Paleoecology and evolution of the carnivorous mammalian faunas of South America from the Late Miocene to the Pleistocene: insights from stable isotopic signatures (13C, 15N, 18O) in fossil bones and teeth

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South America was isolated during most of the Tertiary and developed a very particular mammalian fauna. In contrast to other continents, the carnivore adaptive zone was filled by crocodiles, large snakes and birds, and metatherian mammals (Sparassodonta). Sparassodonta was diverse during the Tertiary with a broad range of sizes (≈ 2-50 kg). This diversity decreased towards the late Miocene and the group became extinct at the middle Pliocene (≈ 3 Ma). The cause of this decline and extinction may have been immigration of placental Carnivores to South America (≈ 6-7 Ma ago), which putatively competed with the sparassodonts (Ecological Competitive Displacement Hypothesis, ECD). This hypothesis was recently criticized and the Ecological Replacement (ER) hypothesis was proposed, which postulates that newcomers (placental carnivores) filled ecological niches left empty after the extinction of previous occupants (marsupial carnivores) due to other causes, such as environmental changes. This subject is currently being studied using morphometrics, which, however, may be biased by a phylogenetic signal. Here we propose using stable isotopes (13C, 15N, 18O) to complement the study of the evolution of the carnivore guild during the late Cenozoic in Argentina. This purely phenotypic approach will yield direct information on actual ecological changes. Under the ER scenario, environmental changes should lead to changes in the isotopic relationships among fossil taxa, while in the ECD scenario, similar isotopic relationships should be observable between extinct marsupial predators and the placental predators that replace them.