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Journal of Paleontology 91(6):1244–1257
Peláez-Campomanes, 2014; Martin, 2016). The rapid dwarfing in Ictidomys from Borchers to slightly higher assemblages, such as Short Haul, Aries A, and Nash 72, may provide another example. As noted above, because there is a dental shape as well as size change, we interpret the post-Borchers early Pleistocene Ictidomys samples to represent immigration of the extant I. tridecemlineatus, perhaps originating from populations of I. meadensis outside of the Meade Basin as a result of natural selection during the post-ashfall environment. This hypothesis is congruent with similar dwarfing in pocket gopher populations of Geomys tyrioni, likely originating from the Borchers Geomys floralindae Martin et al., 2011. Geomys tyrioni was also recov- ered from the Short Haul, Aries A, and Nash 72 assemblages, and was replaced by modern-sized Geomys bursarius (Shaw, 1800) at Rick Forester. It is conceivable that I. tridecemlineatus originated in the Meade Basin, but given the widespread distribution of the Huckleberry Ridge Tuff and the likely wide geographic distribution of I. meadensis, the probability that we have recorded the I. tridecemlineatus speciation event is very unlikely and, in any case, impossible to distinguish from immigration. The important point is that widespread, devastat- ing ash-falls may have contributed significantly to today’s North American rodent diversity. In conclusion, taxonomic revision of the Meade Basin
sciurids results in the recognition of seven genera and at least 14 species, with changing Pliocene–Pleistocene associations con- sistent with environmental reconstructions made previously on the basis of other rodent clades and geochemical environmental proxies. The rapid size shift in Ictidomys and appearance of the modern I. tridecemlineatus provides further circumstantial evidence for the influence of volcanic eruptions and subsequent ash-falls on rodent community structure and cladogenesis.
Acknowledgments
Research in the Meade Basin of southwestern Kansas was sup- ported by theNationalGeographic Society (5963-97, 6547-99) and the National Science Foundation (EAR 0207582, EAR 1338262). The Office of Research and Creative Scholarship at Andrews University funded museum travel for HTG. G. Gunnell and A. Rountrey (UMMP), D. Miao (KU), and R. Zakrzewski (FHSM) provided access to and assistance with relevant collections.
Accessibility of supplemental data
Data available from the Dryad Digital Repository:
http://dx.doi. org/10.5061/dryad.7h398
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