The family Sciuridae includes 273 living species, distributed on all continents except Australia and Antarctica (Wilson and Reeder 1993). In terms of general ecology and body proportions, modern squirrels are divided into three groups—tree squirrels, flying (gliding) squirrels, and ground squirrels (Gurnell 1987). Tree squirrels, including Sciurus carolinensis, the common gray squirrel of eastern North America, and several species of Callosciurus, the large canopy dwellers of South Asian rainforests, are typically slender-bodied with long bushy tails. While tree squirrels can walk, run, and dig on the ground with ease, they spend most of their time in trees, climb head up or head down, and maneuver among small branches with great agility. Flying squirrels have a patagium—a membrane of fur-covered skin—that provides an airfoil for gliding across gaps from tree to tree. This group includes the two species of Glaucomys (northern and southern flying squirrels) common in North America, and several species of Petaurista, the giant flying squirrels of eastern and southeastern Asia. These species are similar in body form to tree squirrels and are obligate forest dwellers. The ground squirrels include the marmots (Marmota), prairie dogs (Cynomys), chipmunks (Tamias), and others. Species in this group have stocky bodies and shorter tails in relation to body length. These species occur in a wide range of non-forested habitats, including grasslands, woodlands, rocky terrain, mountain tundra, and semi-desert (Nowak 1999). Ground squirrels dig burrows and rely on an assortment of terrestrial food resources. Trophically, most squirrels are omnivorous, with a strong reliance on plant foods. The diets of tree squirrels and flying squirrels are dominated by fruits and nuts, whereas the diets of ground squirrels are dominated by seeds, nuts, roots, green vegetation, as well as fruits (Nowak 1999). A recent phylogenetic analysis of squirrels, based on nuclear and mitochondrial DNA sequences, showed that flying squirrels form a monophyletic clade, but tree squirrels and ground squirrels have a complex evolutionary history involving parallel radiations in different continental regions (Mercer and Roth 2003).
Analyses of dental microwear features, such as microscopic pits and scratches on enamel surfaces, have been used in investigations of diet in extinct mammals (Walker et al. 1978; Teaford and Walker 1984; Grine and Kay 1988; Solounias et al. 1988; Solounias and Hayek 1993; Nelson 2003). Wear features on occlusal surfaces reflect mainly the physical properties of the foods most recently eaten by an animal. Other features may represent tooth-on-tooth wear (Rensberger 1978). In addition, teeth may acquire post-mortem modification, including cracking, chipping, and coating with precipitates from sediment matrix; these features may obscure original microwear features. Turnover in microwear features can be rapid, in some cases as little as 24 hours (Teaford and Oyen 1989). Microwear studies of modern species have demonstrated significant differences between folivorous and frugivorous primates, soft-fruit diets and hard-fruit diets in primates, browsing and grazing ungulates, and even seasonal dietary variation in sympatric hyraxes (Walker et al. 1978; Teaford and Walker 1984; Grine and Kay 1988; Solounias et al. 1988; Solounias and Hayek 1993).
Early microwear studies focused on dental factors that might confound dietary interpretations, including microwear patterns resulting from different molar positions and dental facets (Gordon 1982). These studies showed that within a species, different facets and molar positions can yield consistent differences in microwear, including feature ratios, densities, and dimensions, resulting from differences in masticatory biomechanics (Gordon 1982). However, the differences among facets from teeth of the same species are generally much smaller than differences between homologous facets from different species (Teaford and Walker 1984). Also, homologous mandibular and maxillary facets show the same microwear because upper and lower facets break down food by reciprocal action. Thus, these studies indicated that interspecific comparisons on the same facet are reliably informative about differences in diet. Another concern was that differences in microwear features across species may be due to differences in enamel microstructure, or prismatic packing and crystallite orientation (Maas 1991). In a comparison of lemur, sheep, human, and crocodile, Maas (1991) demonstrated that quantitative striation width, which had not been found to discriminate among different diets using a scanning electron microscope (Teaford 1986; Solounias et al. 1988; Ryan and Johanson 1989; Teaford and Robinson 1989), is likely influenced by differences in enamel microstructure. In our rodent analyses, we avoided major differences in enamel microstructure by comparing squirrels to other squirrels. Furthermore, we only used microwear features that have been shown to differentiate diets in large mammals using the light-microscope technique.
