Fossils of the large, swimming reptiles called mosasaurs are common in western Kansas chalk beds, which were deposited in Cretaceous seas. Although mosasaur species varied in length, some grew to be more than 50 feet long. The longest one found in Kansas was about 40 feet long. Mosasaurs had long bodies, sharp teeth, and paddle-like limbs. They died out, along with dinosaurs and many other animals, during the mass extinction at the end of the Cretaceous Period 66 million years ago.