On the evolution of large size in mammalian herbivores of Cenozoic faunas of Southern South America

VIZCAÍNO, S. F.; CASSINI, G. H.; TOLEDO, N.; BARGO, M. S.

Bones, Clones, and Biomes: The history and geography of recent Neotropical mammals

The University of Chicago Press

Lugar: Chicago; Año: 2012; p. 76 - 101

A 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. They include a diversity of taxa within the Xenarthra, Pyrotheria, Astrapotheria, Notoungulata, Litopterna, Cetartiodactyla, Perissodactyla, Proboscidea and Rodentia. We analyze the evolution of the large body size of these mammals in relation to their taxonomic richness.  As the South American mammalian fossil record is largely restricted to southern parts of the continent, with comparatively few Tertiary land-mammal bearing localities outside of Argentina, we limit our samples to faunas from that country. Faunal lists from six different ages were selected. Genera were classified in four body mass categories: (I) less than 100 kg; (II) 100 to 500 kg; (III) 500 to 1000 kg; and (IV) more than 1000 kg. The Pleistocene represents the spectacular climax in terms of body size. In general, but particularly for those faunas in which xenarthrans are dominant, the number of megamammals only 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.  Large size explains most of their vulnerability to extinction at the end of the Pleistocene in South America. Although the impact of the megamammal extinction on the post-Pleistocene evolution of plant communities has not been studied for South America, it is clear that it produced an enormous ecological gap in the herbivorous guild.