Pachyrhinosaurus (Thick-nosed Lizard) is a genus of centrosaurine ceratopsid dinosaur that lived in Alberta, Canada and Alaska, USA, 73.5 - 68.5 million years ago, during the Maastrichtian stage of the Late Cretaceous Period. The species depicted on Prehistoric Planet is Pachyrhinosaurus perotorum,[DN 1] which lived 71 - 69 million years ago in the Prince Creek Formation of Alaska, USA. However, concept art by Jama Jurabaev indicates that the series was, at one point, also planned to feature the type species, Pachyrhinosaurus canadensis, which lived 72 - 71 million years ago in the Horseshoe Canyon Formation of Alberta, Canada.[PhP 2]
Pachyrhinosaurus is a medium-sized ceratopsid that lived in Alberta, Canada and Alaska, USA. There are three known subspecies of it; P. canadensis, P. lakustai, and P. perotorum, the last of which was presented on Prehistoric Planet.[DN 1] It was slightly smaller than P. canadensis, measuring 5 meters (16 feet) in length and weighing around 2 metric tons (2.2 short tons).[1][PhP 1]
Pachyrhinosaurus took two years to reach half its adult size, but would not reach its maximum size until around the age of 20
Based on one specimen described in 2011, Pachyrhinosaurus perotorum reached 28% of its adult size in one year, and became half as big as its adult size in two years. After this, however, its growth slowed down considerably. Because it lived in Alaska, where winters are harsher, P. perotorum decelerated its growth even more during the cold season, as indicated by conspicuous growth banding in its bones, something not seen in the other Pachyrhinosaurus species, which lived in warmer regions further down south.[2] Factors such as a lack of resources and harsh conditions in an area can influence animals to slow down their growth, because the process of growing requires energy, and, consequently, a larger body must meet larger demands, hence, slow growth lets an organism to stay smaller so they would not require as much sustenance as bigger, more mature specimens (which need to put more effort in looking for food and maintaining their dominance over others of their kind for several reasons), and also allows them to conserve the precious energy they do have for more important matters, like searching for a refuge from harsh conditions, escaping from predators, calamities, and other threats more readily, or recovering from illnesses and injuries. In the Maevarano Formation of Madagascar, where the dry and wet seasons are extreme, constant droughts and rain cause Masiakasaurus to grow for 8 - 10 years (40% slower than the normal growth rate of non-avian dinosaurs as small as it, but 40% faster than crocodilians),[4] and Majungasaurus to grow for 20 years,[5] with the harsh conditions likely being one factor that forces it to resort to cannibalism for survival.[6] In the more southern regions of western North America, sometime after Pachyrhinosaurus went extinct, some T. rexes would decelerate their growth in times when resources are less abundant and conditions are harsher,[7] though conversely, more ideal conditions and a greater amount of resources can likely allow a T. rex to grow larger than average. And finally, the harsh winters of Alaska provide enough reason for Pachyrhinosaurus perotorum to slow down its growth during the cold months, allowing the ceratopsian to lower the cost of maintaining and nourishing itself until the impoverished state imposed by winter is replaced by the warmth of spring and summer, when resources like food are more plentiful.[2]
In this same research, Pachyrhinosaurus is shown to have developed pronounced nasal bosses around nine years of age, indicating that this is the time when they reach sexual maturity, gaining the features they need to compete against rivals and attract females. Even after this, however, the ceratopsian would continue to grow. The specimen analyzed in the study died around its 19th year of life, hence, it is presumed that Pachyrhinosaurus reached its maximum adult size around 20 years of age.[2]
Head Ornamentation[]
Two Pachyrhinosaurus males face off
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 are also the pachyrostran members of the pachyrhinosaurini tribe, Pachyrhinosaurus and its earlier relative, Achelousaurus, which do not 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. perotorum, 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. perotorum 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.
