Western Interior Seaway

Scallop, Kingena wacoensis and Brachiopod, Neithea texana
[Grayson Formation, Lake Arlington, Texas]

The Western Interior Seaway was an extensive epeiric sea formed from the high sea levels between the middle and late Cretaceous Periods over 75 million years ago. It once covered most of the Midwest of North America with many present day states once completely submerged. A vast number of marine fossils, from vertebrates to molluscs, have been unearthed in these regions.

Devil's Toenail, Exogyra ponderosa
[Anacacho, Uvalde County, Texas]

The widespread and well-preserved fossils of marine fauna has allowed for the environment of the Western Interior Seaway to be determined. For the majority of its existence, water column stratification gave way to dysoxic to anoxic bottom-water environments. These conditions led to a number of opportunistic and low oxygen adapted taxa to emerge. There was also a predominance of brackish-water conditions on the surface. As the inland sea stretched from the far north down to the tropics, there was a mixing of cooler, less saline waters with warm tropic waters resulting in a limited presence of typical marine organisms.