Identifying California Microtus species using geometric morphometrics documents Quaternary geographic range contractions

Paleontology can provide a deep-time dimension to observations about recent reactions of small mammals to climate change. Obtaining this perspective for voles (Microtus), a common and important constituent of North American mammal communities, has been difficult because species identification based on their dental remains is problematic. Here I demonstrate that geometric morphometrics and discriminant analyses can use commonly fossilized dental features to identify the 5 extant species of Microtus in California: M. californicus (California vole), M. longicaudus (long-tailed vole), M. montanus (montane vole), M. oregoni (Oregon vole), and M. townsendii (Townsend’s vole). Analyses of landmarks on the lower 1st molar (m1) provide more accurate identification than those of the 3rd upper molar (M3), and it is important to use jackknife misidentification metrics to assess the precision of discriminant analyses. Addition of semilandmark curves on m1 does not improve accuracy. The utility of these techniques is demonstrated by identifying Microtus specimens from 2 California fossil localities, Pacheco 2 and Prune Avenue, which provides the first evidence for extralimital presence of M. longicaudus at both localities. The presence of M. longicaudus at these low-elevation sites indicates that pronounced geographic range shifts in this species that have been observed in California over the last 100 years also occurred during previous climate changes. Eventually it might be possible to ascertain whether current range shifts are exceeding those that typified responses to past climate changes.