Pachyrhinosaurus

## PachyrhinosaurusThick-nosed Lizard

* Male
* Juvenile with Mother

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***“ Pachyrhinosaurs, two-tonne herbivores with extravagantly-armored heads. ”***

― **David Attenborough**, *Ice Worlds*
***Pachyrhinosaurus*** (Thick-nosed Lizard) is a genus of centrosaurine ceratopsid dinosaur that lived in western North America 73.5 - 68.5 million years ago, during the Maastrichtian stage of the Late Cretaceous Period. The species depicted on Prehistoric Planet is *Pachyrhinosaurus perotum* from the Prince Creek Formation in Alaska, USA.

## Paleobiology

### Size and Physique

*Pachyrhinosaurus* is a medium-sized ceratopsid that lived in North America. There are three known subspecies of it; *P. canadensis*, *P. lakustai*, and *P. perotorum*, the last of which was presented on Prehistoric Planet. It was slightly smaller than *P. canadensis*, measuring 5 meters (16 feet) in length and weighing around 2 metric tons (2.2 short tons).

### Jaws and Dentition

***“ They seek refuge in the forest, stripping the last leaves and dormant trees, and rooting through ferns or fallen fruit. ”***

― **David Attenborough**, *Ice Worlds*
The front part of a *Pachyrhinosaurus* bill is toothless, while the rest of the mouth is packed with "dental batteries", tightly-packed teeth. Unlike hadrosaurids, which developed their dental batteries into grinding surfaces to grate plants into fine pieces, ceratopsians evolved their teeth to provide cutting surfaces for shearing through tough vegetation, with this difference in function indicating that they evolved their chewing capabilities independently. Ceratopsids are believed to have only fed on plants that were, at most, just one meter (3 feet) in height, while gut microflora broke down plants in their stomach via fermentation.

On Prehistoric Planet, ceratopsians like *Triceratops* and *Pachyrhinosaurus* are depicted with cheeks, in sharp contrast with the other type of marginocephalians, pachycephalosaurs like *Pachycephalosaurus* and *Prenocephale*, which are depicted without 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.

### Head Ornamentation

The centrosaurine ceratopsids, rhinoceros-like in size, are distinguished from their larger and more familiar-looking chasmosaurine relatives by their massive nasal horns, short or nonexistent brow horns, and an ornamented frill (shorter and smaller than the wide, triangular frills of the chasmosaurines) which, in most members, have spike-like ornaments midway up. There are, however, exceptions. Members of the nasutoceratopsini tribe sport a pair of long brow horns much like the chasmosaurines, and usually only have a rounded bump on their nose. There's also the pachyrostran members of the pachyrhinosaurini tribe, *Pachyrhinosaurus* and its earlier relative, *Achelousaurus*, which don't have massive nasal horns. Rather, in the place of this horn is a thick, massive lump of bone called a "nasal boss", along with a smaller boss over the eyes. *Pachyrhinosaurus* also has atop its frill two small horns that curve and point backward. The species depicted on the show, *P. perotum*, has a jagged, comb-like extension at the tip of the boss which is also present in *P. lakustai*, but missing in *P. canadensis*. Also, *P. perotum* has the boss over its eyes separated from its nasal boss by a narrow groove, a trait it shares with *P. canadensis*, but not *P. lakustai*, which has its eyebrow boss and nasal boss kept apart by a wide gap. What truly makes *P. perotorum* unique from the other two species is the narrow dome in the middle of the back portion of its nasal boss, which has a thick, cornified pad on top of it, something not seen in the other species.

### Prehistoric Planet's Depiction

*Main: *Pachyrhinosaurus* on Prehistoric Planet*

Main: *Pachyrhinosaurus* on Prehistoric Planet

Prehistoric Planet depicts *Pachyrhinosaurus perotum* with brown skin and black vertical stripes covering their backs. Younger individuals having blue frills, and older individuals sport splotches of red and green on their face and frill, giving them a striking appearance. Their pupils appear in the form of horizontal, rectangular slits, similar to the eyes of goats.

Adult male *Pachyrhinosauruses* are depicted with keratinized bristles running down their backs and tails. Tail bristles (determined in a 2016 study to be highly modified scales as opposed to feathers due to the different morphology and anatomy of these structures) are known in the primitive *Psittacosaurus*, a basal ceratopsian, and, though there is barely any evidence of such structures in others, some of the later, more advanced members of the ceratopsian clade are occasionally depicted with similar-looking filaments running down the back and tail. This speculative characteristic is sometimes extended to the other marginocephalian clade, pachycephalosaurs (hence why male *Prenocephale* were depicted with bristles on their tails in Badlands, though concept art reveals that the females were initially planned to also have such bristles). These keratinized bristles likely served as an additional means of display.

*Pachyrhinosaurus* appears in the sixth and final segment of Ice Worlds, where a herd of them is shown foraging for vegetation in the polar woodlands of Alaska. The herd then gets attacked by three *Nanuqsaurus*. Though they initially stand their ground and form a defensive circle against the tyrannosaurs, the ceratopsians eventually panic, allowing the predators to bring down an old male. Footage of this segment is also used for the Uncovered segment, "Did Dinosaurs Hunt in Packs?", which focuses on cooperative behavior in large theropods, justifying the depiction of *Nanuqsaurus* as a pack hunter on Prehistoric Planet.

