Variation and evolutionary trends in the dentition of late pleistoceneMcrotus pennsylvanicusfrom three levels in bell cave, Alabama

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Abstract
Dental remains of the meadow vole, Microtus pennsylvanicus, were recovered from three superposed day units (zones) and four excavations (pits) in Bell Cave, Alabama. Bone collagen 14C dates of 26,500 + 480/ ‐ 500 and 11,800 + 480/‐ 500 yr BP. are associated with the bottom and top units, respectively. Length, width, number of closed triangles (five or six), and a measurement to determine the amount of closure of the sixth triangle were taken on the first lower molar (M1) from the three zones in Pits 1 and 4. Length and width of the third upper molar (M3) were also determined for the three zones in Pit 4. Analysis of Variance revealed no effect of pit origin or number of triangles on M1 length or width. A trend from large to small size in M3 is documented, but is not statistically significant The same trend is evidenced by length and width of M1. Size changes in length and width of M1 from Pit 4 are statistically significant, but those from Pit 1 are not Although there is no difference in the lengths or widths of five versus six triangle M1s from any pit or zone, there may be a trend towards increase in isolation of the sixth triangle from the remainder of the anteroconid complex. This trend is statistically significant for zone when ANOVA is performed on the variable, called closure, including both pit and zone origin, but is not significant when teeth from each pit are tested separately. An independent test of the length of M1 from additional zones in Pits 2 and 3 confirms the low probability that directional size change is due to accidental sorting of fossil materials. These data demonstrate statistically significant intraspecific differences during a 15,000 year period of the late Pleistocene, and also provide evidence of variation that can be expected due to the combined effects of depositional reorganization, sampling and either stochastic processes or selection. The slight dwarfing recorded, while not particularly supportive of any evolutionary mode, may be correlated with climatic modification during the Wisconsinan glacial period. The evolutionary rate of dwarfing, as measured by mean reduction in length of the first lower molar, is 4.47 darwins. Although faster than many rates for fossil vertebrates and invertebrates calculated over longer periods of time, it is probably not unusual for Quaternary mammals during the time interval represented.