The Western Interior Seaway was a large inland sea that existed from the middle Cretaceous to the early Paleocene and split North America into Laramidia in the west and Appalachia in the east.  The sea is responsible for the Hell Creek Formation, where many invertebrates, amphibians, fish, dinosaurs, pterosaurs, crocodylomorphs, turtles, squamates, choristoderans, and mammals have been found.

Western interior seaway