The story
Around 100 million years ago, high sea levels and a sinking continental interior let water from the Arctic and the Gulf of Mexico meet across central North America. At its largest the Western Interior Seaway was about 4,800 km long, about 970 km wide and up to 760 m deep. It divided the continent into Laramidia in the west and Appalachia in the east, and withdrew only about 70 to 55 million years ago as the Rocky Mountains rose.
The sea was packed with life. Chalk beds in Kansas, the Niobrara Formation, hold mosasaurs, plesiosaurs such as Elasmosaurus, the 5 m fish Xiphactinus, the giant turtle Archelon, toothed diving birds like Hesperornis, and ammonites and giant clams. On land, the divided continent isolated dinosaurs on the narrow western strip, where horned dinosaurs and tyrannosaurs evolved separately from those in the east. The seabed mud became the Pierre Shale and other rocks that now underlie the Great Plains.
Why it mattered
- It isolated western dinosaurs, producing the tyrannosaurs and horned dinosaurs of Laramidia such as Triceratops.
- Its fossil beds give the best view of Late Cretaceous marine reptiles and fish.
- Its high sea level was part of the Cretaceous greenhouse world.
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