1. Department of Geological Sciences, California State University–Fullerton, McCarthy Hall 254, Fullerton, California 92834, USA, and Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA
2. U.S. Geological Survey, 345 Middlefield Road, MS 901, Menlo Park, California 94025, USA
3. Department of Mammalogy, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA, and Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA