Pachyrhinosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ceratopsidae

Genus: Pachyrhinosaurus

Species: Pachyrhinosaurus canadensis

Introduction

Pachyrhinosaurus was a large herbivorous dinosaur that lived in the forests and plains of Cretaceous North America. A member of one of the most successful herbivorous dinosaur groups of the Cretaceous, Pachyrhinosaurus remains have been located in what is now in northern Canada and Alaska.

During the Triassic and Early Jurassic, the continents of the world were joined into a single supercontinent called Pangaea. This allowed many dinosaur lineages to spread across the world, where they diversified into numerous species that became isolated as Pangaea split into two supercontinents—Gondwanaland in the south and Laurasia in the north—during the Jurassic-Cretaceous.

The horned dinosaurs, including Pachyrhinosaurus, evolved from a common ancestor that lived in Laurasia in the Jurassic. During the Cretaceous, this group radiated to become one of the most diverse lineages of herbivores in North America.

Classification

Scientists used two primary methods to classify organisms into different groups. Linnaean taxonomy classifies organisms according to overall physical similarity, while an alternate method, called “cladistics,” creates groups called “clades” based on the evolutionary relationships between species.

From a Linnaean perspective, Pachyrhinosaurus was placed in the order Ornithischia, or “bird-hipped” dinosaurs, which are united by their backward-facing pelvic structure, similar to modern birds. Further, Pachyrhinosaurus was placed in the infraorder Ceratopsia, the members of which feature horny beaks, armored plates covering the neck and horns and/or bony plates on the skull.

Using cladistic analysis, Pachyrhinosaurus belongs to the clade Genasauria, which includes beaked herbivores, as well as to the Cerapoda group, the members of which feature unique sets of thick, enameled teeth. The animal is further divided into the Centrosaurinae, a clade defined by nasal horns and thick structures on the skull.

As with any dynamic study in the field of science, cladistic analyses serve as hypotheses and can change over time as new specimens are discovered and as the science advances. In 2017 Matthew Baron and his colleagues posited a common ancestor for the ornithischians, such as Pachyrhinosaurus, and theropods, such as Allosaurus, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Pachyrhinosaurus was a large, armored herbivore that, at maturation, reached between 5 and 8 meters (16 and 26 feet) in length and nearly 3.5 meters (11 feet) in height. A mature individual could weigh more than 2 tons (4,409 pounds).

Like most members of the family Ceratopsidae, Pachyrhinosaurus was sturdily built, with short legs relative to its body. Its legs and toes were muscular, with claws that were modified into hoof-like spurs. Pachyrhinosaurus had a large crest extending from the rear of its skull, protecting the neck, and a ridged neck frill. Many members of the family display prominent sharp horns on their skulls, but Pachyrhinosaurus had bony spurs covering its nose, known as a "nasal boss." The nasal boss may have had a thick horn or keratin sheath and changed shape as a juvenile matured into an adult.

The animal's mouth was modified into a sharp, bony beak. Its eyes were set on the sides of the large skull, and the animal possessed enlarged nasal passages. The large body terminated in a stocky tail. Some ceratopsian fossils display impressions of skin with mosaic scales, a trait that might have also applied to Pachyrhinosaurus.

Intelligence

Pachyrhinosaurus was a member of the Ceratopsidae family, the members of which were estimated to possess EQ values in the range of 0.6 to 0.9. Based on this scale, most ceratopsians would have been less intelligent than modern mammals that occupied a similar niche, like cows or wildebeest.

Reproduction and Population

Some paleontologists have suggested that different “species” of Pachyrhinosaurus may actually be the same species that displayed sexual dimorphism, meaning that males and females had different physical characteristics. This might explain the differences in Pachyrhinosaurus skulls: some have “convex” nodes over the nose and others have “concave” bony nodes. This is a trait that occurs is some reptile species and a number of bird species. Sexual dimorphism is associated with certain reproductive behaviors, including male or female competition for mates. If ceratopsians like Pachyrhinosaurus had dimorphism between the sexes, this may indicate a competitive mating system.

Most paleontologists believe that ceratopsians, including Pachyrhinosaurus, were oviparous, or egg layers. Fossilized nests of several species of ceratopsian have been found. Some paleontologists believe that ceratopsians may have bred and laid their eggs in herds. This behavior would be analogous to the herding and synchronized breeding of many modern herd mammals.

