The authors would first like to acknowledge the friendship and inspirational mentorship of C.A. Repenning. "Rep" was a member of the thesis committee of S. May and contributed significantly to this work in many ways, as well as to all of our thinking about mammalian biochronology. We would also like to thank a number of others who contributed to this effort including W.R. Daily and P.M. Sadler (U.C. Riverside); J.H. Hutchison and P. Holroyd (UCMP); D.P. Whistler (LACM); R.H. Tedford (AMNH); D.R. VanAlstine (Applied Paleomagnetics, Inc.); D. Storm (BLM Ridgecrest Field Office); G. Wilkerson (BLM Bakersfield Field Office); P. Renne, G. Scott, and L. Smeenk (Berkeley Geochronology Center); R.E. Reynolds (LSA Associates); R. Hilburn, T. Thomas, N. Thomas, D. Modica, M. Aruta, T. Howe, P. Eby, J. Byrd, P. Lattmer, D. Mckeever, and J. Walkup (Mojave River Valley Museum). The manuscript was improved through the comments of M.M. May and two anonymous reviewers.