CONICET | Buscador de Institutos y Recursos Humanos

One of the major features of the continental Cenozoic faunas of South America is the presence of native lineages of herbivorous mammals, and among them the largest representatives of each fauna. Here, we analyze the evolution of the large body size of these mammals in relation to their taxonomic richness. During the Casamayoran Age (middle Eocene), notoungulates exhibit the greatest diversity of genera with forms larger than 100 kg. During the Deseadan (late Oligocene), notoungulates, astrapotheres and pyrotheres are roughly equally represented (33% each), and only pyrotheres surpassed 500 kg. During the Santacrucian (early-middle Miocene) notoungulates dominated large-bodied forms (66%) and only astrapotheres surpassed 500 kg. During the Laventan (late Miocene), some xenarthrans (Tardigrada) equaled notoungulates and astrapotheres (all 33%), but only tardigrades and astrapothres included members above 500 kg (50% each). During the Ensenadan and Lujanian (Pleistocene) native ungulates declined notably (ca. 10% of the overall diversity), xenarthrans (Tardigrada and Cingulata) exceeded 40% and 50% of the large-bodied forms in each age, most of them being strict megamammals (i.e. above 1000 kg). The remaining taxa are caviomorph rodents and lineages of boreal provenance (proboscideans, artiodactyls and perissodactyls). In general, but particularly for those faunas in which xenarthrans are dominant, the abundance of megamammals distantly related to living counterparts raises problems in interpreting their paleobiology. Particularly for the Pleistocene, communities dominated by megamammals of very low metabolism (xenarthrans) have no counterpart in living faunas. Hence, paleoecological reconstructions lack strict analogues and alternative approaches must be used.

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