High resolution lithostratigraphic and radiometric (40Ar/ 39Ar) correlations of latest Cretaceous and earliest Paleocene local faunas from the Hell Creek and early Paleocene local faunas from the Tullock Formations calibrate the history of the fate of the mammalian fauna at the close of the Cretaceous and its reconstitution during the first million years of the Paleocene. New phylogenetic research provides a more explicit perspective on the shaping of the mammalian faunas during the recovery after the K/T mass extinction, revealing a complex pattern of survival and extinction, and highlighting the significant role of dispersal.

Changes in the mammalian fauna of northeastern Montana and adjacent areas in Canada at the K/T boundary involve a great reduction in taxonomic diversity; the Lancian North American Land Mammal Age (NALMA) local faunas consist of at least 27 species while early Puercan NALMA (Pu1) local faunas contain only 15. Phylogenetic analyses demonstrate that, of the latter, only four species were closely related to Lancian species; the majority of Pu1 mammalian species were immigrants. This relatively depauperate mammalian fauna inhabited the area during the first ca. 400,000 years of the Puercan. In middle and later Puercan (Pu2-3, ca. 660,000 years duration) taxonomic diversity of the mammalian fauna increased to at least 30 species. The fauna is made up of representatives of Pu1 lineages that, in some instances, diversified in Pu2-3 time as well as Lazarus taxa and additional immigrants.

Many of the Puercan taxa found in northeastern Montana also are present in Puercan local faunas from the Ravenscrag Formation of southwestern Saskatchewan and the Nacimiento Formation of the San Juan Basin, New Mexico. Patterns of recovery of the mammalian fauna during the Puercan over this wide area appear to be similar and characterized by a significant amount of dispersal from other regions.

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