LAUSR

ABSTRACT The later part of the Oligocene (Arikareean North American Land Mammal Age) was a time of major faunal change in North America. It is during this time period that archaic mammalian faunas dominated by extinct families started giving way to more modern faunas, including families still extant today. Studies of this faunal transition have so far focused on the Great Plains and the Columbia Plateau. I present here the first quantitative analysis for the northern Rocky Mountains, combining four new radioisotopic dates and almost 1,000 specimens from a series of vertebrate microfossil assemblages through the Cabbage Patch beds of Montana. I demonstrate that the rise of modern mammalian communities was already under way in the Rocky Mountains 30 million years ago, at the base of the beds. However, the major faunal turnover event took place ca. 28 million years ago, at the transition from the lower to the middle units of the Cabbage Patch beds, and led to the disappearance of many archaic mammal taxa that dominated the start of the Arikareean. This event was synchronous across the Great Plains, the Rocky Mountains, and the Columbia Plateau and was driven by the diversification of descendants of immigrants from Eurasia that had reached North America through Beringia. Holdovers from archaic mammalian faunas persist into the upper unit of the beds later than in the Great Plains, but not the Columbia Plateau. Future biogeographic analyses will be necessary to assess the role of topography and environmental change in the rise of modern mammalian communities.