Over 100 million years ago during the cretaceous era, Kansas was covered by a vast ocean, and dramatic natural features such as the Monument Rocks are remains of that ancient seabed. Since the 1870s, fossil hunters have searched the chalk beds and limestone hillsides of the Smoky Hill River region for the remains of ancient creatures and made some of the most significant discoveries in North American paleontology. Among the most astonishing finds were flying reptiles with a 30-foot wingspan known as pteranodons, which surprised scientists because, unlike other flying reptiles, their beaks and hollow bones seemed to make them the ancestors of birds. Paleontologists and bone hunters competed to discover new types of fossils, and the fierce rivalry in the 1870s and 1880s between Professor Edwin D. Cope of Harvard and Professor Othniel C. Marsh of Yale became known as the Bone Wars. The army often provided escorts for scientists, Buffalo Bill once guided Othniel Marsh on an expedition through western Nebraska, and Cody also found fossils and shipped them to museums in the East. Many of the greatest discoveries in western Kansas were made by Charles H. Sternberg and his sons George, Charles M., and Levi, who were the first to find and identify several species of cretaceous life, and fossils excavated by the Sternbergs became part of major museum collections throughout the United States and Europe, including the Fick Fossil and History Museum in Oakley and the Sternberg Museum at Hays.