Most previous microwear analyses have focused on reconstructing the diets of large mammals. Studies of fossil primates, including early hominids (Grine and Kay 1988) and Miocene hominoids (summarized by King 2001), have generally compared fossil taxa to modern apes and monkeys, and attributed general frugivore/folivore/hard-object diets to the fossil taxa. Typically, primate frugivores have higher proportions of pits relative to scratches; folivores, more scratches than pits; hard-object feeders, the most pits (Teaford and Walker 1984. Likewise, microwear analyses of modern and fossil bovids have differentiated between grazers, characterized by many scratches, and browsers, characterized by fewer scratches (Solounias et al. 1988; Solounias and Hayek 1993; Morgan 1994; Solounias and Moelleken 1994).
Microwear on small-mammal teeth has received comparatively little attention. Rensberger (1978) documented microwear on particular molar facets of several rodents (including one squirrel), with emphasis on variation in microwear features in relation to different phases of chewing. Microwear on a molar of Sciurus griseus was dominated by striations, which Rensberger attributed to detritus. Other SEM and light microscope microwear analyses of striations, or scratches, however, have found them to be a feature related to diet and, specifically, to food components such as phytoliths or hard-shelled fruits (TTeaford and Walker 1984; Grine and Kay 1988; Solounias et al. 1988; Solounias and Hayek 1993; Solounias and Semprebon 2002; Godfrey et al. 2004; Semprebon et al. 2004).
These earlier microwear analyses have provided valuable information on dietary preferences of fossil species. These analyses were performed using scanning-electron microscopes and were therefore expensive and time-consuming, thus limiting sample sizes. Solounias and Semprebon (2002), however, developed a new technique using a light microscope to build a large “library” of modern ungulate microwear. This technique has the advantages of being simpler, quicker, and less expensive than using an electron microscope, thus facilitating analysis of much larger sample sizes. Furthermore, because their technique involves analyzing larger surface areas, it obtains a better representation of a dental facet rather than a fraction of it. This method depends upon identification of features such as pits and scratches on the enamel surface without detailed measurements. Features examined under the light microscope complement those examined in SEM studies, but are not equivalent; features examined at 35X are much larger than those examined at 500X (Semprebon et al. 2004). The light-microscope method has been validated for both single-observer and inter-observer reproducibility (Semprebon et al. 2004). Using this method to document microwear of 50 extant ungulates, Solounias and Semprebon (2002) accurately distinguished among browsers, grazers, and mixed feeders. Frugivorous bovids, tragulids, and pigs were characterized by large numbers of pits and coarse scratches, similar to those seen among frugivorous primates.
Using this light-microscope technique, Nelson (2003) analyzed fossil teeth from a wide range of large mammalian herbivores from the Miocene record of Pakistan. The Neogene Siwalik sequence of Pakistan is a long terrestrial record spanning most of the last 20 m.y.r., providing the opportunity to document changes in species’ diets over 105 to 106 years for some temporal intervals (Barry et al. 2002). Many taxa, including a hominoid, suids, tragulids, and some bovids, were as frugivorous as their modern rainforest counterparts (Nelson 2003). Fruit availability appeared to decrease over time, with many frugivores becoming extinct. For species that persisted through the interval of greatest extinction of frugivores, the fruit dietary component was replaced by browse among the remaining frugivores, and browse was replaced by C4 graze (grass) in others. This study contributed to the inference that monsoon forest was replaced by more open habitat in northern Pakistan during the Late Miocene (Quade and Cerling 1995; Barry et al. 2002; Flynn 2003; Nelson 2003).
We applied the light-microscope technique to evaluate the presence of microwear in fossil sciurids. Rodents constitute a significant portion of the Cenozoic mammalian fossil record and yet little is known about their diets. Fossil rodents and other small mammals potentially offer a more fine-grained reconstruction of paleoenvironmental changes than do larger mammals, as do small mammals in many modern communities (Eisenberg 1981). Thus, an approach based on small mammals should usefully complement previous microwear studies involving large mammals. We present an initial determination of whether modern and fossil rodent teeth can be successfully molded and cast for microwear, and of whether microwear features can differentiate among species known to differ in their diets.