Pachycephalosaurus

## PachycephalosaurusThick-headed Lizard

* Male
* Young Male
* Females

* **Skull Thickness** of 25 - 30 centimeters (10 - 12 inches), the thickest skull of any dinosaur.
* Skull is composed of a unique form of fibrolamellar bone, which contains fibroblasts that help heal bone injuries quickly

 **Main** Segments Gallery

MainSegmentsGallery

***“ A herd of pachycephalosaurs, bizarre creatures with thick, dome-shaped skulls. ”***

― **David Attenborough**, *Swamps*
***Pachycephalosaurus*** (Thick-headed Lizard) is a genus of pachycephalosaurid dinosaur that lived in western North America 70 - 66 million years ago, during the Maastrichtian stage of the Late Cretaceous Period. It is known for its skull, which is 25 - 30 centimeters (10 - 12 inches) thick, the thickest of any dinosaur.

The *Pachycephalosaurus* on Prehistoric Planet is based on the "Sandy" specimen, which has pointed horns as opposed to blunt knobs. The taxonomy of *Pachycephalosaurus* is, to this day, subject to continuous debate, with conflicting views regarding the relationship of the spike-crowned form with the more familiar knob-crowned type. Although the former is still officially considered a growth stage of the latter, forming the sole species *P. wyomingensis*, some experts theorize that the horned type is distinct enough to be considered a second species known as *P. spinifer* (previously interpreted as a distinct genus known as *Stygimoloch*, which means "Styx River Demon"), for it is found in rock layers above the knobbed *P. wyomingensis* specimens, indicating that it lived later and may have in fact descended from the knob-crowned form. Hence, the identity of the species on Prehistoric Planet depends on the validity of *P. spinifer* as a second species.

## Paleobiology

### Size and Physique

*Pachycephalosaurus* is the last and largest of the pachycephalosaurids, a group of small, bipedal dinosaurs related to ceratopsians, most known for their thickened skulls. Even so, it stood no taller than the average human, reaching around 1.5 - 1.8 meters (5 - 6 feet) in height, 5 - 7 meters (16 - 23 feet) in length, and 370 - 454 kilograms (816 - 1000 pounds) in weight.

Most of what is known of *Pachycephalosaurus* is inferred from the skull, which has large, rounded eye sockets that faced forward and, coupled with large optic nerves, gave the animal good binocular vision. Large olfactory lobes also indicated that it had a decent sense of smell compared to other dinosaurs. As with most pachycephalosaurs, the post-cranial remains of *Pachycephalosaurus* are poorly known. But based on the fossils of relatives and the few fragments that were found, *Pachycephalosaurus* is believed to have a short, thick neck, short arms that ended in five-fingered hands, broad hips, long legs, and a thick, heavy tail made rigid by tendons that were ossified (turned to bone), though this bundle of bony rods does not necessarily make the tail especially bulky.

### Jaws and Dentition

*Pachycephalosaurus* possesses a short muzzle with a pointed beak, filled with small, rigid teeth with leaf-shaped crowns. These teeth are sharp and serrated, not meant for tough, fibrous plants like the flowering shrubs of its time, but rather for shredding through leaves, seeds, and fruits, though one lower jaw was discovered to have serrated, blade-like front teeth resembling those of carnivorous theropods, indicating a degree of omnivory, which is seen on the show when the pachycephalosaurs fed on insects from a dead tree.

On Prehistoric Planet, *Pachycephalosaurus* and its relative *Prenocephale* are depicted without cheeks, in sharp contrast with ceratopsians like *Triceratops* and *Pachyrhinosaurus*, which are depicted with cheeks. Using the works of Dr. Ali Nabavizadeh as justification, Dr. Darren Naish, lead consultant of Prehistoric Planet, brought up how pachycephalosaurs had simpler, more primitive teeth, and hence chewed food more like a lizard rather than doing so in the more complex manner in which ceratopsians and ornithopods chewed. Regardless of how this debate eventually gets resolved, current understanding has it that dinosaur cheeks aren't like those of mammals, since, contrary to popular belief, dinosaurs did not need cheeks to hold food, and one theory is that they may have had "cheeks" as a result of jaw muscles extending across the parts where true cheeks would be.

