Unveiling 25 million years of dietary and ecological shifts in South American mammals through stable isotopes

SANZ-PÉREZ, DÁNAE; TOMASSINI, RODRIGO; HERNÁNDEZ-FERNÁNDEZ, MANUEL; DOMINGO, LAURA

Madrid

Encuentro; 5th Virtual Conference for Women Archaeologists and Paleontologists; 2025

Association for early-career Women Archaeologists and Paleontologists

Isotopic studies on the South American Cenozoic vertebrates covering a wide time range (e.g., 20 Ma) are scarce. This study provides a detailed isotopic analysis of over 400 samples of tooth enamel and orthodentine, spanning 25 million years, from the Late Oligocene to the Late Pleistocene of Argentina. It addresses critical temporal and geographical gaps in stable isotope research, particularly for the Oligocene-Pliocene lapse. A total of 25 mammalian families were analyzed, including both well-studied groups such as Notoungulata and Rodentia, as well as previously understudied orders like Pyrotheria. The δ13C results reveal significant temporal and ecological shifts. During the Late Oligocene (Deseadan, ~25 Ma), herbivores exhibited diets based on C3 plants, reflecting predominantly woodland and grassland environments. This trend persisted into the Early Miocene (Santacrucian, ~17 Ma), with taxa maintaining this preference for a C3-based diet. However, an increase in dietary variability among different taxa is observed in the Late Miocene (Chasicoan, ~10-9 Ma), being one of the periods with the highest δ13C range (-14.2 to -6.6‰). During the final Late Miocene (latest Huayquerian, ~7-5 Ma), a preference for diets based on C3 environments is again observed. At the final Late Miocene and the Early Pliocene (latest Huayquerian and Montehermosan, ~4.7-4.5 Ma), the expansion of C4 plants introduced a new dietary resource for herbivores, which broadened dietary variability and opened up new ecological opportunities; diets based on mixed C3-C4 plants are recorded in both periods. In the Early Pleistocene, taxa retained C3-based diets, whereas the Late Pleistocene showed the widest δ13C variability of the whole temporal sequence (-12.1‰ to 1.3‰). This variability during the Pleistocene likely reflects a combination of geographic dispersion, habitat diversity, and climatic and environmental fluctuations associated with glacial-interglacial cycles. These results highlight the interplay between environmental/climate changes and mammalian adaptation over the past 25 million years and provide a nuanced understanding of the dietary and ecological evolution of South American mammals.