior for protection and communication is debated due to the lack of scar evidence on fossilized skulls.
  • Paleontologists’ understanding of Pachycephalosaurs evolves with new fossil discoveries, including theories about them possibly being omnivores.

Imagine you are hunting fossils, finding 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 and 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!


What Was The Dinosaur With The Bump On Its Head?

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.

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.

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.

Table of Contents

It might be similar to the combat behavior that bighorn sheep have. Read on to find out more about this dinosaur group, their amazing skull structures, and the 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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Meet the Dinosaur with a Big Bump on its Head – The Pachycephalosaurus

Description 1: The Pachycephalosaurus Was a Herbivore.

Herbivores are animals that eat plants. The Pachycephalosaurus was a herbivore that lived in the mid-Jurassic period to the end of the Cretaceous period. The Pachycephalosaurus had a long snout, large head, and big eyes. The Pachycephalosaurus was a fast and agile animal that could move quickly through the forest floor.

Description 2: The Pachycephalosaurus Lived in The Cretaceous Period.

The Pachycephalosaurus lived during the Late Cretaceous period, which was a warm, tropical period. The Pachycephalosaurus and other animals of the Hell Creek Formation coexisted with other pachycephalosaur species. The Lance Formation and Hell Creek Formation are in Western United States. The climate during the Late Cretaceous period was warm and humid, with a diverse range of vegetation present.

Description 3: The Pachycephalosaurus Was a Bipedal Dinosaur.

Pachycephalosaurus was a large, bipedal dinosaur. Unlike many other dinosaurs of its time, Pachycephalosaurus had a bony dome on its head that was covered in spikes. Not much is known about how Pachycephalosaurus lived, but scientists believe they were omnivorous and had small teeth with noticeable denticles. Pachycephalosaurus was related to the horned ceratopsians and lived around 100 million years ago.

Description 4: The Pachycephalosaurus Had a Thick Skull.

The Pachycephalosaurus had a thick skull, making it one of the toughest dinosaurs. The thick skull meant that the Pachycephalosaurus had a powerful head and could resist impacts and injuries. This made the Pachycephalosaurus a great hunter, as it could easily take down large prey.

Description 5: The Pachycephalosaurus Had a Short Neck.

The Pachycephalosaurus had a short neck which impacted its life. The skull was short, and the eyes were in front, suggesting good vision. The teeth were tiny, and the horns on the back of the head shrank as it grew older. Pachycephalosaurus had a short neck which impacted its life and appearance. Among the pachycephalosaurid (bone-headed) dinosaurs, Pachycephalosaurus probably walked on two legs.

Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short forelimbs, a bulky body, long hind legs, and a heavy tail, which was likely held rigid by ossified tendons. Pachycephalosaurus had been estimated to be about 4.5 metres (14.8 ft) long and weigh about 450 kilograms (990 lb).

Description 6: The Pachycephalosaurus Had a Long Tail.

The Pachycephalosaurus had a long tail that it used for balance and locomotion. The long tail was also used to defend against predators.

Description 7: The Pachycephalosaurus Had Small Arms.

The Pachycephalosaurus had small arms that it used for fighting and hunting. The small arms were used for fighting and hunting and probably helped the Pachycephalosaurus to kill its prey.

Description 8: The Pachycephalosaurus Had Two Clawed Fingers on Each Hand.

Pachycephalosaurus had two clawed fingers on each hand and lived during the Late Cretaceous. They were rare and poorly known among dinosaur groups. Pachycephalosaurus had a large, bony dome on its head which helped protect its brain. The dome was edged with knobs and spikes which may have been blunt.

Only Pachycephalosaurus’s skull remains have been described, so much about the creature is still unknown. The Pachycephalosaurus had a short, rounded skull with large eye sockets. The beak was tiny, and the teeth were shaped like leaves.

Description 9: The Pachycephalosaurus Had Small Legs.

The Pachycephalosaurus had small legs which may have impacted their life in several ways. The small size of the Pachycephalosaurus’ legs may have made it difficult for them to move around, hunt, and escape predators.

This may have caused them to become easy prey for larger dinosaurs and other predators. Additionally, the small size of the Pachycephalosaurus’ legs would have made it difficult for them to travel long distances. This would have limited their access to food and water, which could have impacted their health and survival.

Description 10: The Pachycephalosaurus Had Three Toes on Each Foot.

The Pachycephalosaurus was a bipedal dinosaur with three toes on each foot. It had a small muzzle and a pointed beak, and its head was supported by an “S”- or “U”-shaped neck. The Pachycephalosaurus was famous for its huge dome, up to 25 cm (10 in) thick, and short spikes projecting upwards from the snout. The horns on the Pachycephalosaurus grew and shrank as it grew older, becoming flatter and rounder in shape. Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short forelimbs, a bulky body, long hind legs, and a heavy tail that was held rigid by ossified tendons.

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 looks 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. Visually, you can see the resemblance and even the growth path 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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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

#Species & DescriptionSkull TypeWhere Fossils Found
#1Wannanosaurus yansiensis -Basal or primitive PachycephalosaurFlat, NO bumps or spikesAnhui, China
#2Texacephale - Skull with evidence of headbuttingDome, NO bumps or spikesTexas, USA
#3Pachycephalosaurus - Fossil skulls foundDome WITH bumps and spikesMontana and South Dakota, USA; Alberta, Canada
#4Colepiocephale lambei -Found in 1945, name means "knucklehead"Dome, WITH bumps, NO spikesAlberta, Canada
#5Stegoceras validum - Basal or primitive. Full skeletons foundDome, NO bumps or spikesAlberta, Canada
#6Acrotholus audeti - Predates Pachycephalosaurs in AsiaDome, NO bumps or spikesAlberta, Canada
#7Prenocephale prenes -Similar to Stegoceras. Possibly omnivorousDome WITH small bumps and spikesMongolia
#8Alaskacephale gangloffi -Miniature, half the size of P. wyomingensisDome WITH small spikes surrounding the domeAlaska, USA
#9Amtocephale gobiensis - One of the oldest PachycephalosaursDome, NO bumps or spikesMongolia
Foraminacephale brevis - Found with grooves on skullDome, NO bumps or spikesAlberta, Canada
#Species & DescriptionSkull TypeWhere Fossils Found

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, mixed forests in present-day North America were also mixed.

