The reorganization of North American and European mammalian faunas across the Paleocene/Eocene boundary is attributed to intercontinental dispersal of higher-level taxa that originated in Asia. Hypotheses positing dispersal of these higher-level taxa from a Gondwanan source (either Africa or the Indian subcontinent) are rejected on the basis of their discordance with both phylogenetic and biostratigraphic data sets. Because the only viable routes for dispersal between Holarctic continents during the late Paleocene-early Eocene were land bridges located at high latitudes, global climate change was a decisive factor mediating the nature and timing of faunal interchange. Dispersal of higher-level taxa beyond Asia was episodic and iterative, and seems to have occurred solely via Beringia. Because intercontinental dispersal of terrestrial taxa such as mammals rarely coincides precisely with their phylogenetic origin, geologically earlier first appearances of such taxa on their continent of origin are not unexpected. Given this paradigm, the Gashatan and Bumbanian Asian Land Mammal Ages (ALMAs) are likely to be older than is frequently conceived. At least some Bumbanian faunas, such as that from the Wutu Formation of Shandong Province, China, are correlated with the Clarkforkian North American Land Mammal Age (NALMA), an interval that appears to be undocumented in the European record of mammalian evolution.

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