### Skull Dome and Ornamentation

***“ When they first saw these very thick, domed skulls - and these domes could be up to 30 centimeters in thickness - they were thought to be very strong. It was thought that maybe they were using them for headbutting each other in contests over social hierarchy. ”***

― **Professor Paul Barrett**, *Natural History Museum*
The most prominent feature of *Pachycephalosaurus* is its helmet-like skull dome, surrounded by a crown of spikes, knobs, and horns that traveled all the way down to the snout. These bony domes could be 25 - 30 centimeters (10 - 12 inches) thick, safely cushioned to protect the animal's brain. The way pachycephalosaurs used their domes is still not completely understood. Prehistoric Planet: Uncovered brings up traces of a mesh of fibers on the dome that may indicate the presence of brightly-colored skin to serve as a display, though evidence clearly points to constant, physical use as a battering ram.

***“ Ten inch-thick skulls enable them to survive powerful head-on collisions. ”***

― **David Attenborough**, *Swamps*
Some studies argue that the force of headbutting would break their necks, and they resorted to striking each other's flanks, or swinging their heavy skulls sideways like clubs. However, the likelihood that they adopted proper postures specific for ramming, coupled with constant evidence of multiple lesions on the domes (with one specimen, BMR P2001.4.5, bearing 23 dome scars), injuries that quickly healed, eventually lead to a reanalysis of pachycephalosaur domes, with their CT scans compared to those of modern-day headbutting artiodactyls. Currently, with this evidence, the idea that pachycephalosaurs butted heads is more acceptable than it once was. Aside from being used for intimidation and combat, the pachycephalosaurs can also use their heads for other, more mundane tasks during peacetime, as demonstrated by the dominant male striking and pushing a tree down in order to force out the insects within.

***“ The most distinctive feature about the pachycephalosaurs, of course, is this really weird dome that they have on their head. All around it is a series of very odd tubercles and knobbles and bobbles. They look quite dragon-like. ”***

― **Dr. Susannah Maidment**, *Natural History Museum*
Pachycephalosaur skulls are known for their extreme plasticity, changing in size and shape as the animal grows, with the horns eventually shrinking in size (and consequently, importance) as the skull dome is developed as the more prominent feature. This can be seen on Prehistoric Planet, where, compared to the old, dominant male, the young male appears to have a flatter, less-developed dome, resembling that of *Dracorex hogwartsia* (Dragon King of Hogwarts), which, unlike *P. spinifer*, is now fully acknowledged not as a separate dinosaur, but as a less-mature form of *Pachycephalosaurus*.

### Sounds

The exact vocalizations made by pachycephalosaurs are unknown, though it is plausible to believe that, much like other dinosaurs, they made sounds reminiscent of various modern-day reptiles and birds. Prehistoric Planet uses the singing voice of a black grouse (albeit lowered in pitch to match the animal's larger size) to provide their warbling coos during peacetime, while the calls of herons and hornbills are added to provide harsher, rougher grunts and bellows whenever they engage in more aggressive, more intimidating behavior.

### Prehistoric Planet's Depiction

*Main: *Pachycephalosaurus* on Prehistoric Planet*

Main: *Pachycephalosaurus* on Prehistoric Planet

***“ In a drought-stricken basin where much has perished, an old Pachycephalosaurus bull defends his leadership against a young challenger. ”***

