Introduction

Pachyrhinosaurus (PACK-ee-rye-no-SAWR-us; “Thick-nosed lizard”)

Pachyrhinosaurus is a genus of centrosaurine ceratopsid dinosaur that lived during the Late Cretaceous period, approximately 73.5 to 69 million years ago. Fossils of this herbivore have been discovered in what are now Alberta, Canada, and Alaska, USA. The name Pachyrhinosaurus, derived from Greek words, refers to the large, flattened, bony growth, or boss, on its snout, a feature that sets it apart from many other horned dinosaurs. The first species, Pachyrhinosaurus canadensis, was described by paleontologist Charles M. Sternberg in 1950. Since then, two other species, Pachyrhinosaurus lakustai and Pachyrhinosaurus perotorum, have been identified, showcasing diversity within the genus.

Description and Classification

Pachyrhinosaurus was a large, four-legged plant-eater, typical of the ceratopsid family. Adults are estimated to have reached lengths of 5 to 7 meters (about 16 to 23 feet) and weighed around 2 to 4 metric tons. It had a stocky, powerful build with strong limbs to support its heavy body. The most notable feature of Pachyrhinosaurus was its skull. Instead of a prominent pointed horn on its nose, like its famous relative Triceratops, Pachyrhinosaurus possessed a massive, thick, and often roughened bony boss. Some individuals or species also had a similar boss above their eyes. In some cases, a smaller horn might have been present at the center of the nasal boss.

Like other ceratopsids, Pachyrhinosaurus had a large bony frill extending from the back of its skull. This frill was adorned with various small horns or bumps, called epoccipitals, along its edge. The exact arrangement and size of these frill ornaments, as well as the size and shape of the nasal boss, varied between the different species of Pachyrhinosaurus. For example, Pachyrhinosaurus lakustai is known for often having two distinctive, backward-curving horns on the top edge of its frill.

Pachyrhinosaurus belongs to the family Ceratopsidae, commonly known as horned dinosaurs. Within this family, it is classified as a member of the Centrosaurinae subfamily. Centrosaurines, like Centrosaurus and Styracosaurus, often had more elaborate nasal horns or bosses and shorter, more ornamented frills compared to the Chasmosaurinae subfamily, which includes Triceratops. Pachyrhinosaurus is thought to be closely related to another centrosaurine, Achelousaurus, which also had bosses on its skull, suggesting a possible evolutionary link or shared ancestry.

Distinguishing Features

Pachyrhinosaurus can be identified by several key characteristics that differentiate it from other ceratopsid dinosaurs:

• A massive, flattened or roughened bony boss on the snout where many other ceratopsids had a prominent horn.
• Often, a similar bony boss located above its eyes.
• A relatively large bony frill at the back of the skull, which could be variably adorned with small horns (epoccipitals) and sometimes larger hornlets, depending on the species.
• The frill of some species, like Pachyrhinosaurus lakustai, featured unique, prominent horns or parietal spikes.
• Differences in the size, shape, and texture of the bosses and frill ornamentation distinguish the three known species: P. canadensis, P. lakustai, and P. perotorum.

Paleoenvironment and Diet

Pachyrhinosaurus inhabited North America during the Late Cretaceous. Its remains have been found in sediments that indicate it lived in coastal plain and floodplain environments. Discoveries in Alaska, such as those of Pachyrhinosaurus perotorum from the Prince Creek Formation, show that these dinosaurs lived in high-latitude regions. These environments would have experienced cool temperatures and seasonal variations, including long periods of winter darkness. This suggests Pachyrhinosaurus was well-adapted to such conditions.

It shared its ecosystems with a variety of other dinosaurs. These included hadrosaurs (duck-billed dinosaurs) like Edmontosaurus, other types of ceratopsians, armored ankylosaurs, and predatory theropods such as tyrannosaurids (e.g., Albertosaurus and Gorgosaurus in Alberta, and the smaller Nanuqsaurus in Alaska). The discovery of large bonebeds containing hundreds of Pachyrhinosaurus individuals, like the Pipestone Creek bonebed in Alberta (primarily P. lakustai), suggests they may have lived in large herds, possibly for protection or during migrations.

As an herbivore, Pachyrhinosaurus had a powerful parrot-like beak at the front of its mouth, used for snipping off tough plant material. Further back in its jaws, it possessed complex dental batteries – rows of continually replacing teeth – designed for slicing and grinding vegetation. Its diet likely consisted of low-growing plants such as ferns, cycads, and primitive flowering plants available in its environment.

Significance and Ongoing Research

Pachyrhinosaurus is a significant dinosaur because it showcases the remarkable diversity of skull ornamentation within the Ceratopsidae family, particularly demonstrating an alternative to the prominent nasal horns seen in many of its relatives. The discovery of different species across various locations and geological timeframes, like P. canadensis, P. lakustai, and P. perotorum, helps paleontologists understand evolutionary changes and regional adaptations within a single dinosaur genus.

The Alaskan finds are particularly important for studying how dinosaurs thrived in polar environments. Furthermore, the extensive bonebeds provide valuable information about herd behavior, population structures, and possibly mass death events, offering a window into the social lives of these animals. The function of the large nasal boss is still a topic of discussion among scientists. It might have been used for display to attract mates or intimidate rivals, for species recognition, or in physical contests like head-pushing or flank-butting, rather than direct combat as a sharp horn might imply.

Ongoing research on Pachyrhinosaurus continues to focus on understanding its growth and development (ontogeny) by studying fossils from individuals of different ages. Paleontologists are also working to clarify the precise evolutionary relationships between the different Pachyrhinosaurus species and their links to other centrosaurines such as Achelousaurus. Studies of the bonebeds, using taphonomy (the study of how organisms decay and become fossilized), aim to reconstruct the events that led to these mass accumulations and provide more details about herd dynamics and potential migratory behavior, especially for the northern populations.