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GLOBAL CLIMATIC INFLUENCE ON CENOZOIC LAND MAMMAL FAUNAS 184 11 Global Climatic Influence on Cenozoic Land Mammal Faunas S. David Webb And Neil D. Opdyke University of Florida ABSTRACT The Cenozoic succession of continental mammalian faunas reflects the climatic shift from a "greenhouse" to an "icehouse" world. In midcontinental North America, where the most nearly complete record of terrestrial biotic succession has been assembled, floral and faunal evidence chronicle a broad progression of biome change from tropical forest to savanna to steppe. Thanks to improved land mammal chronologies, the tempo of faunal turnover is now seen to be strikingly syncopated. Stately chronofaunas, made up of stable sets of slowly evolving taxa, persist on the order of 107 yr. Chronofaunas are terminated by rapid turnover episodes in which some native taxa experience rapid evolutionary rates and other taxa appear as intercontinental immigrants. We focus here on pulses of immigration and their correlation with major climatic shifts. The Tertiary record of North American land mammals registers a total of 140 immigrant genera. The chronology of most immigrant arrivals can be placed within the nearest million years (m.y.) or less. The pattern of immigration is decidedly nonrandom, with the largest two episodes involving 15 and 14 genera each within about 1 m.y. We categorize immigrant arrivals into first-, second-, and third- order episodes, with the first consisting of at least nine genera within less than 1 m.y. The seven first-order episodes occur at 58.5, 57, 40, 20, 18.5, 5, and 2.5 m.y. ago (Ma). The first-order episode at ~40 Ma may represent an artifact due to tallying at the end of a long interval of poorly fossiliferous deposits (Uintan). A striking generalization is that the six other first-order episodes occur as three pairs separated by only about 2 m.y. Review of land mammal immigration and rapid turnover episodes on other continents tends to confirm the "universality" of major immigration episodes at ~20, ~5, and ~2.5 Ma. We consider two hypotheses that might causally relate high intercontinental immigration rates to global climate change. The first proposes a correlation between high immigration rates and the availability of land bridges during low sea-level intervals. In this
