An Ultimate Guide to Pachycephalosaurus: Bump Head Dinosaur

Leave a comment / / Updated on: 23rd September 2023

Name Meaning“Thick-headed Lizard”Height1.8 meters (6 feet)
PronunciationPack-ee-sef-uh-low-sore-usLength4.5 meters (14.8 feet)
EraLate CretaceousLate TriassicWeight370 to 450 kg (815 to 992 lb)
ClassificationDinosauria,‭ Ornithischia & PachycephalosauriaLocationMontana, Wyoming, South Dakota, Alberta (North America)

Pachycephalosaurus Pictures

There are two of them now! | MR1805 via Getty Images

The Pachycephalosaurus

Gage Beasley Prehistoric's Pachycephalosaurus Concept
Gage Beasley Prehistoric’s Pachycephalosaurus Concept

If you’ve watched the 1997 The Lost World: Jurassic Park movie, you probably already know what creature we’re talking about. 

You’ve guessed it right – it’s the thick-headed lizard, or, scientifically speaking, Pachycephalosaurus!

This ornithischian dinosaur inhabited the territory we now call western North America. 

It lived during the Maastrichtian stage of the Late Cretaceous, roughly 70-66 million years ago. 

If you live in or have ever visited Montana, South Dakota, Wyoming, or Alberta, you must know you’re walking on the same land the Pachycephalosaurus walked on!

Pachycephalosaurus | CoreyFord via Getty Images

While scientists proposed a second species in the genus, P. spinifer, only the type species is now officially recognized – P. wyomingensis.

Since these dinosaurs are known primarily from their skulls, many aspects of their appearance and behavior remain unknown. 

However, scientists did their best to describe the species as thoroughly as possible.

Without further ado, let’s start our journey to discover the thick-headed lizard!

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Physical Characteristics

Gage Beasley Prehistoric's Pachycephalosaurus Size Comparison Chart
Gage Beasley Prehistoric’s Pachycephalosaurus Size Comparison Chart

Since paleontologists discovered only skull fossils belonging to the Pachycephalosaurus, its overall appearance is poorly known. 

However, based on the skull size, specialists estimate that the members of this genus measured roughly 4.5 meters (14.8 feet) long and weighed approximately 370-450 kilograms (815-992 pounds).

Other sources list completely different numbers – up to 7-8 meters (23-26.2 feet) long and 2 tons (2.2 short tons) heavy! 

That’s why we cannot provide a 100% accurate answer regarding the size of these prehistoric creatures.

Nevertheless, if we examine the size of other members of the Pachycephalosauria suborder, which the Pachycephalosaurus is part of, we’d say that the first estimations are more accurate since the thick-headed lizard is considered the largest member.

Others measured only 2-3 meters (6.6-9.8 feet) long. 

Pachycephalosaurus | slowmotiongli via Getty Images

Take the Stegoceras, for example. It measured only 2-2.5 meters (6.6-8.2 feet) long and weighed only 10-40 kilograms (22-88 pounds) – as much as a dog! 

The Tylocephale, another member of the suborder, was approximately of the same size, reaching 2 meters (6.6 feet) long and weighing 40 kilograms (88 pounds). 

As such, we suspect that the Pachycephalosaurus could not have been much larger.

While we cannot guarantee their size, we can definitely inform you that they had a large, bony dome on their skulls, which could be as thick as 25 centimeters (10 inches)! 

This dome had the purpose of protecting the brain. But that’s not even the most interesting part!

Did you know this dome was adorned with bony knobs and bony spikes on the edges? 

They projected upward and were meant to protect the dome. Scientists suggest that the spikes were rather blunt than sharp.

We also discovered that the skull was short and featured large eye sockets. 

The fact that they faced forward prompted specialists to suspect binocular vision, which indicates that this creature probably had good eyesight. 

Pachycephalosaurus skull
Pachycephalosaur Skull | Eden, Janine and Jim via Flickr (CC BY 2.0)

It is believed that binocular vision is found primarily in predators; that is if we discuss modern animals. 

This makes the Pachycephalosaurus even more interesting! 

After all, it’s considered a herbivorous ornithischian, right? Well, keep reading to find the solution to the mystery!

If you think that’s the end of curiosity, you’re wrong! 

We’ve got more! 

Besides its bony dome, the Pachycephalosaurus also had a pointed beak, tiny teeth, and a thick, U-shaped neck. 

It had a bulky body, short forelimbs, long hindlimbs, and a heavy tail.

