Biostratigraphy and biochronology of late Cenozoic North American rodent assemblages

Late Cenozoic rodent assemblages from western North America were added to a previous database limited to the central and eastern United States to develop a continental correlational system for the past 5 million years. Three hundred and twelve species representing 124 rodent assemblages were included, from which 12 Cenozoic Mammal Zones (CMZs) were constructed. Replacement chronologies for rodent assemblages based upon superposition, radiometric dates, paleomagnetic profiles, and evolutionary stages of dentitions from various depositional basins formed the skeleton for the chronological ordering of assemblages, and those lacking one or more information sources were sequenced based upon greatest concordance with available data. Arvicoline cricetids provided the most useful information for sequencing assemblages, followed by neotomine and sigmodontine cricetids and geomyids. The appearance of modern-sized cotton rats in CMZ 4, followed by the immigration of Allophaiomys across Beringia in CMZ 3, heralds the shift to the dominance of cotton rats and arhizodont voles in North American grassland ecosystems. No rapid bouts of significant turnover are associated with the beginning of the Pleistocene at 2.58 Ma (million years ago), but a pronounced turnover event was observed in the Meade Basin of southwestern Kansas immediately following the Huckleberry Ridge ash-fall at 2.07 Ma. Preliminary observations suggest two categories of rodent turnover; low-level background rotation determined by stochastic short-term regional and long-term global environmental change, and short-term turnover spikes mediated by catastrophic events such as volcanic eruptions.