We thank Will Downs for his friendship and inspiration and for screen-washing and preparing the fossil specimens that inspired this study. L. Flynn loaned us Siwalik fossil squirrel teeth to cast and provided useful suggestions for the study. All Siwalik fossil specimens are on loan from the Geological Survey of Pakistan. N. Beranek provided valuable research assistance in the later phase of this study. We thank L. Flynn and an anonymous reviewer for helpful comments and suggestions. A grant from the National Science Foundation (EAR-0125663) supported this work.