Wednesday, May 24, 2017

The Long, Complicated History of Appalachian Ceratopsians

North America during the Campanian
The title is a joke. Finding ceratopsian remains east of Texas is an incredibly recent phenomenon. Oh sure, there are plenty of (fragmentary) dinosaurs known from the eastern half of the United States, but those taxa tend to be tyrannosauroids, nodosaurs, ornithomimids, and hadrosaurs. I should mention at this point that, during much of the Late Cretaceous, North America was cleaved into three islands by the presence of a shallow interior sea called the Western Interior Seaway (WIS): Laramidia to the west, Nunavut to the north, and Appalachia to the east. Laramidia’s eastern north-south shoreline is where we get most of North America’s dinosaurs—Alberta, Montana, Utah, and (to a lesser extent) Texas are all hotbeds of dinosaur action, and that includes ceratopsians.

Appalachian dinosaurs differ sufficiently from their Laramidian cousins to imply a lengthy period of isolation—all of Appalachia’s dinosaurs must have migrated over there prior to the appearance of the WIS. Once the WIS receded at the end of the Campanian (it was gone completely by the Maastrichtian), dinosaurs from Laramidia and Appalachia were free to comingle—if they did at all, but poor sampling in the eastern United States (there’s a paucity of Late Cretaceous exposures) is an omnipresent frustration.

The  North Carolina ceratopsian
Anyway, in 2016, Longrich published on a partial ceratopsianmaxilla from North Carolina (paywalled). It is very small and there aren’t any teeth in it, but Longrich ascribes it to the Leptoceratopsidae. However, Andy Farke has told me it could also simply be from a basal neoceratopsians like Aquilops. To me, this diagnosis is the safer bet. Saying it’s a leptoceratopsid demands that leptoceratopsids evolved prior to the appearance of the WIS and that they migrated not just from Asia to North America but from Asia to Eastern North America by the Campanian. Given that the rest of Appalachia’s dinosaurs are seemingly from more plesiomorphic stock than their Laramidian counterparts, it makes more sense to—at this juncture—call it a basal neoceratopsian and leave it at that.


The Mississippi ceratopsid tooth
Today, we got another Appalachian ceratopsian from Farke& Phillips (2017). This time, it’s a late Maastrichtian specimen from Mississippi. The downside is that the specimen is a single tooth. Thankfully, ceratopsian teeth are quite distinct, and the authors are able to show that it is more derived than the teeth you find in Zuniceratops or Turanoceratops. However, because chasmosaurines and centrosaurines did not differ in terms of teeth, that’s as far as they can classify it (Ceratopsidae).

This Mississippian ceratopsid is found in rocks deposited after the WIS began to retreat, so it may represent an early dispersal of Laramidian dinosaurs into Appalachia. Is there a reason it couldn’t represent a relict ceratopsid, marooned in Appalachia after the WIS cut the country in two? In fact, there is! The earliest ceratopsid fossils—guys like Diabloceratops and Albertaceratops—turn up after the WIS had already separated Laramidia from Appalachia. So unless some plucky ceratopsid swam across a surprisingly wide body of seawater (unlikely considering ceratopsids didn’t do well when submerged), the Mississippi ceratopsid must have been a western immigrant.

2 comments:

  1. I would have to agree that the Late Cretaceous ceratopsian remains from Appalachia could be descended from leptoceratopsids and ceratopsoids that immigrated from Laramdia about 100 million years ago because the pre-Turonian fossil record of North American ceratopsians is poor (Aquilops is the only mid-Cretaceous horned dinosaur from Laramidia) and the early evolutionary history of Neoceratopsia is complex.

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  2. Looking at the map, I'm struck by the enormous amount of extra land area left by the retreat of the WIS. Do we know if that had an climatic effects?

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