What Lived in North America's Ancient Inland Sea?

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Some people consider the Great Plains of America to be "flyover country." The truth of that pejorative falls outside the realm of science. It is true, however, that 80 million years ago one could not cross the heart of North America without flying... or swimming. That's because a great, shallow ocean once stretched from what is now the Gulf of Mexico all the way to the Arctic Ocean.

Remnants of the Western Interior Seaway can be found above and below the ground. In Gove County, Kansas, the Monument Rocks jut magnificently seventy feet up from what is otherwise a mostly flat and featureless terrain. The remarkable earthen structures are made of carbonate rocks which formed on the seafloor over the ocean's sixty-million-year lifespan. Standing near the formations, backdropped by a bright blue sky, one can almost imagine standing on the prehistoric seabed back in the Cretaceous Period, 2,500 feet below the ocean's surface.

That's about as deep as the Western Interior Seaway got, which is positively shallow compared to the average depth of oceans today, roughly 12,100 feet. This, however, meant that the Sun's life-giving rays touched a significant portion of the water column, and so, the ocean teemed with all sorts of marine creatures. Paleontologists have unearthed fossils of plesiosaurs and mosasaurs, giant marine reptiles that grew up to sixty feet long. They've also dug up the remains of huge sharks, turtles the size of cars, and clams six feet in diameter, the largest to ever exist.

The Western Interior Seaway formed about 100 million years ago when the mountains that now define western North America lifted up as a result of tectonic forces. Those same forces flexed the land east of the mountain range downward. Waters from the Arctic Sea and the Tethys Sea (now the Gulf of Mexico) flowed in to fill the lowlands.


For about half of the Cretaceous Period, North America was essentially two islands, Laramidia to the west and Appalachia to the east. This put new selective pressures on the dinosaurs dwelling there, sparking rapid speciation. Paleontologists today are thus treated to a diverse array of unique animals to dig up.

The Western Interior Seaway was present during one of the warmest periods on Earth, when the poles were devoid of ice and sea levels were 500 feet higher. Abrupt global cooling, likely triggered by a massive asteroid, not only doomed the dinosaurs but the ocean as well. Over millions of years, the waters of the Western Interior Seaway dwindled and evaporated to be locked up in far-off frozen glaciers. Today, we're left to wonder at the ocean that was.

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