Fossil Mammals, Carbon Isotopes, and Evolution of Cenozoic Terrestrial Ecosystems in Argentina

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MacFadden 9506550 This proposal seeks NSF grant support for a three-year project to investigate the ancient feeding ecology and evolution of Cenozoic mammalian herbivore communities of southern South America using the classic 60-myr sequence from Argentina. The research will address three interrelated themes of fossil mammal paleoecology: (1) the long-standing enigma of why extinct notoungulates acquired high-crowned teeth, and presumably grazing diets, earlier in South America than most northern hemisphere high-crowned herbivorous mammals; (2) endemic/immigrant mammal feeding ecology with the goal of a better understanding of biotic interactions during the Plio-Pleistocene Great American Interchange; and (3) the evolution of C3 an C4 photosynthesis and timing of grassland spread in southern South America. This study will be accomplished by (1) determining the stable carbon isotope ( 13C) values of tooth enamel and (2) analyzing molar tooth morphology (i.e., relative unworn crown height). The data from carbon isotopes combined with tooth-crown height will determine whether a particular extinct herbivore taxon was a C3 browser, C3 grazer, mixed feeder, or C4 grazer. (During the middle and late Cenozoic, pure C3 feeders have mean tooth enamel apatite 13 values of ca. -13 parts per mil, whereas pre C4-feeders have mean tooth enamel apatite values of ca. 0%. In general, low-crowned herbivorous mammals are predominately browsers, whereas high-crowned herbivorous mammals are predominately grazers. These two methods, which until recently have not been used together, are a powerful new tool in fossil mammal paleontology to understand long-term ecosystem evolution and global change through geologic time. Funds are requested for (1) museum travel to obtain additional specimens for isotopic analysis and to construct a data base on mammalian herbivore hypsodonty at La Plata (1 trip), the American Museum of Natural History (1 trip), and the Field Museum of Natural History (1 trip); of Utah; (3) one trip for my Argentine colleague (Prado) to visit the Florida Museum of Natural History; and (4) one graduate student assistantship.