Jaws and Dentition[]
A female Pachyrhinosaurus trying to strip the last leaves from a snow-covered plant
“ They seek refuge in the forest, stripping the last leaves and dormant trees, and rooting through ferns or fallen fruit. ”
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.[8][TH 1] Ceratopsids are believed to have only fed on plants that were, at most, just one meter (3 feet) in height,[9] while gut microflora broke down plants in their stomach via fermentation.[10]
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,[DN 2] and hence chewed food more like a lizard rather than doing so in the more complex manner in which ceratopsians and ornithopods chewed.[AN 1][DN 3] Regardless of how this debate eventually gets resolved, current understanding has it that dinosaur cheeks are not like those of mammals, since, contrary to popular belief, dinosaurs did not need cheeks to hold food,[AN 2] 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.[AN 1][TH 2]
A veteran Pachyrhinosaurus intimidates two younger males
Prehistoric Planet depicts Pachyrhinosaurus perotorum 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)[11] 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).[PhP 3] These keratinized bristles likely served as an additional means of display.
Pachyrhinosaurus herd facing a Nanuqsaurus pack
Pachyrhinosaurus perotorum 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 from this story 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.
Concept art by Jama Jurabaev showing Pachyrhinosaurus canadensis rolling around in the ashes of a burnt forest
The type species, Pachyrhinosaurus canadensis, appears to have originally been planned for the fifth segment of Forests, which depicts a forest fire in the Horseshoe Canyon Formation, with the area's residents taking advantage of unique opportunities in the aftermath of the disaster. Concept art by Jama Jurabaev shows one individual taking advantage of the newly-burnt forest by rolling around in ash,[PhP 2] which not only helps it get rid of parasites, but also potentially allows it to spread its scent and pheromones on the ground as a means to mark its territory. The reason this idea was dropped from the final product is unknown, but it is possibly due to time constraints, and another animal in the segment, the saurornitholestine dromaeosaur Atrociraptor, is already shown relying on the leftovers of a forest fire to rid itself of parasites, albeit in a more peculiar manner, utilizing the smoke of a smoldering twig rather than taking a bath in ash.
A Pachyrhinosaurus surrounded by three Nanuqsaurus in the open fields of Alaska
“ 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. ”
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 (though P. lakustai is only known from the portion of the area in Alberta). The type species, 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. perotorum, 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[]
“ Within the snow-covered forest, a tense standoff develops between ancient rivals, Pachyrhinosaurus and Nanuqsaurus. ”
― The official Apple TV+ episode synopsis for Ice Worlds
Pachyrhinosaurus perotorum would have coexisted with animals like Edmontosaurus, Ornithomimus, Alaskacephale, some dromaeosaurs like Dromaeosaurus and Saurornitholestes, as well as the most common animals of the Prince Creek Formation, troodontids.[12] Given their large size, adult Pachyrhinosaurus perotorum 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 and Agonistic Behavior[]
Two Pachyrhinosaurus males battle to establish which one of them is superior
“ 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. ”
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 type species, Pachyrhinosaurus canadensis, 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 perotorum itself was not found in bonebeds, it is assumed that it also came in herds and gave parental care to juveniles. Pachyrhinosaurus perotorum has very low rates of disease and injury compared to ceratopsians in southern regions (as it is hard to tell which features on the limited number of specimens are pathologies, post-mortem damage, abnormalities, or simply the result of individual variation or other non-pathological causes), suggesting that their Alaskan habitat is not as harsh as it first appears to be on them, with the high-latitude environment not imposing any notably unique hardships on these particular centrosaurines (at least, based on fossil evidence).[3]
The wound on the right hind leg of the old, dominant bull Pachyrhinosaurus is taking longer to heal during winter
“ Battling males barely have time to eat or rest. With food now scarce, it's hard to rebuild strength, and injuries take longer to heal. To make matters worse, the Arctic's most powerful predator is always on the prowl... ”
However, this does not mean that P. perotorum lived mostly free of injuries, as skull lesions indicate that they still engaged in headbutting behavior expected of ceratopsians, though some of these lesions are actually on the back end of their nasal bosses which likely did not make contact with opposing forces as much as the front part of the nasal bosses, implying that some of their injuries were not caused by combat.[3] As depicted on Prehistoric Planet, P. perotorum has some form of social hierarchy amongst the members of the herd. Maintaining one's high position in this hierarchy means risking injury, an inconvenience that may end up affecting the animal in other matters, like fighting or evading predators.
