Body Size Regulates Niche Overlap Asymmetry in the Subtropical Andes Rain Shadow: Isotopic Paleoecology of Oligocene South American Ungulates

The Oligocene is a key period in South American mammalian evolution with diversification in groups such as notoungulates, sloths, and primates. While isotopic studies are abundant for the Neogene-Quaternary, Oligocene faunas remain understudied, with previous studies on Salla (Bolivia) and Gran Barranca (Argentina). This work presents δ¹³C and δ¹⁸O_CO3 isotope values from tooth enamel of several mammals (notoungulates, pyrotheres, and marsupials) from Quebrada Fiera (Mendoza, Argentina) to infer diets and to reconstruct the paleoenvironmental and paleoclimatic conditions; hypsodonty and body mass estimates are used for contrasting paleoecological inferences. Herbivores δ¹³C values (from -10.5‰ to -6.2‰) indicate a mosaic landscape including woodland-mesic C₃ grasslands and open woodland-arid C₃ grasslands. Nevertheless, annual precipitation estimates (∼203 mm/year) suggest a crucial role for (semi)permanent bodies of water to sustain such a patchy environment and a rich community of herbivores. The mean δ¹⁸O_CO3 value (25.5‰ ± 1.6‰) is intermediate between the values from Bolivia and Patagonia for the same period, suggesting the existence of a latitudinal gradient with more open and drier environments in lower latitudes. This would be associated with a stronger incidence of the rain shadow effect in the Central than the Southern Andes, likely reflecting regional differences in moisture sources. Importantly, this study reveals an overlap asymmetry in the isotopic niche associated with body size: smaller species exhibit significantly greater isotopic niche overlap, while larger species occupy more distinct isotopic spaces. These findings highlight the role of size-related ecological strategies in shaping community organization in the subtropical Andes rain shadow.