1. Department of Mammalogy, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, U.S.A.
2. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A.
3. Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, U.S.A.,
4. Florida Museum of Natural History, University of Florida, 1659 Museum Road, Gainesville, Florida 32611, U.S.A.