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INTRODUCTIONCharles A. Repenning preferred to be known as Rep to everyone. Rep was always kind and encouraging about research in the geosciences, especially to students, and this is the way I met him. He introduced the fascinating world of microtine evolution and diversity to the students of Everett Lindsay at the University of Arizona, Tucson, where I was part of the graduate program. Rep was profoundly interested in microtines – those muroid rodents with prismatic molars, which radiated during the later Neogene. He was particularly interested in the patterns of intercontinental dispersal of microtines, as evidenced in his subdivision of the late Cenozoic of North America (Repenning 1987; Repenning et al. 1990) and subsequent analyses tracing the biogeography of diverse subgroups (Repenning 1998, 2003). Repenning (1988, 1992) well knew that the prismatic structure of the cheek teeth was likely independently derived in various microtine lineages (for example Arvicolinae versus Ondatrinae versus Lemminae [voles-muskrats-lemmings]), and he consistently used the term "microtine" as a convenient label for all of them, without implying close systematic relationship. Here, it is important to note that, while the microtine radiation covers perhaps no more than the last 7 m.y., the various microtines may well have a genetic coalescence that is deeper in time and monophyletic with respect to most other extant muroids. Rep documented the evolution of detailed dental, cranial, and mandibular structures in microtines. He explored subtle changes in dentine tracts, expansion of the anterior cap of m1 and posterior expansion of M3, addition of triangles, spacing of triangles and thickness of their enamel, and development of cementum. He illustrated biogeographic dispersal routes through North America, coupled with an unsurpassed familiarity of the natural history of the western part of the continent, including its changes over time. This introduction to the world of voles, bog lemmings, muskrats, and their kin left an indelible imprint, such that years later, when I had fossil microtines at my disposal, I naturally turned to Rep as an authority and potential coauthor. Frick Microtines from ChinaFossils described here are conserved at the American Museum of Natural History (AMNH), New York, as part of the Frick Collection. Childs Frick was famous for amassing a large collection of fossil vertebrates mainly from Cenozoic deposits of the western United States. In the 1930s he also paid collectors to work in the late Tertiary sedimentary basins of Shanxi Province and elsewhere in the People's Republic of China. Kan Chuanpo ("Buckshot" of the earlier Central Asiatic Expeditions of the AMNH) was engaged to acquire material representative of the later Cenozoic of China in central to southern Shanxi, including Yushe Basin. Fifty years later, Richard H. Tedford and Qiu Zhanxiang led another expedition to Yushe Basin, of which I was a member. The specimens collected by Buckshot came to be housed in the Frick Wing at the AMNH in the late 1970s. For the first time, Frick collections became available for general study. Typically, Frick had favored dramatic fossil representatives of carnivores and ungulates. Interesting rarer taxa inevitably were also found. Despite a bias against small mammals, several were inserted into the crates of fossils returning from the field, from both the western United States and China. Shanxi Province produced a number of excellent leporid and ochotonid specimens (see Erbajeva et al. 2006) and a rich microfauna from Pai Tao Tsun (as it was known in the 1930s). Few but diverse small mammals also came from Yushe Basin (Table 1) and many of these were attributed to the general collecting area Nan Zhuang Gou. Two microtines in the Nan Zhuang Gou assemblage attracted my attention, and I enlisted the help of Charles Repenning in their description and interpretation. Our research led to the conclusion that the museum collection from Nan Zhuang Gou was a composite of material coming probably from a large area and held until buyers (typically dragon bone drug dealers) came to purchase them. Realizing that the collection in itself would not advance understanding of microtine biogeography and biochronology, by 1989 we abandoned a manuscript on their significance. However, Rep had prepared a beautiful drawing of the two specimens, and his description remains sound. Subsequent field work has made a return to analysis of these specimens relevant to fuller knowledge of the fossil record of Yushe Basin. In the following, I borrow from our fragmentary manuscript and add new data.
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