 The result was an increased understanding of the natural history of North America, including the discovery of the Western Interior Sea that had covered Kansas and much of the rest of the Midwestern United States during parts of the Cretaceous, the discovery of several important fossils of primitive birds and horses, and the discovery of a number of new dinosaur genera including "Allosaurus", "Stegosaurus", and "Triceratops".Much of this activity was part of a fierce personal and professional rivalry between two men, Othniel Marsh, and Edward Cope, which has become known as the Bone Wars.Two 20th century developments in geology had a big effect on paleontology.