DIETARY PLASTICITY OF MUROID RODENTS FOR CONTINENTAL-SCALE COMPARISONS OF CARBON ISOTOPES IN THE LATE MIOCENE OF PAKISTAN AND SPAIN

In a general trend of cooling with increasing seasonality, C₄ grasslands replaced C₃-dominated vegetation globally, but not necessarily synchronously, at low- to mid-latitudes between 8 and 6 Ma. In Pakistan, many lineages of frugivores and browsers disappeared during this unidirectional ecological change. Preceding this South Asian mammalian turnover by <1.5 million years, a progressive disappearance of forest-adapted mammals occurred in Spain, coupled with immigration of faunal elements associated with more open habitats. A comparison of faunal change between the two regions can be useful to differentiate complex ecological responses of terrestrial mammals caused by the global-scale climate change in contrast to regional environmental change.

Here, we present the very first broad-scale comparisons of small mammal diet in relation to late Miocene climate change, utilizing stable carbon isotopes of muroid rodents. In a previous study, we have shown that carbon isotopes of Pakistani murine rodents closely follow those of soil carbonates, indicating high degrees of dietary plasticity of the group. In this study, we added nearly 230 samples of cricetid rodents, ranging from 12.5 to 9.1 Ma, to examine continental-scale trends (northern Pakistan vs. Catalonia, Spain) and differential dietary behaviors (murine vs. cricetid rodents).

Compared to Pakistani rodents, carbon isotope values of Spanish cricetids are consistently more negative, indicating that cricetid rodents in Spain are mostly pure C₃ consumers and that the ecosystems in the region provided more closed or wetter habitats than in Pakistan throughout the examined time interval. In Pakistan, carbon isotopic results would suggest that cricetid rodents preferred more closed habitats than murine rodents and that general feeding behaviors of extant murine rodents were acquired through evolution. Prior to a remarkable positive shift of carbon isotopes starting at ~8 Ma, there is a noticeable negative shift (1.5 to 2 ‰) of carbon isotopes in these rodent groups, suggesting a temporary spread of wetter/more closed environments prior to the ecological change. Ecological responses of Pakistani and Spanish rodents to climate and vegetation change over the Neogene offer valuable comparisons to the diversification history of North American rodents.