The first lower molar length, width, and posterior loop width were analyzed for ten Cosomys primus populations from horizontal deposits ranging in elevation from 3,000 to 3,295 feet. Sedimentation rates based on radiometric dates for two ashes and rates derived in previous studies suggest that these sediments span an interval of time that ranges from 45,000 to 164,000 years ago. Nonmetric data included the presence of enamel pits, prism folds, and crenulations; the number of triangles; and relative dentine tract heights. Coefficients of variation for the metrics were low, 4 to 7, and a one-way ANOVA revealed no significant differences between any of the populations. Changes in the mean phenotype were less than two phenotypic standard deviations and were not correlated with elevation. The presence of enamel pits, prism folds, and crenulations correlated only with less occlusal wear.

Thus, these microtine rodent populations appear to exhibit stasis over a time period of at least 45,000 and possibly as much as 164,000 years, unless the deposits accumulated more rapidly than previously estimated.