Dental microwear unveils environmental control of feeding ecology in Miocene squirrels from the Iberian Peninsula

Reconstructing the dietary and habitat preferences of extinct taxa is essential to understand the selective pressures that drive morphological and behavioral evolution, which provides a necessary framework for understanding macroevolutionary patterns and how extinct lineages navigated ecological shifts across time and space. This study examines the degree to which environmental changes influence dietary intake by analysing dental microwear in Heteroxerus rubricati , an extinct xerine squirrel from the Middle Miocene (Aragonian) of the Iberian Peninsula. We examined 85 upper molars from multiple sites across two environmentally distinct geographic regions in Spain (the Madrid Basin and the Calatayud-Montalbán Basin), during a time of significant climatic change coinciding with the Middle Miocene Climatic Transition (15.0-13.5 Ma). Four microwear variables (fine scratches, wide scratches, small pits, and large pits) were quantified to proxy the consumption of grasses, seeds/dust, nuts, and arthropods. Notwithstanding the consistency of a mostly granivorous diet in both locations, H. rubricati exhibited notable spatiotemporal shifts in microwear signatures. While the Madrid Basin sample exhibited a high frequency of small pits, correlating with elevated environmental aridity and the subsequent ingestion of exogenous dust, the Calatayud-Montalbán Basin sample also showed a greater importance of fine scratches and large pits, which correlate with a more mesic, forested habitat with a higher dietary component of grasses. Additionally, signal variation related to the consumption of grasses shows temporal changes in both basins associated with variations in the amount of herbaceous vegetation in the landscape during this time interval. Therefore, the dietary signal of H. rubricati was not static; rather, it suggests a high degree of dietary plasticity fundamentally governed by localised resource availability in response to external environmental controls. Notably, the correlation between small pits and arid conditions highlights the utility of microwear as a high-resolution proxy for paleoclimate. Ultimately, these findings underscore the role of environmental aridity as a primary macroevolutionary force, shaping the ecological niche and evolutionary trajectory of small mammal lineages by modulating the interaction between the organism and its available food resources.