Distinguishing the diets of coexisting fossil theridomyid and glirid rodents using carbon isotopes

Carbon isotope analyses were conducted on the teeth of four species of rodents and associated plant fossils from the Late Eocene–Early Oligocene Solent Group of the Hampshire Basin, UK. Results indicate that there is no detectable difference in the overall mean δ13C values between permanent cheek teeth of the two species of theridomyid, Thalerimys fordi and Isoptychus sp. This accords with their very similar teeth that indicate comparable diets. However, the teeth of the two species of glirid (dormouse), Glamys priscus and Glamys fordi, have distinctly more negative mean δ13C values than those of either T. fordi or Isoptychus sp., with which they co-existed. This indicates that both glirids had diets significantly different from those of the theridomyids. This dietary distinction is consistent in all three levels studied spanning at least 3 million years. Carbon isotope analysis of associated plant fossils combined with independent evidence from dental morphology and gnaw marks on Stratiotes seeds shows that seeds of the open water, free-floating aquatic plant Stratiotes formed a significant proportion of the diet of G. priscus and by inference that of G. fordi. In contrast calculated dietary δ13C values of T. fordi and Isoptychus sp. overlap the δ13C values of marginal freshwater aquatic plant seeds and thick-walled plant tissues. This evidence, combined with independent taphonomic and palaeoecological information, suggests that these theridomyids and glirids foraged in close association with large water bodies. This supports the use of tooth enamel from these rodents as a proxy for freshwater oxygen isotope values, in palaeoclimate reconstruction.