Journal of Paleontology, 91(6), 2017, p. 1244–1257 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.59
Ground squirrels (Rodentia, Sciuridae) of the late Cenozoic Meade Basin sequence: diversity and paleoecological implications
H. Thomas Goodwin1 and Robert A. Martin2
1Department of Biology, Andrews University, Berrien Springs, MI 49104 ⟨
goodwin@andrews.edu⟩ 2Department of Biology, Murray State University, Murray, KY 42071 ⟨
rmartin@murraystate.edu⟩
Abstract.—The Meade Basin, SW Kansas, yields a rich vertebrate fossil record from the late Cenozoic. Here, we review fossil ground squirrels (Sciuridae) from the region as a contribution to the broader Meade Basin Rodent Project. We recognize 14 species in seven genera: two species of giant ground squirrels (Paenemarmota Hibbard and Schultz, 1948) from the early Pliocene, and at least 12 species in six extant genera (Ammospermophilus Merriam, 1892; Otospermophilus Brandt, 1844; Ictidomys Allen, 1877; Poliocitellus Howell, 1938; Urocitellus Obolenskij, 1927; Cynomys Rafinesque, 1817) from the Pliocene–Pleistocene sequence, including the first regional records of Ammospermophilus. Based on dental morphology and the ecology of modern congeners, we interpret faunal change through the sequence as primarily reflecting a shift from a Pliocene assemblage of “southwestern” taxa with granivor- ous/omnivorous diets (relatively low-crowned, transversely narrow cheek teeth) in warm and at least occasionally dry shrub or shrub-steppe habitats (Ammospermophilus, Otospermophilus, Ictidomys meadensis [Hibbard, 1941a]), to a Pleistocene temperate assemblage of grazing taxa that either exhibited relatively high-crowned, transversely wider cheek teeth (Urocitellus, Cynomys, Ictidomys tridecemlineatus [Mitchill, 1821]) or were otherwise dependent on grassland habitats (Poliocitellus). The early Pleistocene Borchers assemblage was transitional in this regard, heralding a “revolution” observed as well with other clades in the Meade Basin rodent community. This interpretation is broadly congruent with evidence of Pliocene climatic change and the staged development of regional grasslands, with the modern proportion of C3/C4 plants established in the Meade Basin during the early Pleistocene.
Introduction
This paper represents a contribution of the Meade Basin Rodent Project (MBRP), a long-term research program designed to examine the history of rodent diversity on the Central Great Plains. The Meade Basin, a depositional trough running through part of southwestern Kansas and the panhandle of Oklahoma, covers an area of ~50 ×150km and includes episodic exposures from the late Miocene to Late Pleistocene (Figs. 1, 2). The primary goal of the MBRP is to create a species-level database from which a number of analyses can be generated, testing a variety of ecological and evolutionary hypotheses. For example, the database has been used to examine patterns of diversity in cotton rats and pocket gophers (Peláez-Campomanes and Martin, 2005; Martin et al., 2011) and diversity dynamics of the entire rodent community through the late Neogene and Quaternary (Martin and Peláez-Campomanes, 2014). While the taxonomy and diversity of most rodent groups in
the Meade Basin are reasonably resolved, the taxonomy of Sciuridae (squirrels and allies) has not been reviewed. Prior studies recognized the early Pliocene giant ground squirrel Paenemarmota and an average of two smaller ground squirrels (previously assigned to Spermophilus Cuvier, 1825) and one prairie dog (Cynomys)ofunknown affinities (Martin et al., 2008; Martin and Peláez-Campomanes, 2014). Here, we review the Sciuridae within the Meade Basin, representing the culmination
of a study by the senior author of both newer material collected through the MBRP and material housed at the University of Michigan and University of Kansas collected previously by C.W. Hibbard and his students. A more comprehensive phylogenetic analysis of extinct and extant ground squirrels would be helpful, but was beyond the scope of this analysis, which is primarily concerned with taxonomy of the Meade Basin species. This review recognizes seven genera and possibly 14 species of sciurids in the Meade Basin record. The increase in number of genera reflects both recognition of additional species and application of a revised taxonomy of Spermophilus to existing material. Helgen et al. (2009) divided the paraphyletic Spermophilus into eight presumably monophyletic genera, and we recognize four of these genera from the Meade Basin fossil sequence. Morphometric comparisons among fossil ground squirrels
from the Meade Basin revealed multiple phenetic clusters of tooth size and relative tooth width. In part, these clusters helped to separate and identify species, but they also revealed a pattern of dental morphological change in Ictidomys during the early Pleistocene that mirrors previously reported size shifts in other Meade Basin rodents, such as the cotton rats and pocket gophers (Peláez-Campomanes and Martin, 2005; Martin, 2016). Consequently, in addition to providing basic taxonomic descriptions of the Meade Basin sciurid remains, we present some graphic comparisons suggesting that the Huckleberry
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