Because of the wound on his right hind leg, the dominant male is unable to outrun the Nanuqsaurus pack, forcing him to face them in an unfavorable battle
“ As the chase goes on, one tiring bull fails to keep up with the rest, until all he can do is turn and face his attackers. ”
Even so, because of their increased size and bulk, ceratopsians like Pachyrhinosaurus are very resilient to damage. A Triceratops horridus specimen known as "Big John" was 60 years old when it died, according to Flavio Bacchia, who supervised the fossil's assembly.[13] It bore a partially-healed keyhole-shaped gouge on the right side of its frill that is around 20 centimeters (8 inches) long and 5 centimeters (2 inches) wide, and it is believed that this was the result of a rival Triceratops attacking "Big John" from behind, further supporting the use of their horns in intraspecific combat. Based on how long modern reptiles heal similar traumatic injuries, as well as the size of the lesion and the extent to which it healed (with the margins of the wound being porous and disorganized, indicating new bone growth and remodeling), "Big John" likely died six months after being wounded, albeit due to causes unrelated to the injury.[14] An even closer relative of Pachyrhinosaurus, Centrosaurus also shows the importance of herding behavior as a factor in an animal's increased chances of survival. In 2020, the first case of osteosarcoma (bone cancer) in a non-avian dinosaur was found in Centrosaurus specimen TMP 1989.018.0108 (which, as indicated by the specimen's name, was found in 1989, 31 years before its condition was discovered), left limping due to an aggressive tumor in the fibula (calf bone) of its back right leg, making the animal an easy target for hunters. However, because it lived in a herd, TMP 1989.018.0108 survived longer than expected despite its severe condition. The individual did not have time to pass away due to the cancer, as it drowned in a flood along with the rest of its herd.[15] Finally, when it comes to Pachyrhinosaurus itself, several specimens, each one bearing notable lesions, have been cataloged by the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta, Canada. One of these, Pachyrhinosaurus lakustai specimen TMP 1989.055.1234, has a massive hole stretching across the centrosaurine's face from the front of the right eye to the part below the right eye, coupled with a short, round growth by the left branch of the maxilla (upper jaw) and lesions on the ceratopsian's left squamosal (part of the cheek), hence, the specimen was nicknamed "Harvey" by the technicians who prepared the fossil, referencing the Batman villain Two-Face (the alter-ego of former Gotham district attorney Harvey Dent, who got half his face hideously scarred and disfigured by acid, albeit on the left side as opposed to the right, which is the case for TMP 1989.055.1234).[16] Despite all this, it has been determined that none of these injuries lead to the animal's death. Up to the time when it and the rest of its herd mates met their demise, TMP 1989.055.1234 was not only alive, it was also able to keep up with the rest of its group rather than straggling behind,[17] mirroring the life and death of Centrosaurus specimen TMP 1989.018.0108. These cases not only prove the tremendous durability of ceratopsians, but also implies how formidable and capable predators like tyrannosaurs must be to tackle and take down these powerful animals.
Defensive Tactics[]
Pachyrhinosaurus herd in a standoff against a Nanuqsaurus assault
“ Out here, the herd will be able to close ranks and be able to form an impenetrable wall of armored heads and muscle. ”
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.
Pachyrhinosaurus herd panics and flees from Nanuqsaurus pack
“ 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. ”
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.[18] 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.[19][20]
Gallery[]
Join us for a story you've never heard, on a scale you've never witnessed
