The Western Interior Seaway was a vast inland sea that existed during the late Cretaceous period, approximately 100 to 70 million years ago. It split the continent of North America into two landmasses, known as Laramidia to the west and Appalachia to the east. This seaway extended from the Arctic Ocean in the north to the Gulf of Mexico in the south, covering large areas of what are now the central United States and parts of Canada. The seaway was home to a diverse range of marine life, including mosasaurs, plesiosaurs, and various species of fish and invertebrates. Fossils found in regions like Kansas and Montana provide valuable insights into the ecosystems that thrived in this ancient sea. Over time, geological processes caused the seaway to gradually recede, leading to the formation of the land we see today.