Habitat and Distribution

Most Pachycephalosaurus fossils were recovered from the Lance Formation in Montana and Wyoming, as well as from Montana’s Hell Creek Formation. 

Some material was also discovered in the Scollard Formation in Alberta.

These three formations were all part of the western shore of the Western Interior Seaway, dividing North America into Laramidia and Appalachia. 

As such, they all had similar climates and habitats.

More precisely, they likely had a subtropical climate with no cold seasons. 

Lance Formation along Cow Creek, Niobrara County, Wyoming | Anky-man via Wikipedia (CC BY-SA 3.0)

However, while some sources list that the Lance and Hell Creek Formations were abundant in precipitation, others focusing on Canada’s formations indicate that Alberta was a rather arid region due to a so-called rain shadow effect.

Moreover, since they were close to the Western Interior Seaway, these formations consisted of floodplain, lacustrine, fluvial, estuarine, and swampy environments.

Studies on the Hell Creek Formation indicate that the territory was abundant in plants like conifers, ferns, ash trees, palmettos, etc.

Behavior and Diet

Restoration of head-butting subadults | Ryan Steiskal via Plos One (CC BY 2.5)

The Pachycephalosaurus was a bipedal ornithischian dinosaur.

Evidence shows that these creatures were likely capable of interspecific and intraspecific communication.

Males are thought to have engaged in head-butting behavior similar to that observed in modern busk oxen and bighorn sheep. 

This is backed up by the fact that the members of the Pachycephalosauria carried evidence of anatomical combative behavior, such as spinal rigidity and strong neck musculature.

On the other hand, some specialists suggest that the Pachycephalosaurus didn’t exhibit head-butting behavior because, supposedly, the skull roof couldn’t sustain a ramming impact. 

Above this, fossilized skulls showed little to no evidence of scars that could be associated with head-butting. 

Pachycephalosaurus skull holotype
The holotype adult skull of Pachycephalosaurus “reinheimeri” (DMNS 469). | John R. Horner, Mark B. Goodwin via Plos One (CC BY-SA 3.0)

Moreover, studies on the function and structure of pachycephalosaur skulls and domes suggest that they weren’t suitable for this type of combat.

Another theory is that they engaged in flank-butting behavior. 

An individual stood either parallel to or face-to-face with its rival and engaged in intimidation displays. 

If the intimidation failed, it bent its head downward and to the side. 

Afterward, it struck the rival on its flank. 

Specialists argue that the wide torso was intended to protect the internal organs from impact.

Pachycephalosaurus | leonello via Getty Images

Other studies focused on cranial pathologies discovered in members of the Pachycephalosauridae revealed that many lesions can be associated with osteomyelitis. 

This is an infection of the bone that results from trauma that might be associated with aggression between individuals.

As such, while not fully confirmed, things are now a bit clearer in terms of behavior. 

But what about the diet? What did the Pachycephalosaurus eat?

The truth is scientists don’t fully know this yet. 

While they suppose that these creatures were herbivorous and fed on tough, fibrous plants, the serrated blade-like teeth similar to those of carnivorous theropods made them wonder whether these creatures weren’t, in fact, omnivores and ate meat as well.

Hopefully, future discoveries will reveal other details about the Pachycephalosaurus diet.

Life Cycle

Pachycephalosaur Dome Close-up | slowmotiongli via Getty Images

Pachycephalosaur domes are known to have played a role in sexual selection.

Like all dinosaurs, Pachycephalosaurus reproduced by laying eggs. 

While modern birds lay one egg at a time because they have only one functional oviduct, dinosaurs laid two eggs at a time, thanks to their two functional oviducts.

Since many dinosaurs were precocial after hatching, meaning they did not require much adult help, we may suppose that baby Pachycephalosaurus specimens were also precocial.

There’s direct evidence suggesting that the appearance and physiology of their domes changed over the years. 

Casts of three skulls, representing possible growth stages, Museum of the Rockies | Tim Evanson via Flickr (CC BY-SA 2.0)

More precisely, some fossils belonging to juvenile individuals indicate that their skulls were flat, the dome was reduced, and the spikes were larger. 

Supposedly, as they aged, their domes grew larger. 

Moreover, the arrangement of the spongy bone found in juvenile pachycephalosaur skulls indicates that they grew relatively rapidly.

Evolution and History

Pachycephalosaurus dinosaur | leonello via Getty Images

The Pachycephalosaurus is part of the Ornithischia clade, under the Marginocephalia clade and the Pachycephalosauria group. 

Members of Ornithischia evolved during the Jurassic period and went extinct in the Late Cretaceous. 

