In my post a few days ago about scary predators, I mentioned a curious feature of the prehistory of North America -- the Western Interior Seaway, which for a bit over thirty million years in the mid- to late-Cretaceous Period split the continent in half, connecting the Arctic Ocean to the Gulf of Mexico.
This meant that a broad strip of land from current-day Alberta to east Texas was underwater. In fact, Kansas -- which seldom comes to mind when you think of the ocean -- is one of the best places in the world to find late-Cretaceous-age marine fossils like mosasaurs, plesiosaurs, and scary-ass enormous carnivorous fish like Xiphactinus.
The Seaway is thought to have formed because the Laramide Orogeny -- the combination of uplift and volcanism that created the Rocky Mountains -- caused downwarping of the continental crust to the east, allowing the ocean to flood inward. The Laramide uplift eventually would be the Seaway's undoing, however; the upward push gradually shifted eastward, lifting what is now the American Midwest and leaving it high and dry. (Of course, this final stage happened right around the same time as the Chicxulub Impact occurred, so living things at that point had other worries; but fossil beds in North Dakota that preserve evidence of the actual impact show that most of what had been the Seaway had already broken up into swamps, rivers, and shallow lakes.)
As you can see from the map, the Western Interior Seaway split North America into two continents, a western one (Laramidia) and an eastern one (Appalachia). What's curious is that we know a great deal more about the paleontology of Laramidia than we do of Appalachia. Most of what come to mind as the big, charismatic dinosaur species of the late Cretaceous, such as T. rex and Triceratops and Parasaurolophus, lived in Laramidia; and just this week, a paper appeared in PLOS One about one of the Laramidian mammals, a muskrat-sized marsupial called Heleocola.
So what was happening in Appalachia?
The answer is "we're not really sure," because the evidence is so slim. A rapidly-rising mountain range, such as what Laramidia was experiencing at the time, results in a lot of eroded sediments and volcanic ash with which to bury recently-deceased organisms, making the western parts of North America prime places for hunting fossils. The part of the continent east of today's Mississippi River is, on the whole, made up of rocks of far greater age. (For example, where I live -- a bit down and to the right of the letter "C" in "Appalachia" on the map -- has rocks of Devonian age, which were already about three hundred million years old when the late Cretaceous dinosaurs were lumbering around.)
So old, stable crust with gentle topography = much less eroded sediment, and little to no formation of the sedimentary rock where you find fossils.
There have been a few finds here and there, even if nowhere near the fossil riches in the western half of the continent. We know there were species from some of the familiar groups -- tyrannosaurids, hadrosaurs, coelurosaurs, ornithomimids, and lambeosaurs -- but on the whole, they were more like their ancestors (i.e. they had changed less over time, and still resembled the "basal" or "stem" lineages). Why this happened is unknown. There's a general rule that slow environmental change and low selection leads to very slow rates of evolutionary change (thus the oft-quoted statement that sharks have barely changed in overall form in two hundred million years, which is only true if you pick and choose which species to look at). So were the inhabitants of Appalachia simply in a more congenial environment, as compared to the ones in the tectonically-active, rapidly-rising mountains of Laramidia?
It's certainly a possibility, but it's hard to make any real determinations based on a lack of evidence. As I've pointed out before, even with the most favorable of conditions, only an extremely small fraction of organisms ever become fossils; what we don't know about the past vastly outweighs what we do know. Still, it's mind-boggling to think about a time when things were so very different. My home territory of the Finger Lakes Region of New York, now cool hardwood forests where the scariest denizens are foxes and black bears, were then warm, humid subtropical jungles, with a climate more like Central America, and populated by a huge assemblage of dinosaurs we're only beginning to understand.
Just as well things have changed, really. I have a hard enough time keeping bunnies out of my vegetable garden, I can't imagine how I'd deal with my lettuce plants being munched by hadrosaurs.
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