Ammonite fossils found in the park show that this area was under water 75 million years ago. Some ammonites grew to more than three feet across and served as a food source for giant mosasaurs and other predators. The Western Interior Seaway once split North America in half, and in some places this vast inland sea was 2,500 feet deep and 600 miles wide. As the sea receded, ammonites could no longer survive in this area. Ammonites were relatives of octopuses and squid but had coiled shells over their bodies, and all ammonites went extinct 65.5 million years ago.