Kuhl) was changing the face of the North American continent, creating and modifying wetlands by building dams. The family Castoridae arose in the Oligocene, and the genus


arose in the Pliocene (Jenkins and Busher 1979). Beavers have existed throughout the Pleistocene, and giant beavers the size of bears existed as recently as the deglaciation of the North American continent (Erickson 1962).