Goodwin and Martin—Sciurid rodent diversity of the Meade Basin
1251
Figure 7. Upper M3 fossils, all displayed as right (indicated if reversed). Scale bar = 1mm. (1) I. meadensis from Borchers (FHSM VP-18122; L reversed). Abrupt deflection of posterior cingulum(arrow) distal to short ridge coursing buccodistally from protocone (dotted line) noted in text. (2) I. tridecemlineatus from Short Haul (FHSM VP-18902; L reversed). (3) P. franklinii from Sunbrite (UMMP 39596). (4) Urocitellus cf. U. richardsonii from Adams (UMMP 34705; L reversed). (5) Cynomys hibbardi from Rick Forrester (FHSM VP-14104).
character frequency and subtle quantitative change in length of p4 as phyletic change within this lineage. We assign the Meade Basin squirrel material described in
this section to the genus Ictidomys, but this assignment is complicated by conservative and similar dental morphology of Ictidomys and Xerospermophilus (Merriam, 1892), both of which occur in the Meade Basin today (the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, and spotted ground squirrel, Xerospermophilus spilosoma [Bennett, 1833]; Helgen et al., 2009). Cheek teeth of Xerospermophilus tend to be relatively narrow (Goodwin, 2009), and the hypoconid of p4 tends to be more rounded and less elongate buccally than in I. tridecemlineatus, but there is overlap in both features. In addition, the M1s–M2s of extant Xerospermophilus usually lack a mesostyle (present in one of 50 teeth observed in extant species of Xerospermophilus), a feature more commonly observed in extant species of Ictidomys (15 of 46 teeth observed). Unfortunately, fossils assigned to I. meadensis typically display narrow cheek teeth and a more rounded hypoconid on p4 (Fig. 4.4, 4.5) as in Xerospermophilus, but also may exhibit a mesostyle on M1–M2 (7 of 23), as in Ictidomys. Better material may clarify assignment of the fossils, but for now we assign the material to Ictidomys.
Ictidomys tridecemlineatus (Mitchill, 1821) Figures 3.1, 7.2
1821 Sciurus tridecem-lineatus Mitchill, p. 248 [For listing of synonyms in the literature of modern Ictidomys tridecemlineatus, see Streubel and Fitzgerald, 1978b, p. 1].
Holotype.—Not designated. Type locality fixed as “Central Minnesota” by Allen (1895, p. 338).
Remarks.—During the early Pleistocene, an abrupt and significant reduction in tooth size between Borchers
I.meadensis and slightly younger fossils of Ictidomys from the Borchers Badlands (length of p4: t = −6.90, df = 30, p<0.001; length of P4: t = −9.89, df = 30, p<0.001; Fig. 5.1, 5.2) was accompanied by increase in relative trigonidwidth of p4 (t = 2.88, df = 28, p<0.01; Fig. 5.3) and decrease in frequency of themesostyle onM1–M2(from30% [7 of 23] to 8% [2 of 26]). These simultaneous changes in tooth size, relative width, and mesostyle frequency indicate the appearance of a new species in the Meade Basin, which we identify as Ictidomys tridecemlineatus based on size and dental proportions, although extant I. tridecemlineatus more frequently exhibits a mesostyle on M1–M2 (~33%). Early Pleistocene I. tridecemlineatus usually displays a relatively elongateM3, and never displays the discontinuity between the ridge extending posterodistally from the protocone and the posterior cingulum described previously for I. meadensis from Borchers (compare Fig. 7.2 with Fig. 7.1). We interpret the combination of abrupt change in size,
shape, and qualitative characters as indicating immigration rather than in situ phyletic evolution. The abrupt size reduction in local Ictidomys immediately post-Borchers mirrors the appearance of the dwarf pocket gopher Geomys tyrioni at Short Haul (Martin, 2016), and may reflect dramatic restructuring of regional communities after the massive Huckleberry Ridge ash-fall (Martin and Peláez-Campomanes, 2014). Pleistocene I. tridecemlineatus from the Meade Basin
varies temporally in size (Fig. 5.1, 5.2), significantly so for length of p4 (F = 13.72, df = 2, 17, p<0.001), but not length of P4 (F = 2.02, df = 2, 21). There was no significant stratigraphic variation in relative p4 trigonid width (Fig. 5.3; F = 0.31, df = 2, 14). In length of p4, both early and Late Pleistocene were significantly smaller than Middle Pleistocene fossils from Cudahy and Sunbrite (Tukey HSD, p<0.001).
Genus Poliocitellus Howell, 1938 Poliocitellus franklinii (Sabine, 1822) Figures 3.2, 4.6, 7.3
1822 Arctomys franklinii Sabine, p. 587 [For listing of synonyms in the literature of modern Poliocitellus franklinii, see Ostroff and Finck, 2003, p. 1].
1976 Spermophilus lorisrusselli, Hibbard, p. 282, fig. 2.
Holotype.—Not specified; type locality restricted to Carlton House, Saskatchewan, Canada by Preble (1908, p. 165).
Remarks.—Poliocitellus franklinii (Sabine, 1822) is an extant occupant of tall-grass prairies of the northern and central Great Plains that does not occur in the Meade Basin today, with closest populations ~200km to theNNE (Ostroff and Finck, 2003). It is the sole extant member of its genus. Hibbard (1976) described Poliocitellus lorisrusselli (Hibbard, 1976) from the Cudahy- equivalent Wilson Valley local assemblage of NE Kansas, which he characterized as smaller than but otherwise similar in dental morphology to extant P. franklinii. Multiple fossils from the Meade Basin can be assigned to
Poliocitellus. These fossils differ from I. tridecemlineatus in larger size (Fig. 5.1, 5.2) and the presence of a pronounced
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