Taxonomy, diet and geographical provenance of fossil species of Sigmodon from west central Mexico during the late Pleistocene

The information of diet, habitat and geographic location of animals obtained from fossil deposits is important to understand the environmental changes of a place that occurred through time. Most of the paleoecological research in Mexico is centered on large mammal taxa. A minimal research effort is allocated to small vertebrates which due to their relative brief longevity and small territories, provide fine-scale paleoenvironmental information. In the present work, the occlusal morphology of the first molar was analyzed, as well as the dental microwear marks and the 87Sr/86Sr isotopic ratio in fossil and extant relatives of the genus Sigmodon from La Cinta-Portalitos locality, situated on the northern portion of the Cuitzeo Basin, central western Mexico. Goals included determining species richness within Sigmodon, as well as dietary behavior and geographic origin. Based on the comparison of the occlusal morphology of fossils and modern teeth, three fossil species were distinguished: S. hispidus, S. leucotis and S. alleni. Microwear patterns showed that S. hispidus population could be associated to a grazer diet, inhabiting open areas, similar to the extant populations. The species S. leucotis exhibited a microwear signature associated to a grazer diet as well, and S. alleni revealed a fruit–grass–insect feeding behavior. Finally, in the case of S. hispidus, the 87Sr/86Sr isotopic signature proved to be like that of the bedrock, so the S. hispidus population was local and a suitable indicator of paleoenvironmental conditions of La Cinta-Portalitos.