― The official Apple TV+ episode synopsis for *Swamps*
On Prehistoric Planet, the pachycephalosaurs are depicted with a yellow and brown color combination, with their domes partly covered in dark scales that may be battle scars and long-healed lesions, gashes, and bruises from previous battles, and blade-like bony protrusions jutting from the sides of their lower jaws. The females have paler scales compared to the males, which have prominent markings in the form of green and black speckles and stripes across the body, orange highlights by the throat area, and domes that come in a vibrant shade of red. The dominant male is noticeably larger and more heavily-built than the other members of his herd, with a tall, prominent dome and large back horns with some of the tips chipped off, serving as proof of his fighting experience. By contrast, the younger male's dome seems less prominent, less rugged, and not as heavily-marked with dark scales, given his age and lesser experience. Dr. Darren Naish designed the facial features of *Pachycephalosaurus* based on the skeletal reconstructions of Dr. Scott Hartman, while the color variants were worked on by David Krentz, ensuring that, while the males have more vibrant coloring, the females were, to a degree, also striking in looks.

***“ During the age of the dinosaurs, spectacular species like this pachycephalosaur attract the most attention. But the prehistoric world was also home to a wide variety of other quite different species. ”***

― **David Attenborough**, *Prehistoric Planet: Uncovered - What Else Lived Alongside The Dinosaurs?*
*Pachycephalosaurus* is the main focus of the fourth, penultimate segment of Swamps, which centered around an old bull facing off against a young upstart in his herd. The species was also further discussed in "Was Pachycephalosaur Really A Headbutter?", a Prehistoric Planet: Uncovered segment which presents evidence of its headbutting capabilities and concluded that the dome of *Pachycephalosaurus* was useful as a display and as a weapon. The skull of a *Pachycephalosaurus* also briefly appears in the opening of the Uncovered segment "What Else Lived Alongside The Dinosaurs?", where it is used as an example of a spectacular species that attracts the most attention during the age of the dinosaurs.

## Paleoecology

### Paleoenvironment

*Main: Scollard Formation*

Main: Scollard Formation

***“ A devil toad can only survive here because every year, the land is flooded by seasonal rains. However, there are places where changing climate means the rains are less-certain. This inland basin in North America was, in the past, several feet deep in water. But after a decade of drought, it's now bone-dry. Nonetheless, some dinosaurs still linger here. ”***

― **David Attenborough**, *Swamps*
*Pachycephalosaurus* lived 70 - 66 million years ago in Laramidia, an island continent that would one day become western North America, which, by that time, was split in half by an inland sea. Fossils have been excavated mostly from the Hell Creek Formation in Montana and South Dakota, the Lance Formation in Wyoming, while indeterminate remains have been uncovered from the Denver Formation in Colorado, but evidence of its presence further north indicates that it was cosmopolitan, not confined to certain faunal provinces. In fact, the fourth segment of Swamps, which is focused on *Pachycephalosaurus*, is confirmed by official press notes to have taken place in the Scollard Formation, located in what is now southwestern Alberta, Canada, with the animal's presence indicated by a domed parietal found in the area. In any case, the aforementioned formations show that *Pachycephalosaurus* lived by the the eastern edge of Laramidia, where rivers, forests, and coastal plains were kept constantly humid by ample rainfall and a subtropical climate warmer than those areas are today, rendering those regions devoid of a cold season.

Due to how little of their bodies have fossilized, it was once believed that pachycephalosaurs dwelled in areas of high elevation like mountains, where the conditions needed for the fossilization process are poor, with their heavy skulls rolling down to lower regions where they can be preserved. Prehistoric Planet depicts the pachycephalosaurs as running through a dry, open inland basin said to have once been several feet deep in water, rendered bone-dry by a decade of drought. However, Dr. Darren Naish clarified that such an environment would not have been appropriate for *Pachycephalosaurus* (it was only shown in such an area because of filming constraints). Instead, *Pachycephalosaurus* would have more likely thrived in wetter, more heavily forested lowlands like the one shown in the fifth and final segment of Swamps.

