Pentaceratops aquilonius
Pentaceratops aquilonius is another of these museum discoveries- I was rummaging around in the Canadian Museum of Nature, looking for material of Mojoceratops, and ran across some frill pieces from the Manyberries area of southeastern Alberta. Wann Langston described the animal in 1959 and noted (correctly) that it most closely resembled Anchiceratops compared to the ceratopsian material that had been described.
In fact, it was far more similar to Pentaceratops. The problem was that the frill of Pentaceratops- although discovered- hadn't been described yet. Langston did the best one could with what was available. Years later, when I took a look at it, the Pentaceratops frill had been described, so it was pretty clear that the Manyberries animal was actually related to it.
My initial assumption was that this was actually P. sternbergi. However, the hooks on the back of the frill, although upturned, didn't turn forward. The notching on the back of the frill was also very weak. These are primitive features, suggesting it was not only more primitive than Pentaceratops, but more primitive than Utahceratops as well- sister to the group formed by the two southern species.
The specimen comes from pretty high up- in the Dinosaur Park Formation- in the Lethbridge Coal Zone which produced Chasmosaurus irvinensis. The dinosaurs found here seem to be distinct from the ones lower down in section, so Pentaceratops probably didn't coexist with animals such as Centrosaurus and Styracosaurus found lower down; it was probably moving into Alberta later, and was part of a different, slightly younger fauna.
This animal ended up getting a different name- Pentaceratops aquilonius- to emphasize that it's part of a more primitive, northern lineage. Aquilonius means 'northern'. I got some questions about why I chose that name- because it sounds nice, of course- but the obvious implication was that people didn't think it sounded very good. Well, tough. You don't like it, go name your own dinosaur. I thought it had a nice ring to it.
Whether these are all separate species, or merely subspecies... hard to say. There are consistent morphological differences between northern and southern populations of bison, for example. Would a paleontologist consider them separate species? Perhaps. It's a bit arbitrary, where one species ends and another begins, what's a species and what's merely a subspecies.
But the fossils do show something interesting- different dinosaur lineages, closely related, in the north and in the south. How is it that this can happen? The conventional explanation for speciation is that some kind of isolating mechanism separates the populations, and they evolve in different directions. The problem: there is no isolating mechanism. Dinosaurs clearly could walk between the north and south- otherwise the same genera- Pentaceratops, Parasaurolophus, and Stegoceras, for example- wouldn't be found in both places. But frequently they did not exchange, leading to populations diverging and producing distinct morphologies in the north and in the south: genetic isolation without physical isolation. How is that possible?
We speculate about climate being involved, but some dinosaurs were able to move from the South up into the Arctic Circle, then into the Gobi Desert from there, so their climactic tolerances were clearly pretty wide. Perhaps the flora? But herbivores again, could disperse from the South to the Arctic and then the Gobi- encountering (and eating) radically different plants on the way. Dinosaurs weren't that particular about what they ate. Mountains? No good either: mountains won't stop tiny squirrels and foxes today, how could they slow down huge dinosaurs? Oceans- plausible, except there's no evidence for a seaway separating the north and south.
After thinking about it, the only obvious mechanism for preventing northern dinosaurs moving south, or vice versa, is other dinosaurs. The main barrier to species moving their ranges is rarely climate or flora or whatever- it's your competitors. I call this the biotic barrier to dispersal- it is probably the main thing driving endemism in continental environments.
Of course, it can't be operating all the time- or dinosaurs (and other species) couldn't disperse from one area to another. So what causes it to break down? My guess is that environmental shifts- severe climactic disruptions, prolonged droughts, volcanic-induced warming or cooling- can wipe out local species, or cause them to be poorly adapted to their habitats. That allows outside species to move in. They become well adapted to the environment, evolve into a new species, and prevent competitors from moving in- at least until the next environmental disturbance.
Paper here: http://www.sciencedirect.com/science/article/pii/S0195667114001293