Pentaceratops aquilonius is a horned dinosaur from the Late Cretaceous of Southern Canada, and the northern cousin of New Mexico's Pentaceratops sternbergi. It comes from the Dinosaur Park Formation of Alberta, Canada, which has produced related horned dinosaurs such as Chasmosaurus and Mojoceratops. However it comes from fairly far south- near the town of Manyberries, in southern Alberta- and comes from somewhat further up in the formation, in the Lethbridge Coal Zone. It is therefore one of the youngest horned dinosaurs known from the formation. I chose the name aquilonius because I thought it sounded pretty, like a spell Hermione Granger might say to tidy up a messy room. Apparently nobody else thinks so, because everybody asked why I chose that name and implied they didn't like it. Well, too bad. When you find your own dinosaur, you can name it whatever you want. So there.
When the specimens were originally described, Pentaceratops was known, but the frill had yet to be described. Of the species that had been described, it seemed most similar to Anchiceratops, and so it was put into that genus and stayed there for many years. During the process of describing Mojoceratops, I traveled to Ottawa to have a look at the Mojoceratops specimens there, and ran across the specimens. What struck me was that it wasn't all that similar to Anchiceratops at all. In Anchiceratops, there is a pair of hornlets on the frill that stick forward, instead of backwards. In the Manyberries specimen, there are a pair of hornlets that stick forward. However, the way they attach is different. In Anchiceratops, they are stuck onto the front surface of the frill; in the Manyberries specimen, they arise from the back edge of the frill, and then curl forward. That's more similar to Pentaceratops.
Based on this, and the larger holes in the frill, it's clear that the specimens are related to Pentaceratops instead of Anchiceratops. However, there are some subtle differences- the openings in the frill aren't quite as big, the hornlets don't hook forward as strongly, the V-shaped notch in the back of the frill is weaker. In every respect, the specimens look more primitive than Pentaceratops, or for that matter, Pentaceratops' close relative, Utahceratops. So it seems to be something different- a primitive relative of Pentaceratops sternbergi and Utahceratops gettyi.
What's interesting here is that we have different dinosaurs in the north and the south- but they're not necessarily that different; they're sometimes similar enough you'd put them the same genus. There is Pentaceratops aquilonius in the north and Pentaceratops sterbergi in the south, Parasaurolophus walkeri up north and Parasaurolophus cyrtocristatus in the south.
What got me puzzling is why the dinosaurs in the north and south are different. The standard explanation for this kind of thing is that some sort of barrier forms and then the species evolve in separate directions, voila, speciation. A mountain range, or a sea could separate them. But we have no evidence that such a sea existed. And modern animals quite happily go over or around mountain ranges- even small animals like squirrels and foxes are found on either side of the Rockies. So how could an ancient Rocky Mountain range stop a giant dinosaur? Climate is another possibility. Maybe dinosaurs in the South can't tolerate the cooler north, or northern dinosaurs can't tolerate southern heat? But dinosaurs were pretty tolerant of a range of climates. Tyrannosaurs crossed from southern North America into Mongolia, and to do that they had to go through the frigid (or in the Cretaceous greenhouse climate, at least fairly cool) Arctic Circle to cross the ancient Bering land bridge. So that doesn't make sense. Climate might stop a cold-blooded lizard, but not a warm-blooded dinosaur. What about food supply? Perhaps they are specialized to eat different things. But when duckbills and ceratopsians went from North America to Asia, they had to go from one kind of plants in the south, to another in the north, and then yet another as they moved south into Asia... so that doesn't make sense.
Clearly, at certain times dinosaurs can move long distances and tolerate different climates and food sources. And at other times they are able to move from one climate and one flora to a totally different climactic zone. So what prevents them from moving and pushes the populations apart? After thinking about it, the only thing I could think of was that the dinosaurs themselves must be creating the barriers to migration.
Think of it this way. Initially a dinosaur species spreads out across the continent. Once populations reach different parts of the continent, however, they adapt to it. They adapt to the climate. Change their colors to blend into the local flora and landscape. Become adapted to parasites. Become better adapted to feeding on the local vegetation. Soon, you have populations with distinct adaptations for their habitats. Once this has happened, the dinosaurs in the North can still tolerate the climate in the south, eat the southern foods, deal with southern parasites and diseases. But not as well as dinosaurs that have been there for hundreds of thousands of years, adapting to the local conditions. Northern individuals attempting to move south will find themselves poorly adapted. Likewise, southern species trying to move north will have trouble competing with the locals. Once the populations specialize and diverge, they prevent each other from moving. The barrier to dispersal isn't physical, or climactic, or food sources- it's the dinosaurs themselves. Competition.
Of course, for all this to happen in the first place, a species must first be able to spread out to many different habitats. So how does that happen? My guess is that something disturbs the equilibrium. The climate shifts, there is a long drought, a volcanic eruption, a new predator... something leaves the well-adapted populations poorly adapted, or wipes them out. All of a sudden, the ecosystem is ripe for colonization from outside.
It is a nice name. Isn't it?