Pachyrhinosaurus

About Pachyrhinosaurus

Characteristics and Anatomy of Pachyrhinosaurus

The most prominent feature of Pachyrhinosaurus is the massive, thick bony bump called a "boss" covering its snout and above its eyes. It did not have the sharp horns that typical ceratopsians possess.

In the past, there was a hypothesis that "a huge, rhino-like horn might have grown on top of this bony boss as a base." However, as a result of examining the fine structure of the fossil surface in detail, this theory has now been rejected.
Comparing it with modern animals, it was found that the bony boss of Pachyrhinosaurus was covered with a thick, hard keratinous pad (a mass of skin), much like a muskox. This thick pad functioned as a shock absorber (cushion) when they forcefully butted heads or pushed against each other in tests of strength.

Dental Battery and Brain

It had a sharp, parrot-like beak at the tip of its mouth, and a chewing organ called a "dental battery" with hundreds of densely packed teeth at the back of its jaw. This allowed it to efficiently grind and eat hard, fibrous plants.

Also, as a result of examining the inside of the skull using CT scan technology, it is known that their brains were relatively small, and their sense of smell and hearing were not particularly well-developed.
On the other hand, the semicircular canals, which maintain body balance, were well-developed, suggesting that they adopted a posture suitable for eating low-lying plants while simultaneously pointing their frills forward to display to their companions.

Three Species and Diversity of Head Morphology

The genus Pachyrhinosaurus includes three main species depending on the age and region of the geological formation where they were discovered. The way the bosses spread and the shape of the spikes on the frill differ depending on the species.

Growth and Bosses

The massive bosses of Pachyrhinosaurus were not present from birth. When they were young, they had low "half-horns" split into left and right instead of huge bosses.
It is known that when they grew to about two-thirds the size of an adult, a large amount of bone tissue was formed between the split half-horns, undergoing a dramatic change to complete the thick boss.

Continuous Evolution: From Horns to Bosses

Over millions of years, the evolutionary process of ceratopsians (Centrosaurinae) on the North American continent changing the shape of their heads is becoming clear.

• Styracosaurus: Approx. 75.5 million years ago: A close relative of the ancestor. It had a long, straight horn on its nose.
• Einiosaurus: Approx. 74.5 million years ago: The nasal horn is heavily curved and tilted forward.
• Achelousaurus: Approx. 74.2 million years ago: The horn itself disappeared and began to be replaced by rough bony bosses.
• Pachyrhinosaurus: Approx. 73.5 to 68.5 million years ago: The bony bosses maximized and became complex until they covered the entire head.

The change from horns to bosses implies a change in their "tactics" when fighting with each other. It is thought that they shifted from a dangerous fighting style of stabbing opponents with sharp horns to a fighting style of butting heads with blunt weapon-like bosses or hitting flanks, avoiding fatal wounds to each other.

Group Life in the Harsh Polar Regions

Pachyrhinosaurus was highly social and lived in huge herds ranging from hundreds to thousands of individuals.
At Pipestone Creek in Alberta, Canada, a massive bonebed of Pachyrhinosaurus lakustai caught in a flood has been discovered. It contains individuals of various ages, from juveniles to adults, suggesting that they had a social structure where multiple generations coexisted.

How did they survive the Alaskan winter?

The newest species, Pachyrhinosaurus perotorum, lived in high-latitude regions in Alaska, close to the Arctic Circle. Alaska at the time was warmer than it is today, but even so, winter brought months of complete darkness (polar night) where the sun did not rise, and temperatures dropped.

In the past, there was a debate: "Did Pachyrhinosaurus migrate long distances south to warmer areas with food during the winter?"
However, as a result of examining a large number of fossils discovered in Alaska, it was found that there are surprisingly few paleopathologies (traces of diseases and injuries) such as bone fractures due to overwork or infections. If they had been making harsh long-distance migrations or fighting over extreme food shortages, there should be traces of trauma left on their bodies.
This "lack of injuries" is considered evidence that Pachyrhinosaurus did not make harsh migrations, but settled in Alaska to overwinter. During the winter, they adapted to the harsh environment by minimizing their activity and conflicts.

A group of Pachyrhinosaurus migrating in Alaska during the Late Cretaceous period about 70 million years ago was depicted in the 2013 UK-US-Australia co-production film "Walking with Dinosaurs". Currently, seasonal long-distance migration of Pachyrhinosaurus is denied.

Discovery and Naming

Between 1945 and 1946, skull fossils were first excavated from the Horseshoe Canyon Formation in Alberta, Canada. Later, fossils were also unearthed from the St. Mary River Formation in the same province, and in 1950, it was described as a new genus and species, Pachyrhinosaurus canadensis, by American-Canadian paleontologist Charles Mortram Sternberg.

Radiometric dating measured the strata where these were discovered to be from 74 to 67 million years ago, indicating that it was a humid subtropical climate with widespread wetlands and floodplains at the time.
Later, with the discovery of the lakustai species at Pipestone Creek in Canada, and the perotorum species in the far north of Alaska, it became clear that Pachyrhinosaurus prospered across the vast North American continent (the continent of Laramidia at the time) with varying climates.

Pachyrhinosaurus Stamp and Fossil Gallery