### Paleofauna

*Pachycephalosaurus* coexisted with other well-known dinosaurs like *Ankylosaurus*, *Edmontosaurus*, *Triceratops*, and *T. rex*, which served as the apex predator of the regions where *Pachycephalosaurus* lived in. *Pachycephalosaurus* also coexisted with two smaller pachycephalosaurs, *Sphaerotholus* and *Platytholus*. In a 2011 study that estimated the proportions of large-bodied dinosaurs in the Hell Creek Formation, it was revealed that *Pachycephalosaurus* only made up 1% of the dinosaurs found in Hell Creek, though many other dinosaurs were just as rare, if not even more so, with some not even included in the breakdown of Hell Creek's paleo-population.

As one of the last non-avian dinosaurs, *Pachycephalosaurus*, along with most of the animals it coexisted with, would have bore witness to the K-Pg Extinction Event 66.043 million years ago, a catastrophe that would ultimately wipe out three-quarters of all life on Earth, including all non-avian dinosaurs.

### Feeding Preferences and Adaptations

***“ They prefer to feed on fruit and leaves. But now, they're forced to make do with roots and insects. ”***

― **David Attenborough**, *Swamps*
While usually viewed as plant-eating dinosaurs, pachycephalosaurs possess serrated, bladed teeth much like those of theropods, indicating that they may have eaten and processed just enough meat to be considered omnivorous rather than mostly herbivorous. Given their omnivorous diet, the pachycephalosaurs are shown to be quite resilient, able to adjust their dietary habits by subsisting on roots and insects in the absence of their preferred food, fruits and leaves, sometimes using their heads to get the sustenance they want, as shown when the dominant male headbutted a tree to the ground to feed on the insects within it.

### Social Habits and Agonistic Behavior

***“ The herd is lead by a large male. He keeps order, so everyone is able to search for food in peace. But trouble is brewing within the herd. The young male is throwing his weight around. It's time for the dominant male to put him in his place, or kick him out. ”***

― **David Attenborough**, *Swamps*
Nothing is known about pachycephalosaur social habits, but based on their ceratopsian relatives and similar modern-day analogues, the adults likely had a social hierarchy, and parents took care of their young, at least, until the young males, upon reaching maturity, try to gradually assert their dominance by bullying more submissive members of the herd. In response, the large male leading the herd would confront the troublemaker to prevent further disruption of order within the herd, since, if the younger male is allowed to continue disregarding the herd's social hierarchy, he may gain enough courage and strength to eventually challenge and even overthrow the leader himself. Such potential contenders are left with two choices; they can either submit to the dominant male's authority and keep their place within the herd, or face the leader in a duel, with the consequence of their defeat being their eviction for disrupting peace and order within the herd.

***“ He flaunts his colorful headgear, trying to intimidate the youngster. But he's not going to back down that easily... ”***

― **David Attenborough**, *Swamps*
As with most animals, the pachycephalosaurs try to intimidate their rivals first with a vigorous display of their prominent features. When these warnings via show of force are ignored, the rivals move on to physical blows. Their fighting style is based on the idea that, while they did headbutt as indicated by evidence, they did not rush at each other, head lowered, in the same way as musk oxen do. Instead, as Prehistoric Planet depicts it, rushing is done by the aggressor within a shorter distance, the combatants prefer to bash and shove each other up-close, swing their heads like clubs (which the dominant male attempts to do in order to knock his retreating opponent off-balance), and even lock the horns on the back of their head in an attempt to stab each other. It is shown that the older, more powerful, and more experienced male is more willing to go on the offensive from the start, initiating most of the headbutts, recovering from collisions quicker, and continuing to push his enemy even when blocked, allowing him to slam through the defensive efforts of his enemy and maintain the pressure on him. When a fight goes on for long enough, rivals would resort to wrestling with each other much like monitor lizards, doing anything to force the duel in their favor. The younger male also showed some inexperience by failing to confirm if his older rival was indeed unable to continue the fight, allowing the older male to exploit his premature celebration, knock him down to the ground, and have him evicted from the herd for his transgressions and insubordination.

