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The generally well developed and understood stratigraphic record associated with fossil mammals in North America is combined with independent chronological data sets that foster the development of high-resolution geochronology in nonmarine sequences. An updated chronology for all North American mammal ages (or subdivisions) is utilized to examine the tempo and mode of overland mammal immigration/emigration episodes during the Cenozoic Era. In addition to the thirty or more "background" dispersals involving only a few taxa, ten major immigration/emigration episodes are recorded during the Cenozoic Era in North America. All are important for evaluating the dispersal pattern, as well as for mammal age boundary definition. For the Paleogene interval, major immigration/emigration episodes define the following mammal ages (or intervals): Clarkforkian, Wasatchian, late Uintan, and Chadronian, with the Wasatchian and late Uintan being especially noteworthy. The interval that embraces the late Arikareean mammal age is the first immigration episode of the Neogene interval, but the events recognized in the early and late Hemingfordian mammal ages, respectively, are the most impressive. An important, "medial" Clarendonian emigration is reflected in the North American basis for the Old World "Hipparion" Datum. The events that define the beginning of the Hemphillian and late Blancan mammal ages also are founded on important immigrant first occurrences, but for the first time in the Neogene interval involve taxa from South America as well as from Asia. At other times, either there is no effective immigration or, if present, it involves only a few (four or less) taxa (= "background"). In certain intervals, apparently lowered sea level had no effect on dispersal, but in an even larger number of cases immigration took place in spite of what appears to have been times of relative sea-level highstand. Thus tectonic, climatic, and other factors must be considered to account for North America’s dispersal history during the Cenozoic Era.

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