Goodwin and Martin—Sciurid rodent diversity of the Meade Basin


Kansas (Goodwin, 1995b), but apparently did not expand its range southward until the early Pleistocene, essentially coincident with the Microtus Schrank, 1798 immigration event in the Meade Basin (Martin et al., 2008). The composite Middle Pleistocene record from Cudahy


and Sunbrite yields a diverse sciurid assemblage distinct from that of the early Pleistocene. Ictidomys tridecemlineatus persisted, but was larger in size than early Pleistocene populations, and both Urocitellus cf. U. richardsonii and Franklin’s ground squirrel, P. franklinii, were present (Table 1; Fig. 5) well south of their current ranges. Prairie dogs were absent from the rich Cudahy fauna, but two prairie dog species were present at the coeval Sunbrite locality: a large form probably representing an extant lineage, but known only from a single heavily worn upper molar, and a diminutive form representing a new species (Fig. 4.9). The roughly time- equivalent Robin’s Roost assemblage from northwestern Oklahoma likewise yielded a large prairie dog. Absence of prairie dogs from Cudahy is puzzling; Cudahy is <25km from Sunbrite and fossils collected from both localities were excavated from the same limited 20cm horizon beneath the primary ash-fall of the Lava Creek B Tuff (0.627±0.015 Ma; Mark et al., 2017). Both assemblages share the same species of Microtus and other rodents (Martin and Peláez-Campomanes, 2014), suggesting similar paleoecology. We speculate that the absence of prairie dogs from Cudahy may be an artifact of localized taphonomic sampling, which by chance did not encompass a prairie dog town. Late in the Pleistocene, I. tridecemlineatus persisted, but


returned to the same smaller size as during the early Pleistocene, Urocitellus cf. U. richardsonii remained abundant, and P. franklinii was present at least intermittently (Table 1). Both black-tailed (C. ludovicianus) and white-tailed (C. niobrarius) prairie dogs were present, but were not found in the same assemblages (Table 1). A single fossil from the Adams locality demonstrated at least a temporary Late Pleistocene presence of Otospermophilus in the Meade Basin, perhaps representing the extant rock squirrel (O. variegatus) that today occurs <100km to the SW. Two of five Late Pleistocene ground squirrels (Urocitellus cf.


U. richardsonii and P. franklinii) disappeared from the Meade Basin near the end of the Pleistocene, but persisted farther north on theGreat Plains, and athird (C. niobrarius) became globally extinct at this time. The modern Meade Basin ground squirrel community was assembled from the remaining two Pleistocene species (I. tridecemlineatus and C. ludovicianus), along with the spotted ground squirrel (Xerospermophilus spilosoma), with the latter reaching the eastern margin of its modern range in southwestern KS (Streubel and Fitzgerald, 1978a). While the dentition ofX. spilosoma cannot reliably be distinguished fromthat of I. tridecemlineatus, the consistency of size and characters in Pliocene and Pleistocene Ictidomys from the Meade Basin and the current predominantly western and southwestern distribution of X. spilosoma suggest that the spotted ground squirrel, like the hispid cotton rat (Sigmodon hispidus Say and Ord, 1825) and fox squirrel (Sciurus niger Linnaeus, 1758), may be a Holocene immigrant to the basin. Overall, the record of ground squirrels is congruent with two early Pliocene to Pleistocene trends in paleoenvironments


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inferred from stable isotopic and faunal evidence in the Meade Basin. First, carbon isotopic investigation of paleosol soil carbonates demonstrates a step-wise increase in regional abundance of C4 vegetation (mostly grasses) leading to emplacement of essentially modern-type grassland ecosystems in at least some habitats during the early Pleistocene (Fox et al., 2012b). Pliocene squirrels of the Meade Basin were likely granivorous to omnivorous (they lacked dental specializations of small grazers, such as high-crowned, transversely wide cheek teeth; Goodwin, 2009), and, based on modern analogs, at least Otospermophilus and Ammospermophilus probably occupied shrub to shrub-steppe environments (Oaks et al., 1987; Best et al., 1990a, 1990b; Belk and Smith, 1991). In contrast, species with high-crowned, transversely wider cheek teeth known to occupy grassland ecosystems abruptly appeared in the early Pleistocene (Fig. 5.3), at or just aboveBorchers, andwere dominant in later Pleistocene assemblages (Ictidomys tridecemlineatus, Urocitellus,and Cynomys). A similar pattern is evident in arvicolid rodents—Microtus, with ever-growing cheek teeth, first appears in the Borchers Badlands at Short Haul, dated to ca. 2.0Ma (Martin et al., 2008). Second, oxygen isotopic analysis of paleosol carbonates


suggests an association between the stepwise increase in C4 abundance and a combination of cooling temperatures, increased winter precipitation, and/or increased soil moisture, with particularly significant change across the Pliocene– Pleistocene transition (Fox et al., 2012a)—a change reflected in the replacement of a more southern-adapted rodent commu- nity with one more adapted to temperate conditions (Martin et al., 2008). Based on the distribution and ecology of modern representatives, the same pattern holds for sciurids; Pliocene species with modern congeners either permit (I. meadensis)or support (O. rexroadensis and Ammospermophilus sp.) warmer and/or more-arid paleoenvironments, whereas Pleistocene faunas incorporate species of cooler, more-temperate aspect (Urocitellus spp., Cynomys spp., I. tridecemlineatus, and P. franklinii). The presence of Ammospermophilus at two early Pliocene


localities slightly younger than Fox Canyon (XIT 1B, Wiens B) may indicate an episode of significant aridity in the early Pliocene. All extant species of Ammospermophilus occupy desert habitats (Best et al., 1990a, 1990b; Belk and Smith, 1991), thus it seems probable that fossil Ammospermophilus were likewise arid-adapted. This interpretation is consistent with the absence of cotton rats from the Meade Basin during the early Pliocene (Peláez-Campomanes and Martin, 2005), although cotton rats were present at the later Pliocene Rexroad Locs. 2 and 2A, from which Ammospermophilus was also tentatively identified. Today, Sigmodon arizonae Mearns, 1890 can be found in limited mesic habitat along small streams coursing through Arizona deserts, so the association of Ammospermophilus with Sigmodon minor Gidley, 1922 during the middle Pliocene could still be consistent with an arid period.


Ictidomys and the Huckleberry Ridge Tuff.—There is growing evidence that eruptions from the Yellowstone Caldera, resulting in huge accumulations of volcanic ash hundreds of km from the source, contributed to disruption of community structure and influenced cladogenesis in the Meade Basin rodents (Martin and


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