Tracing 25 million years of mammalian dietary and ecological evolution in South America: insights from Argentina

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

Santana do Cariri

Encuentro; PALEO-NE & VII IMERP; 2025

Museu du Paleontologia Plácido Cidade Nuvens

Stable isotope analyses provide a powerful tool to investigate long-term interactions between climate, vegetation, and mammalian ecology, yet South American isotopic studies remain fragmentary and geographically biased. In this contribution, we present a detailed isotopic dataset from Argentina that encompasses more than 400 samples of enamel and orthodentine spanning since the Oligocene to the Pleistocene. Samples were pre-treated to remove organic matter and secondary carbonates using 30% H2O2and 1 M acetic acid–calcium acetate. Stable isotope compositions (δ13C, δ18O) were measured on a Thermo MAT 253 isotope ratio mass spectrometer coupled to a Kiel IV carbonate device at the University of California, Santa Cruz (USA). The standard deviations for repeated measurements of international and in-house carbonate standards across runs was ±0,04–0,12‰ (δ13C) and ±0,06–0,22‰ (δ18O). These data document ecological patterns across 25 mammalian families, including both diverse and well-studied orders such as Notoungulata and Rodentia, as well as previously understudied groups like Pyrotheria. The results reveal significant temporal dietary shifts. During the late Oligocene (Deseadan SALMA stage, ~25 Ma) and early Miocene (Santacrucian SALMA stage, ~17 Ma), herbivores predominantly exploited C3 resources from woodland and grassland environments (δ13C range from -10,4 to -6,6‰, Vienna Peedee belemnite). In the late Miocene (Chasicoan SALMA stage, ~10–9 Ma), an increase in dietary breadth is observed (δ13C range of -14,2 to -6,2‰, possibly suggesting the presence of C4 plants prior to their global expansion), and during the late Miocene (Huayquerian SALMA stage, ~7–5 Ma), a preference for C3 diets is again registered (δ13C range of -13,4 to -7,4‰). The expansion of C4 vegetation during the latest Miocene–early Pliocene introduced novel dietary opportunities, reflected in mixed C₃–C₄ feeding strategies (δ13C range from -12,5 to -3,5‰) during the latest Huayquerian and Montehermosan SALMA stages (~4,7–4,5 Ma). By the Pleistocene (Ensenadan-Lujanian SALMA stages), isotopic values exhibit the widest range of the entire sequence (δ13C range from -12,1 to 1,3‰), a variability that likely reflects the wider geographic dispersion of sampling localities, but also larger environmental heterogeneity, and the glacial–interglacial fluctuations. These findings reveal how climate and environmental shifts shaped mammalian adaptations over the past 25 million years, offering detailed insights into the dietary and ecological evolution of South American mammals. The future combination of this solid isotopic regional evidence along with an ongoing construction of a paleobiological dataset, will allow us to explore the interaction between ecological and evolutionary dynamics of South American mammals over the past 25 million years.