First we wish to thank Will Downs, for his energy and good nature in promoting all aspects of the work, and for his approval of Y797 as one of the best microsites in the Siwalik sequence. E. Lindsay's productive Zinda Pir project formed the basis for comparison of diatomyid samples and interpretation of diatomyid evolution. Also indispensable were open exchanges with P. Mein (who kindly supplied a cast of Diatomys liensis) and L. Marivaux. J. Barry has helped in many ways, including initial sampling and washing of Y797 sediment, and identification of fossils. S.M. Raza visited the locality and provided helpful contextual information. Discussions with C. Badgley, A.K. Behrensmeyer, and D. Pilbeam are greatly appreciated. Reviews, especially A. Winkler's thoughtful critique, improved the manuscript. We thank the many members of the Harvard-Geological Survey of Pakistan project for their support and camaraderie in the field. Financial support was derived from a number of grants from the National Science Foundation, the Smithsonian Foreign Currency Program, the Wenner-Gren Foundation for Anthropological Research, and the American School for Prehistoric Research.