Diet

Paleontologists believe that all ceratopsians, including Pachyrhinosaurus, were herbivores. Pachyrhinosaurus had a sharp beak, which was probably used to cut through vegetation as the animal grazed.

Given the types of vegetation discovered from botanical fossils in the area, Pachyrhinosaurus would have had a variety of ferns, palms, cycads, and conifer plants to eat within its habitat. Pachyrhinosaurus also had a large abdominal cavity, which may have evolved to hold a ruminating stomach in which plant matter could ferment to speed digestion.

Behavior

Paleontologists have discovered fossil beds containing remains of multiple adult Pachyrhinosaurus and juveniles, indicating some social behavior within the group. Because of these discoveries and the animal's resemblance to modern ruminants (animals that regurgitate and chew partially digested food as part of their digestion process), some have suggested that ceratopsians were herding herbivores, perhaps migrating to follow seasonal vegetation. More evidence is needed to support the herding hypothesis, however.

Herding and other group behaviors would have afforded ceratopsians like Pachyrhinosaurus better defense against formidable predators. Some paleontologists have also suggested that the horns, spurs, and crests that define the family may have also been used in conflicts within the species, such as competition over mates, as seen in many modern animals.

Habitat and Other Life Forms

During the Cretaceous, the center of North America was covered by a shallow inland sea surrounded by swamps, flood plains, and forests that blended into arid scrub. The dominant plants were gymnosperms, seed-bearing plants such as conifers and cycads; angiosperms, or flowering plants, were also beginning to spread across the continent, most appearing as small weeds and shrubs.

A variety of small reptiles, amphibians, and mammals lived alongside Pachyrhinosaurus in Late Cretaceous Canada, many closely resembling modern species. There were also early birds in the area, though the pterosaurs were still the dominant flying vertebrates of the day.

Other members of the Ceratopsia in Cretaceous Canada included the genera Anchiceratops and Arrhinoceratops. There were also a large number of hadrosaurs, or “duck-billed” dinosaurs, in the area, including Brachylophosaurus and Edmontosaurus. Another successful herbivorous dinosaur group in the area was the ankylosaurs, including Euoplocephalus , which was known for its heavily armored back and clubbed or spiked tail.

There were many predatory dinosaurs in Cretaceous Canada, including large theropods (“beast-footed” carnivorous dinosaurs) like Daspletosaurus and Gorgosaurus, both relatives of Tyrannosaurus. Smaller dromeosaur predators, sometimes called “raptors,” also hunted in Canada, including genera like Dromeosaurus. There were several species of ornithomimids, or “bird mimics,” living in Cretaceous Canada, including the genus Dromiceiomimus, which has been found close to Pachyrhinosaurus fossils.

Research

The first Pachyrhinosaurus was discovered by paleontologist Charles Sternberg in 1946 from a formation in Alberta, Canada. Later discoveries included specimens of a variety of sizes in other formations in northern Canada. A formation at Pipestone Creek in Alberta revealed more than fourteen skulls, as well as remains from dozens of animals.

The Pipestone Creek formation also revealed that Pachyrhinosaurus displayed some degree of morphological differences between individuals, suggesting the possibility of sexual dimorphism between the sexes. The individuals ranged widely in age as well. The Pipestone Creek fossils have become one of the most important bone beds of ceratopsian fossils, providing thought-provoking evidence about the social lives of the horned dinosaurs.

The several species of Pachyrhinosaurus have been distinguished by the differences in their beak, nasal boss, and frills. The type species remains P. canadensis, which dates to about 71 to 72 million years ago (Ma). Members of P. lakustai (84.9–70.6 Ma) were found at Pipestone Creek in the mid-1970s and identified as a separate species in 2008. Those earlier pachyrhinosaurins notably had a central horn that grew from the frill behind their eyes. In the mid-2000s P. perotorum (70–68.5 Ma) was found in Alaska, the farthest north of the Pachyrhinosaurus species, and was named in 2012. Its definiting traits were a larger nasal boss and thicker frill spikes. The discovery of multiple P. perotorum trackways bolstered the herding hypothesis.

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