Epoxy casts of squirrel teeth suitable for documenting microwear were constructed, and microwear features were visible on 73% of the modern specimens. Microwear was also visible on 26% of the fossil specimens, and preliminary analysis of teeth from Pliocene Spermophilus rexroadensis suggested that its diet was similar to that of modern species of Spermophilus. Our study suggests that the Solounias and Semprebon (2002) light-microscope technique works with squirrel teeth. Despite the small number of modern species and small sample sizes for some of them, we found statistically significant differences in microwear features of frugivorous tree squirrels and herbivorous ground squirrels. These data suggest that the overall approach can be developed for both characterizing the dietary variability of extant taxa and inferring the dietary habits of fossil squirrels.
In order to develop this method to its potential, several future efforts are needed. First, a greater number of modern species, each with a sample size of ≥20 specimens (to capture individual variation and for robust statistical tests), is needed for the reference library. It will be especially valuable to include species whose dietary habits have been documented in detail. Because it is unclear which physical properties of food and detritus create which microwear features, it would also be useful to experiment with captive animals fed on controlled diets followed by casting of their teeth. Second, other groups of rodents with teeth of different sizes and shapes should be evaluated to determine if epoxy casts reproduce microwear. Third, as Gordon (1982) documented for primates, different facets of the same tooth and different molars in the same jaw should be examined to determine how consistent microwear features are in the teeth of the same individual; similar studies would be useful for samples from within and among populations to characterize ecophenotypic variability in microwear. For fossil rodents, large samples of teeth should be examined to characterize the variability present within and among fossil localities. The effects of different taphonomic histories and of acid-washing on fossil enamel, as mentioned above, need to be evaluated. Once the application of this method to fossil teeth is better understood, then it will be possible to study variation in microwear features over the duration of small-mammal lineages, as Nelson (2003) did for large mammals. In combination with microwear data for large mammals and other paleoecological approaches, the study of small-mammal microwear will expand our knowledge of mammalian ecomorphology in ecosystems of the present and the past.