We thank P. Hester and S. Landon of the U.S. Bureau of Land Management for permits and providing field assistance. We also thank U. Denetclaw, L.J. Flynn, R. Haskin, S. Libed, W. Slade, Y. Tomida, K. Tremaine, A. Weil, S. Williams, R. Williamson, and T. Williamson for field and lab assistance. Numerous personnel including R. Fox and C. Scott, of the University of Alberta, Edmonton, J. Meng and J. Galkin of the AMNH, F. Jenkins and J. Cundiff of the MCZ, Harvard, A. Henrici and A. Tabrum of the Carnegie Museum of Natural History, Pittsburgh, P. Holroyd, W. Clemens of the UCMP, E.H. Lindsay of the University of Arizona, and D. Bohaska of the National Museum of Natural History, Washington, D.C. provided discussion, hospitality, and access to specimens, and for that we are grateful. We thank K. Davies and R. Cifelli, University of Oklahoma, Norman, for casting and M. Spilde, University of New Mexico for assistance with the SEM. T.D. Carr provided discussion and important feedback on an earlier draft. This research was supported by an NSF grant (EAR 0207750 To TEW).