Analyses of fossil mammal faunas from 2945 localities in the United States demonstrate that the geographic ranges of individual species shifted at different times, in different directions, and at different rates in response to late Quaternary environmental fluctuations. The geographic pattern of faunal provinces was similar for the late Pleistocene and late Holocene, but differing environmental gradients resulted in dissimilar species composition for these biogeographic regions. Modern community patterns emerged only in the last few thousand years, and many late Pleistocene communities do not have modern analogs. Faunal heterogeneity was greater in the late Pleistocene.
Worldwide late Pleistocene terrestrial mammal faunas are characterized by stratigraphic associaiicns of species that now have exclusive geographic ranges. These have been interpreted as either taphonomically mixed or representative of communities that no longer exist. Accelerator mass spectrometry 14C dates (n = 60) on single bones of stratigraphically associated fossil micromammals from two American and two Russian sites document for the first time that currently allopatric mammals occurred together between 12,000 and 22,000 yr B.P. on two continents. The existence of mammal communities without modem analogs demonstrates that Northern Hemisphere biological communities are ephemeral and that many modern biomes are younger than 12 ka. Future climate change may result in new nonanalog communities. CHEEK BEND CAM. TENNESSEE: Stratum II PECCARY CAVE. ARKANSAS: Unit C 0 w 17.m 17,500 b M m l u s xenthognethus Desypos bellus * Clethriommys gepperi rn Geomys bursarius 0 P h e n m y s inlemdius 0 Blanne &dinensis o Neolome floridena 0 Synapkmys borealis 0 Scalopus equelicus
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