Under the Cretaceous Sea

Would you believe me if I said that the Midwestern US was not always so barren, that the lush cornfields of Kansas were once a vast ocean that stretched from Appalachia to the Rockies, that this ocean was teeming with larger than life predators and one of the most active ecosystems on the planet? Well, you better believe it, cause I'm here to tell you about one of my favorite periods of prehistory: the Cretaceous Western Interior Seaway of North America. 

Western Interior Seaway of North America, circa 75 MYA

Modern North America: 3,000 miles of uninterrupted highway, punctuated by large cities, varied landscapes, and more than the occasional cow. The east coast boasts beautiful, flat landscapes and shorelines that span its length. As you move west past the ancient Appalachian mountains, weathered by millions of years of wear and tear, you enter the flat and seemingly unassuming Midwest. This area of North America seems to stretch for months rather than days (especially in a Uhaul truck moving all your wordly possessions like I did), but beauty nonetheless abounds as you encounter vast temperate grasslands interrupted by the occasional bustling metropolis. Further west still and elevation begins to climb as you pass through the towering giants of the Rocky Mountains, young and yet unaffected by the forces steadily eroding Appalachia. Finally, the west coast looms ahead, sporting an incredible range of biomes from rich, arid desert of the Southwest and Mexico to the gloomy, never-ending rains and snow of the Pacific Northwest and Canada. In today's world, the geography of North America teems with diversity, providing homes to a variety of flora and fauna supported by the different biomes and habitats stretching between the coasts. 

However, North America was not always this endless gradient of earth, stretching for thousands of uninterrupted miles. As the dinosaurs roamed the land and pterosaurs stalked the skies, North America was sliced in half by a relatively shallow intercontinental sea: the Western Interior Seaway. 

In the mid-Cretaceous period, some 90 million years ago or so, a significant change began on the continent of North America. Almost synchronously, an arm of the Arctic Ocean began spreading south and a portion of the Tethys Sea near the Gulf of Mexico began spreading north. Eventually they collided, forming what we know today as the Western Interior Seaway. As time marched on, the seaway continued to expand, creating a rich marine ecosystem that effectively clubbed North America in two. The resulting landmasses were Appalachia to the east and Laramidia to the west. At its largest, the Western Interior Seaway stretched approximately 1,000 km or 620 miles wide and was only 2,000-3,000 feet deep, which is quite shallow as seas go. 

Throughout the history of the seaway, sea levels rose and fell multiple times, connecting and separating the two landmasses over and over. As a result, barriers to gene flow were created and torn down again and again. Populations that were separated by the seaway were once again able to mix and interact as sea levels fell. As sea level rose again, those barriers were replaced, preventing any further interactions between the populations. As a result, speciation rates were high in this region during the Cretaceous, giving us rich diversity and active ecosystems. 

Western Interior Seaway through time, 92 MYA to 65 MYA

The rich ocean waters of the Western Interior Seaway were filled with a variety of marine life, from the largest predators to the smallest bottom feeders. Perhaps most amazing of the Cretaceous marine life were mosasaurs, huge aquatic predators that could grow up to 60 feet long. Mosasaurs were the apex predators of these ecosystems and directly competed with giant sharks and terrifying fish. With predators like these swimming around, there had to be a rich diversity of prey to feed upon and sustain their populations. My personal favorite, the ammonites, were quite abundant during this time and dominated the open ocean, catching small fish and other invertebrates to prey on. This seaway was also home to an incredible number of other mollusks, plankton, birds, and echinoderms. 

Kansas mosasaurs of the Western Interior Seaway

Fossilized Xiphactinus, the killer fish of the Cretaceous 

Life in the Western Interior Seaway

As we neared the end of the Cretaceous and the inevitable mass extinction event at the K-T boundary, the Western Interior Seaway shrank and eventually receded. The same uplifting that dried up the intercontinental sea began to create the beautiful giants in the west: the Rocky Mountains. As the earth's crust shifted and rose, the seaway disappeared; those pieces of continental crust then collided to create the beautiful Rockies we have today. This uplifting continues to this day, growing the mountains of the Rockies inch by inch every single year. 

I hope you enjoyed learning about this amazing point in history as much as I enjoyed writing it! My Master's research focused on the biogeography of mollusk species during this time period, and it is a topic I love talking about! If you want to learn more about this amazing, ancient ocean, be sure to check out the book and website by Michael J. Everhart: Oceans of Kansas. In addition, check out the Digital Atlas of Ancient Life (and the new app version!), managed by my Master's advisor at KU, to check out the fossil evidence collected throughout Kansas and other parts of the Midwest that gave us all this amazing information about the WIS. Stay curious my science friends!