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

Pachycephalosaurs are mostly considered 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.

Headbutting Behaviors of Pachycehpalosaurs

Pachycephalosaurus and its relatives were likely headbutting equivalents to bighorn sheep or musk oxen. According to other theories, pachycephalosaurs cannot have used their dome heads in this manner. Head-butting behavior has been argued against because of the lack of evidence of scars or damage on fossilized Pachycephalosaurus skulls. The neck was carried in an “S”- or “U”-shaped curve, rather than a straight orientation, which is not fit for head-butting.

The head of a Pachycephalosaurus was shaped differently than other dinosaurs, which may have resulted in different headbutting behaviors. In the case of Pachycephalosaurus and other pachycephalosaurid genera, flank-butting may have been an alternative means of combat.

The headbutting behavior of pachycephalosaurs was likely used to protect vital organs from trauma. Sues proposed flank-butting in 1978, and Ken Carpenter has expanded upon it.A study in 2012 showed that cranial pathologies in a P. wyomingensis specimen were likely due to agonistic behavior.

Osteomyelitis is an infection of the bone which was found in 22% of all Pachycephalosauridae domes examined. Osteomyelitis may be responsible for the damage observed in other pachycephalosaur specimens previously explained as taphonomic artifacts or bone absorptions.

The headbutting behavior of pachycephalosaurs was used for intra-specific combat. Pachycephalosaurus had a high rate of pathology, lending support to the hypothesis that they used their domes for combat. Pachycephalosaurs had dome-shaped heads which were resistant to headbutting.

The dome-shaped head of a pachycephalosaur prevented them from being headbutted, which may have been important in their social lives. Pachycephalosaurs had unique bone structures which helped them heal wounds quickly and efficiently.

Pachycephalosaurs likely used their headbutting behaviors to compete with other dinosaurs. The frequency of lesion distribution and bone structure support this hypothesis. Pachycephalosaurs differed from other dinosaurs in their headbutting behaviors. Their headbutting behavior may have been used to compete for food or mates.

What Did the Big Bump on The Head of The Pachycephalosaur Do?

The big bump on the head of the Pachycephalosaur was used for protection and communication. The bumps on the heads of pachycephalosaurs were likely used as a form of protection from head-on collisions.

Interestingly, other dinosaurs that had bumps on its head were used for other purposes. For instance, the Brachiosaurus had a bump on its head, but it was used instead as nostrils, based on the fossil findings.

The domes on the head of the Pachycephalosaur were used for identification. Some experts think the bumps were used as billboards, while others believe they were more durable and could have been used for battering.

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. Interestingly, when it comes to the closest living things that are related to the Pacychephalosaurs, it is not the headbutting mammals but birds and reptiles.

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. 

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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. 

Dome-Headed Dinosaurs – Fossils of Adults Give Clues About Habitat

The fossils of adult pachecephalosaurs can give clues about their habitat. The shape and placement of injuries on the skulls depended on the shape of the skull. This study provides evidence that injuries may have been a common occurrence in the habitat of adult pachecephalosaurs. The fossils can help us understand the pachecephalosaurs’ lifestyle and environment.

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, which 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 our knowledge grows and adjusts as we continue to find more and more fossils of these dinosaurs. The verdict is still out whether a Dracorex, 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.

Frequently Asked Questions

What was the dinosaur with the bump on its head?

The dinosaur with the bump on its head is called the pachycephalosaurus, also known as the hard-headed dinosaur.

What are some other names for the pachycephalosaurus?

The pachycephalosaurus is sometimes referred to as the dome-headed dinosaur or the dome-headed dino.

Was the pachycephalosaurus a herbivore or a carnivore?

The pachycephalosaurus was a herbivore, which means it only ate plants.

Did the pachycephalosaurus walk on two legs?

Yes, the pachycephalosaurus was a bipedal dinosaur, meaning it walked on two legs.

What is the significance of the bump on the pachycephalosaurus’ head?

The bump on the pachycephalosaurus’ head is believed to have served as a protection during head-butting contests with other dinosaurs, similar to how triceratops used its horns.

When did the pachycephalosaurus live?

The pachycephalosaurus lived during the Late Cretaceous period, approximately 70 million years ago.

Are there any surviving pachycephalosaurus fossils?

Yes, several pachycephalosaurus fossils have been discovered by paleontologists.

What other dinosaurs lived during the same time as the pachycephalosaurus?

Some other dinosaurs that lived during the same time as the pachycephalosaurus include the tyrannosaurus rex and the triceratops.

How thick was the dome-shaped skull of the pachycephalosaurus?

The dome-shaped skull of the pachycephalosaurus was approximately 20 times thicker than the skulls of regular dinosaurs.

Did the pachycephalosaurus have any unique features?

Yes, the pachycephalosaurus had a third eyelid called the nictitating membrane, which protected its eyes while it engaged in head-butting behavior.