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## References

### General

1. ↑ 1.0 1.1 Pachycephalosaurid (Dinosauria: Ornithischia) cranial remains from the latest Cretaceous (Maastrichtian) Scollard Formation of Alberta, Canada
2. ↑ 2.0 2.1 "3.4 Colorado, United States; 6. Denver Formation," in Weishampel, et al. (2004). Page 581.
3. ↑ 3.0 3.1 *The Princeton Field Guide to Dinosaurs*
4. ↑ 4.0 4.1 Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur *Pachycephalosaurus*
5. ↑ 5.0 5.1 Vegetarian dinosaur may have actually eaten meat, skull suggests
6. ↑ Cranial Pathologies in a Specimen of *Pachycephalosaurus*
7. ↑ Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae)
8. ↑ Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur *Stegoceras validum* (Ornithischia, Dinosauria) and Combative Artiodactyls
9. ↑ Dinosaur Census Reveals Abundant *Tyrannosaurus* and Rare Ontogenetic Stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA
10. ↑ Dinosaur extinction battle flares

### Dr. Ali Nabivazadeh

1. ↑ New Reconstruction of Cranial Musculature in Ornithischian Dinosaurs: Implications for Feeding Mechanisms and Buccal Anatomy
2. ↑ Dinosaurs did not need cheeks to hold their food since they don't chew from side to side the way mammals do.

### Dr. Darren Naish

1. ↑ 1.0 1.1 1.2 Prehistoric Planet's *Pachycephalosaurus* is based on the Sandy specimen, and its taxonomy is still not sorted out.
2. ↑ 2.0 2.1 Regardless of whether they are a separate species or not, the spike-crowned *Pachycephalosauruses* are depicted as a distinct population, given how they are found in a rock layer different from that of the non-spiked variants.
3. ↑ Pachycephalosaur tails had unusual intermuscular bones bundled around their tail, though these bony rods do not necessarily make the tail especially bulky.
4. ↑ Pachycephalosaurs likely lacked cheeks because of their primitive teeth.
5. ↑ 5.0 5.1 After basing off of Dr. Ali Nabavizadeh's works regarding pachycephalosaur chewing styles, the decision was made to depict pachycephalosaur anatomy as being more fit for lizard-style chewing as opposed to the more complex chewing styles of ceratopsians and ornithopods, further showcasing the anatomical diversity of ornithischians.
6. ↑ A lot was done for the color variation of the pachycephalosaurs, ensuring that, while the males are more showy, the females were also quite showy.
7. ↑ *Pachycephalosaurus* should have been featured in well-watered, densely forested lowlands, and its habitat as portrayed on Prehistoric Planet is inappropriate, having resulted from the constraints of making the show.
8. ↑ Little - or rather, esssentially nothing - is known about the social habits of pachycephalosaurs, though some assumptions can be made based on their ceratopsian relatives and extant analogues. It is therefore reasonable to assume that herds controlled by large adult males would evict young males once they are more mature and more likely to assert their dominance.
9. ↑ Pachycephalosaurs did not butt heads like musk oxen, though numerous skull injuries indicate that they did bash heads, just in a manner different from traditional portrayals. They would have fought up-close, pushing, shoving, swinging their heads, and even using their spikes to stab, with the show basing their fighting styles off of lizards like marine iguanas, crocodylians, and even mammals like giraffes.

### Dr. Thomas Holtz

1. ↑ 1.0 1.1 1.2 Holtz Dinosaur Genus Appendix
2. ↑ Lecture 17 Thyreophora Defense! Defense! DEFENSE!!

### Prehistoric Planet

1. ↑ 1.0 1.1 1.2 I Know Dino: The Big Dinosaur Podcast, Episode 445: Dinosaurs in Swamps: Featuring Prehistoric Planet 2
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 As stated in Prehistoric Planet: Uncovered - Was Pachycephalosaur Really A Headbutter?
3. ↑ 3.0 3.1 3.2 As stated in Swamps.