Beginning about 100 million years ago, a large inland sea known as the Western Interior Seaway covered most of the middle of the modern-day countries of the United States and Canada. It stretched from the Gulf of Mexico to the Arctic Ocean and was deep and wide. A wide range of dinosaurs, including "Ankylosaurus", "Pachycephalosaurus", "Thescelosaurus", "Triceratops", and "Tyrannosaurus", inhabited the area. The combination of extensive prehistoric fauna and a shallow inland sea led to significant preservation and fossilization of animal and plant remains.