Sixteen mammalian assemblages from the middle Paleocene (Torrejonian) to early Eocene (Wasatchian) of western North America have been studied to determine their species composition and diversity. Torrejonian and Tiffanian faunas are dominated by small mammals of archaic aspect, while larger forms predominate in Clarkforkian assemblages and remain common in the Wasatchian. Both Clarkforkian and Wasatchian faunas are characterized by immigrant taxa, including representatives of more modern groups of mammals. Torrejonian assemblages have many species, and species abundances are relatively equitably distributed. Early and middle Tiffanian samples reveal a significant drop in both species richness and evenness, with predominance of one or two species; but diversity may have increased in the late Tiffanian. Clarkforkian assemblages resemble early-middle Tiffanian ones in their low species richness and evenness. Wasatchian assemblages, however, are somewhat richer in species and show much greater equitability of species abundances, thus resembling Torrejonian samples. The hypothesis is proposed that the observed pattern of species diversity is in part related to climatic disturbances (specifically changes in temperature) in the northern Western Interior. Consistent with this interpretation is the record of Paleocene-Eocene megafloras, which shows similar changes in species diversity. Thus, high diversity in the Torrejonian and Wasatchian coincided with warm, equable conditions. Lower diversity in the intervening Tiffanian and Clarkforkian was probably related to cooler temperatures, although a slight warming trend seems indicated in the Clarkforkian. Species diversity in the Clarkforkian and Wasatchian was also influenced by competition and predation directly related to major faunal immigrations.
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