We present here the initial results of a new approach to gathering ecological and paleoecological information from small mammals. We adapted methods of microwear analysis, originally developed for the study of dental microwear in large mammalian herbivores, to the teeth of modern and fossil squirrels (Sciuridae). Our results demonstrate that microwear features commonly present on large mammalian teeth are also observed on teeth of modern and fossil squirrels. Furthermore, a preliminary analysis of microwear on the teeth of eight extant species suggests that different microwear features are associated with different diets.
Modern mammals provide a set of potential models for the feeding habits of fossil mammals, although some fossil mammals have teeth unlike those of any modern species. An important approach to the inference of paleodiets involves documenting microwear on mammalian cheek teeth, where most chewing of food occurs. Dental microwear features are microscopic pits and scratches in tooth enamel, and they mainly reflect the physical properties of the foods most recently eaten by an animal. Different diets produce different wear features in modern mammals, and for this reason, microwear analysis has been used to infer paleodiets for a wide range of herbivorous fossil mammals (Walker et al. 1978; Teaford and Walker 1984; Grine and Kay 1988; Solounias et al. 1988; Solounias and Hayek 1993; Morgan 1994; Nelson 2003).
We have focused on squirrels for ecological and practical reasons. Squirrels are taxonomically and ecologically diverse today and throughout much of their evolutionary history (Mercer and Roth 2003). The extant species occupy a wide range of biomes, from tundra to semi-desert to rainforest. They include canopy dwellers, ground-dwellers, and burrowing species. Also, squirrel molars are more similar to the general tribosphenic molar pattern than are the molar teeth of most other rodents, thus facilitating comparison of microwear data from squirrels with prior results from a diverse set of larger mammals. In addition, squirrels have a substantial fossil record, and we have particular interest in some of the Neogene sequences in which fossil squirrels occur. The broader purpose of this study is to develop and assess methods that will be suitable for a wide range of rodents and other small mammals.
Small mammals are those that weigh ~1 kg or less as adults. Almost three-fourths of the roughly 4600 species of living mammals (Wilson and Reeder 1993) qualify as small mammals. Most small mammals are rodents and bats; others include shrews, moles, hedgehogs, tenrecs, tree shrews, small primates, and small marsupials. In most undisturbed modern mammalian faunas, over half of the species are small mammals. Also, the fossil record of small mammals is rich and increasingly well documented (Luckett and Hartenberger 1985; Black and Dawson 1989; Korth 1994; Qiu and Li 2003), despite taphonomic biases against small-mammal remains, as well as the intensive efforts required to recover those remains.
This subject is fitting as a tribute to Will Downs. Will devoted a substantial portion of his professional life to recovering and preparing remains of small vertebrates, including some of the fossil specimens in this study. He enlarged our knowledge about the evolution and ecological diversity of small mammals. He also encouraged and facilitated new empirical and theoretical approaches in vertebrate paleontology, so it is an honor to dedicate this paper to him.
This paper describes microwear analysis as adapted for squirrel teeth and evaluates initial results from this approach. For modern and fossil specimens, our goals were to 1) determine the feasibility of casting small-mammal teeth so as to be suitable for microwear analysis, 2) determine if microwear features were present on the casts, 3) develop a replicable approach for documenting microwear features on squirrel teeth, and 4) investigate the relationship between dietary habits and microwear features among selected modern taxa for the purpose of inferring dietary habits from fossil specimens. In the next section, we present information about the modern and fossil squirrels examined in this study, and review previous approaches to microwear analysis of mammalian teeth. A summary of our approach follows, including preparation of specimens and documentation of microwear features. Results are presented as photographs and statistical summaries of microwear features from modern and fossil specimens. We interpret the microwear data in relation to the dietary habits of selected modern squirrels. The results suggest several directions for future work. An appendix lists all of the specimens studied and data gathered for this study.