## Paleoecology

### Paleoenvironment

*Main: Prince Creek Formation*

Main: Prince Creek Formation

***“ In the coldest months, the far north of the Arctic can freeze into a spectacular winter wonderland. Not all dinosaurs choose to leave, or to seek shelter. The largest have the strength to tough it out. ”***

― **David Attenborough**, *Ice Worlds*
*Pachyrhinosaurus* is known from several of the northernmost areas of the North American continent. *P. lakustai* lived 74 - 72 million years old in the Wapiti Formation, a Canadian formation stretching from northwest Alberta to northeast British Columbia. *P. canadensis* is known from the Horseshoe Canyon Formation in southwest Alberta, around 72 - 71 million years ago. The one depicted on Prehistoric Planet is the last species, *P. perotum*, which lived 71 - 69 million years ago in the Prince Creek Formation, a polar woodland that experienced 120 days of winter darkness, with temperatures dropping as low as 2 - 4 degrees Celsius (36 - 39 degrees Fahrenheit) during the cold months.

### Paleofauna

*Pachyrhinosaurus perotum* would have coexisted with animals like *Edmontosaurus*, *Ornithomimus*, some dromaeosaurs like *Dromaeosaurus* and *Saurornitholestes*, as well as the most common animals of the Prince Creek Formation, troodontids. Given their large size, adult *Pachyrhinosaurus perotum* would have only needed to be wary of one hunter, the apex predator of the region it lived in, *Nanuqsaurus*, a tyrannosaur roughly matching them in body mass.

### Social Habits and Agonistic Behavior

***“ The autumn rut is over. But less-dominant males still try to push their way up the social hierarchy. But they've forgotten who's in charge. This old bull is still a formidable force, and one that few dare to challenge. ”***

― **David Attenborough**, *Ice Worlds*
Centrosaurine ceratopsians are commonly found in bonebeds, massive fossil sites full of specimens, indicating herding behavior and mass death (due to events like crossing a river during a flood). The earliest species, *Pachyrhinosaurus candensis*, is known from such bonebeds, some of which show four distinct age groups from juveniles to adults, also indicating that centrosaurines cared for their young. While *Pachyrhinosaurus perotum* itself wasn't found in bonebeds, it is assumed that it also came in herds and gave parental care to juveniles. Prehistoric Planet also depicts a form of social hierarchy amongst the members of the herd. Maintaining one's high position means risking injury, an inconvenience that may end up affecting the animal in other matters, like fighting or evading predators.

### Defensive Tactics

***“ Out here, the herd will be able to close ranks and be able to form an impenetrable wall of armored heads and muscle. ”***

― **David Attenborough**, *Ice Worlds*
While there is no concrete proof of such behavior, it is believed that, much like musk oxen today, ceratopsians like *Pachyrhinosaurus* formed a defensive circle when faced with predators, protecting the juveniles in the center while occasionally charging out at a predator that comes too close.

***“ In this test of nerve, all it takes is for one animal to break ranks, and panic quickly spreads. With the herd on the run, the predators have a chance. ”***

― **David Attenborough**, *Ice Worlds*
This formation is not as invincible as it appears to be. Grizzly bears have been known to break the defensive circles of musk oxen and force each member to run off in every direction. It is possible that predators like *Nanuqsaurus*, which are actually roughly as heavy as the centrosaurines themselves, could have done the same. In terms of individual defensive capabilites, while a fully-grown *Pachyrhinosaurus* would have been capable of taking on predators, their advantage in battle is not guaranteed. Despite *Nanuqsaurus* being depicted at two-thirds of its actual length in the sixth segment of Ice Worlds (which can be justified by the idea that the three of them may be adolescents), there is evidence that even young tyrannosaurs are capable of attacking fully-grown ceratopsians successfully. One of the most significant fossils of any dinosaur is the "Dueling Dinosaurs" specimen of Hell Creek, which preserves an adolescent *T. rex* locked in combat with a *Triceratops horridus*, proving that even adolescent tyrannosaurs are capable of taking down large ceratopsians, a feat that has more chances of success when done by a pack.

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Female *Pachyrhinosaurus**Pachyrhinosaurus* eatingOld Bull scratching the treeThe *Pachyrhinosaurus* herd moves to the open ground
## References

### General

1. ↑ 1.0 1.1 1.2 *The Princeton Field Guide to Dinosaurs*
2. ↑ A functional analysis of jaw mechanics in the dinosaur *Triceratops*
3. ↑ Feeding height stratification among the herbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada
4. ↑ "Resource Exploitation and Habitat", Sampson (2001); page 268.
5. ↑ Feathers or Scales?
6. ↑ The giant troodontid dinosaurs of Alaska
7. ↑ A Brown Bear Attack on Musk Oxen
8. ↑ 'Dueling Dinosaurs' fossil, hidden from science for 14 years, could finally reveal its secrets
9. ↑ North Carolina Museum of Natural Sciences to receive the "Dueling Dinosaurs"

### Dr. Ali Nabavizadeh

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. ↑ Pachycephalosaurs likely lacked cheeks because of their primitive teeth.
2. ↑ 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.

### Dr. Thomas Holtz

1. ↑ GEOL 104 Dinosaurs: A Natural History - Marginocephalia: That's Using Your Head!
2. ↑ Lecture 17 Thyreophora Defense! Defense! DEFENSE!!

### Prehistoric Planet

1. ↑ 1.0 1.1 As stated in Ice Worlds.
2. ↑ Gabriel Ugueto, one of the designers who worked on Prehistoric Planet, shows *Prenocephale* concept art and confirms that the tail bristles of the females were removed on the show, making the bristles a male-exclusive feature.