Laramidian Endemism, Part 2

Last week we saw that, during a good portion of the Late Cretaceous, North America was divided into two unequal halves by the shallow Western Interior Seaway (WIS). Those dinosaurs on western Laramidia were boxed between mountains to the west and the fluctuating coastline of the WIS to the east. They lived on a narrow, vertical strip of land running from Alaska’s North Slope to Mexico. On the other hand, everybody had beachfront property. We can’t say much about Appalachia’s Mesozoic composition, as much of its fossil-bearing rocks were destroyed by the last Ice Age. However, tyrannosaurs and hadrosaurines who were isolated prior to the formation of the WIS evolved along different lines than their western relatives. Dryptosaurus and Appalachiosaurus are the island continent’s aberrant tyrannosaurs. Hadrosaurus and a few potentially dubious genera (Lophorhothon, Hypsibema) represent the area’s hadrosaurine population. It’s ironic that, despite the area’s drastic surface area deficit, Laramidia is ridiculously rich in dinosaur fossils whereas Appalachia is not at all.

We will focus on ceratopsids because that’s what my Secret Project is about and I therefore have some modicum of experience dealing with their living situation on Laramidia.

The short version is that ceratopsians originated in Asia, initially diversified, and crossed over to North America during the Albian, at the end of the Early Cretaceous. The Ceratopsidae evolved during the Campanian on Laramidia—after the establishment of the WIS. Diabloceratops, the oldest ceratopsid at 79 mya, is from Utah. We’ll call its locality Southern Laramidia. As far as ceratopsids are concerned, Southern Laramidia consists of Utah, New Mexico, Texas, and Mexico. Northern Laramidia, again in terms of ceratopsids, consists of Alaska, Alberta, and Montana.

Where Campanian ceratopsids lived, from Sampson et al. (2010).

What’s interesting is that Northern Laramidia and Southern Laramidia seem to have different sets of ceratopsids. Diabloceratops, Nasutoceratops, Utahceratops, Kosmoceratops, Pentaceratops, "Titanoceratops,” Coahuilaceratops, Agujaceratops, and “Torosaurus” utahensis are all from Southern Laramidia. Up in Northern Laramidia, you’ve got Chasmosaurus, Vagaceratops, Mojoceratops,” Anchiceratops, Arrhinoceratops, Pachyrhinosaurus, Centrosaurus, Styracosaurus, and more. It may seem like the North is more diverse than the South, but remember two things: Southern Laramidia has only recently seen a boom in paleontological exploration, while people have been prospecting Alberta and Montana since the Bone Wars. Second, ceratopsids went through lightning-fast (geologically speaking) replacement rates. A single species might last a half a million years.*

Just to be clear, this North/South dichotomy was present in other Campanian dinosaurs, too. Southern Laramidian tyrannosaurs Tetraphoneus, Bistahieversor, and Lythronax are mirrored by Northern Laramidia's Nanuqsaurus, Gorgosaurus, and Daspletosaurus. You don’t find Albertosaurus in Utah, and you don’t find Tetraphoneus in Montana. Duckbill dinosaurs and ankylosaurs also follow this endemic pattern.

I want to stress that all this endemism, and in fact the Laramidia/Appalachia separation as a whole, was a Campanian event. Once the WIS drained away during the Maastrichtian, dinosaur diversity in North America plummeted. Triceratops, Tyrannosaurus, and Edmontosaurus dominated their respective guilds, seemingly flushing out all competitors.

According to Wick (2014), Tyrannosaurus even got into the Lone Star State, where game was even bigger (Alamosaurus).

As Sampson et al. (2010) write, “of the dozens of species of Campanian dinosaurs described from this landmass, none can currently be placed with confidence in both the northern and southern provinces.”** So what caused Laramidian endemism in the first place? 

Current thinking is that a geographic barrier must have existed which separated the southern and northern provinces, although exactly what form that barrier took remains mysterious. Sampson et al. suggested a mountain-building event, further incursion of the WIS, or the existence of a persistent river system, although they lament that none of these options appears probable. More likely, climactic conditions may have differed considerably between the two provinces.

Interestingly, once the WIS drained to the south and Laramidia was once again joined up with Appalachia, it was the southern ceratopsids who spread north and evolved into familiar Maastrichtian taxa such as Anchiceratops, Triceratops, and Torosaurus. Everyone else got the boot, although Pachyrhinosaurus escaped by traveling ever-farther north and eventually settling down at the top of the world. It's certainly possible that more southerly species apart from Triceratops (or "Ojoceratops") existed but haven't been found yet.

Next up, some possible cross-pollination between the northern and southern provinces!

*Pentaceratops and Anchiceratops are the longest-living species, each lasting between 1.5 and 2 million years.

**Although, in the next post, we’ll see that might not be the case.