Among the first vertebrates to develop jaws and teeth, sharks have been around for at least 400 million years. Sharks shed and regrow teeth throughout their lives, so countless teeth were shed in seas that covered parts or all of Kansas off and on over hundreds of millions of years. The Cretaceous rocks of western Kansas are an especially good source of sharks’ teeth. Because the rocks are eroding, teeth once encased in solid rock are sometimes now just lying on the ground. They also can be found in the Pennsylvanian-age and Permian-age rocks of eastern Kansas, although they are not as abundant there. Other shark skeleton parts have been found, but they are much rarer than teeth.