CONCLUSIONS

Of the composite fauna in the Frick Collection (Table 1), several taxa quite possibly came from nearby Nan Zhuang Gou (but could be younger elements): Ochotona, Spermophilus, Sus, and Nyctereutes. Others are known from the Mazegou Fm. or younger deposits: Erinaceus olgai, Mesosiphneus praetingi, and Axis shansius. Modern field work duplicated M. praetingi in the base of the Mazegou Fm. Higher in the formation, new collections include M. paratingi and Ochotonoides close to O. complicidens. Possibly Mimomys orientalis could come from this interval.

Some Nan Zhuang Gou taxa first appearing in upper levels of the Mazegou Fm., e.g., Ochotonoides, E. olgai, and elephantids, range into younger Nihewanian localities of Yushe and elsewhere. Other elements (Lepus wongi, Cromeromys gansunicus, Cervus elaphus, and true Ochotonoides complicidens) appear to be strictly Nihewanian and would be additions from the overlying loess or from the reversely magnetized Haiyan Formation, about 7 km to the northwest of Nan Zhuang Gou.

Previously, Rep and I had the impression that Nan Zhuang Gou fossils collected in the 1930s came from two levels: the local early Pliocene strata and the overlying loess. It was apparent that the two microtines did not belong in the same assemblage, and that certain F:AM taxa were mutually exclusive (e.g., M. praetingi and O. complicidens). It is now evident that the two-horizon hypothesis does not explain the observed temporal spread. Based on occurrences of taxa determined from field work, my present conclusion is that the Frick fossils would have been acquired from multiple horizons, not all in the vicinity of the local exposures. It is apparent now that Nan Zhuang Gou was a point of sale for material that came from a considerable area of Yushe exposures, encompassing a large span of time.

Although the Nan Zhuang Gou assemblage acquired in the 1930s sheds little light on East Asian biochronology, it nonetheless includes important specimens and is a product of the history of collecting in the area. It also presents a clear example of why precise provenance is important in biostratigraphy. Charles Repenning was a champion of resolved biostratigraphies that are relevant to correlating regions of continental and intercontinental scale. The IVPP-AMNH program in Yushe Basin was designed in that vein – construction of a precise local biostratigraphy that could be extended to a broad region for biogeographic and chronologic analyses. Modern work in Yushe Basin enabled resolution of taxon ranges in time, thereby extending the local biostratigraphy to regional biochronology. Careful fieldwork also specified likely provenance of key fossils collected earlier in the 20^th century. Elsewhere in the Holarctic, that level of precision enabled Rep to build a comprehensive interpretation of microtine evolution and dispersal on a global scale.