New faunal and stratigraphic evidence documents the validity of the North American Clarkforkian Land-Mammal Age. Its limits are defined by the initial appearances of immigrant taxa. In the type area (northwestern Wyoming), the Clarkforkian is subdivided into three zones. Clarkforkian faunas are approximately correlative with latest Paleocene-earliest Eocene faunas of the Paris Basin of Europe and in part with the Gashato-Naran Bulak faunas of Mongolia.
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