Their closest relatives are the members of the Thyreophora group, but they can be distinguished from one another by several physiological characteristics, including teeth number.

However, their history goes far beyond these clades until it reaches the Ornithischia clade. 

Recent studies have questioned ornithischians’ evolution and taxonomic classification, suggesting that these creatures likely appeared later than previously thought.

Until 2017, it was widely recognized that the Pisanosaurus was the oldest known ornithischian, having lived during the Late Triassic, thus indicating that ornithischians, including the Pachycephalosauria, date from the Late Triassic.

Pachycephalosaurus | MR1805 via Getty Images

Now, however, scientists aren’t as certain that the Pisanosaurus was an ornithischian, suggesting it was a member of the Silesauridae

This prompted the studies mentioned above regarding the origin and evolution of saurischians.

Returning to Pachycephalosaurus, we must mention that the first fossils were discovered in the early 1850s. 

This major paleontological event was followed by many other notable discoveries that helped scientists name and describe the species. 

The genus was established in 1943, and the species was named Pacycephalosaurus wyomingensis in 1983. 

Their placement in Marginocephalia makes them closely related to ceratopsians.

Interactions with Other Species

There are two of them now! | MR1805 via Getty Images

Based on our research on the formations the Pachycephalosaurus fossils were recovered from, it’s likely that these creatures shared their habitat with the following types of animals:

  • Hypsilophodontids
  • Ankylosaurids
  • Other pachycephalosaurids
  • Troodontids
  • Dromaeosaurids
  • Various fishes and mammals
  • Crocodiles
  • Amphibians
  • Birds

The most common dinosaurs in the habitat were likely Triceratops, Tyrannosaurus, and Edmontosaurus.

Pachycephalosaurus | MR1805 via Getty Images

In short, the Pachycephalosaurus probably had a lot of fun living among so many creatures! 

Whether they confronted each other and engaged in combat is a completely different story. 

Since it shared its habitat with the Tyrannosaurus, which is among the world’s strongest prehistoric predators, it’s likely that the small Pachycephalosaurus occasionally served as prey for the almighty T-Rex.

Besides this, since many dinosaurs were herbivorous, there might have been competition for food.

Cultural Significance

Pachycephalosaurus wyomingensis “Sandy” specimen, Royal Ontario Museum | IJReid via Wikipedia (CC BY 4.0)

Since Pachycephalosaurus is the largest and best-known genus in the Pachycephalosauridae family, its discovery aroused the curiosity of hundreds of specialists. 

Even today, specialists publish scientific articles and research papers discussing aspects of this creature’s appearance, distribution, and behavior. 

However, since it is known primarily from fossilized skulls, new paleontological discoveries are a must to complete this dinosaur’s profile.

Pachycephalosaurus in The Lost World: Jurassic Park | Photo via Jurassic Park Wiki

Let’s not forget about Pachycephalosaurus’ appearances in the media! 

This creature was a notable character in The Lost World: Jurassic Park, as well as in the Jurassic Park: Operation Genesis video game. 

In Dinosaur Revolution, this prehistoric animal is portrayed as being hunted by two Troodon. It also appears in Bizarre Dinosaurs and Dinosaurs Decoded.


Having lived between 70 and 66 million years ago in western North America, the Pachycephalosaurus is now one of the most popular and well-studied pachycephalosaurs. 

It is most known thanks to the tall, bony dome on its skull decorated with knobs and spikes.

This bipedal creature had short arms and strong, long hind limbs. 

It had a bulky body that might have protected the internal organs in combat.

The function of the distinctive dome has been the subject of many debates, and scientists suggest it might have served as an aid for head-butting, flank-butting, or sexual and social selection.

If you haven’t watched The Lost World: Jurassic Park or other movies and documentaries the Pachycephalosaurus appeared in, choose one and enjoy their uniqueness on screen! 


Is Troodon a Pachycephalosaurus?

The Troodon and the Pachycephalosaurus are different dinosaurs. 

The former is a saurischian theropod, while the latter is an ornithischian pachycephalosaur.

How did Pachycephalosaurus go extinct?

The Pachycephalosaurus went extinct during the Cretaceous-Paleogene extinction event. 

It is one of the last non-avian dinosaurs to have lived before the extinction event.

What is the Pachycephalosaurus‘ closest relative?

If we’re discussing in terms of scientific classification, the closest relative of the Pachycephalosaurus is the Alaskacephale

However, Dracorex and Stygimoloch are often associated and synonymized with the Pachycephalosaurus.


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