The muskrat (Ondatra spp.), a common element in late Kansan and post-Kansan deposits, provides a valuable Stratigraphic tool for deposits of glacial age, in terms of the chronocline proposed by Semken (1966). However, specimens of interglacial age commonly appear out of sequence in the regression. In addition, the nearly ubiquitous distribution in North America of modern muskrats has precluded paleoecological interpretations other than for the presence of permanent water.

Statistical analysis of 147 fossil first-lower molars of Ondatra substantiates and refines the established chronocline. Additionally, it reveals that specimens taken from deposits regarded as interglacial in age have a significantly lower length/width ratio than have those collected from deposits regarded as glacial in age. A statistical examination of 403 Recent first-lower molars from 26 of the United States and from Manitoba, Canada, indicates the following. (1) Both O. zibethicus zibethicus and O. z. cinnamonius found in southern regions exhibit a significantly lower length/width tooth ratio than do those found in northern regions. (2) The ratios, when plotted geographically, form a predictable north-south cline, which, although progressive, has a point of inflection at 42° N. lat. (Nebraska). Specimens collected to the north give a positive residual and southern specimens a negative residual to a least-squares regression. (3) O. z. zibethicus (eastern form) and O. z. cinnamonius (plains variety) are indistinguishable on dental parameters at any given latitude. In conclusion, the data suggest that the length/width ratio is a reliable tool in distinguishing between “warm” and “cool” associated members of the population. Corresponding differences in ratios in glacial and interglacial specimens support this hypothesis. The Hay Springs local fauna, as preserved in the American Museum of Natural History, probably represents a mixed collection, taken from more than one quarry, in a section where rocks of glacial and interglacial age are preserved.

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