Biochronology and Evolution of Early and Middle Paleocene Nonmarine Mollusca of the Northern Great Plains

非海相软体动物是北美西部古新世陆相地层中最丰富的大型动物化石。然而，它们的生物年代学潜力尚未实现，主要原因是缺乏研究集中在确定厚层序中分类群的生物地层范围，这些范围是通过与化石哺乳动物的对比来暂时控制的。本研究将完成蒙大拿州Crazy Mountains盆地的Puercan和Torrejonian地层（早古新世和中古新世）的生物地层分带，其中软体动物和哺乳动物的数量特别多。该带将与蒙大拿州佩克堡地区以及萨斯喀彻温省的柏树山和大沼泽地区年龄相仿的软体动物和哺乳动物地层进行生物年代学对比。Puercan、Torrejonian和早期蒂芙尼亚时代地层复合剖面将与北达科他州西南部小密苏里河剖面富含化石的蒂芙尼亚（古新世上）地层在时间上重叠。该合成剖面将为北美古新世提供第一个完整的非海洋软体动物生物年代学，并将特别作为大平原北部软体动物和哺乳动物的参考剖面。

Nonmarine Mollusca are the most abundant animal macrofossil of Paleocene continental strata in western North America. Their biochronologic potential, however, has yet to be realized due primarily to the lack of study focused on the determination of the biostratigraphic ranges of taxa in thick sequences of strata temporally-controlled by correlation with fossil mammals. This study will complete a biostratigraphic zonation of Puercan and Torrejonian Strata (early and middle Paleocene) in the Crazy Mountains Basin, Montana where the number of molluscan and mammalian localities are particularly numerous. This zonation will be biochronologically correlated with mollusk-and mammal-bearing strata of similar age in the Fort Peck area of Montana, and the Cypress Hills and Big Muddy areas of Saskatchewan. The Puercan, Torrejonian, and early Tiffanian age strata composite section will be temporally overlapped with the richly fossiliferous Tiffanian (upper Paleocene) strata of the Little Missouri River section in southwestern North Dakota. The composite section will provide the first overall nonmarine molluscan biochronology for the Paleocene of North America, and will specifically serve as a reference section for both mollusks and mammals in the northern Great Plains.