Grasslands, which today form ~1/4 of Earth?s vegetative cover, are thought to have developed earlier in South America than on other continents, based on floral, faunal, and sedimentological evidence at Gran Barranca, Argentina, and nearby. However, the timing and rate of change remain unclear, as well as causal mechanisms. We will determine the timing and nature of climatic, vegetational, and faunal events related to the evolution of Earth?s earliest grassland ecosystems in southern Argentina by integrating floral, stable isotopic, and faunal data through the long, well-dated, and nearly continuous sedimentological record at Gran Barranca (~42.0 to ~18.5 Ma). We will focus on evaluating whether: (1) The rise to dominance of open-habitat grasses in South America occurred in parallel with their appearance, (2) An increase in openness of habitats occurred in parallel with the rise to ecological dominance of open-habitat grasses, (3) Changes in vegetation were correlated with climate change, and (4) Hypsodonty in South American mammalian herbivores responded rapidly to the spread of open, grass-dominated habitats. Fossil phytoliths (plant silica) will be analyzed to provide detailed, continuous floral records of grasses as well as other climate- or ecology-sensitive plant groups (e.g., palms, sedges). Fossil teeth will be analyzed for stable isotope compositions, focusing on bulk d13C values and isotopic zoning of d13C and 18O values. Together with phytolith assemblage data and sedimentology, this isotopic information will be used to infer changes in habitat openness, aridity, and seasonality in terms of temperature and precipitation. Comparison with records of faunal community composition and functional morphology will elucidate the patterns of grassland expansion and faunal responses. Our data will additionally help answer the following two questions: (1) How does the timing and rate of climate change in Argentina compare with the marine record? (2) How do patterns of fauna-flora-climate evolution in South America compare with putatively similar changes that occurred later, e.g., in central North America? The proposed project incorporates several avenues for disseminating the research to the public through the Burke Museum, including an in-museum exhibit, materials for traveling study kits on the evolution of grasslands, and development of a phytolith database with images accessible on-line through the Museum?s website.
The purpose of this grant was to investigate the climatic controls on grassland evolution in southern Argentina from ~40 million years ago (Ma) to ~20 Ma, and to see how mammals evolved in response to that ecosystem change, especially with respect to the length of their teeth. Many modern mammals have very long teeth ("hypsodont" teeth, or "hypsodonty"), which is viewed as an evolutionary response to abrasive diets, particularly the consumption of grasses in dusty environments. In contrast to modern mammals, ancient and more primitive mammals have short teeth ("brachydont" teeth). Our work involved collecting and interpreting 3 datasets: the record of plant microfossils to identify the initiation and spread of grassland ecosystems, the geochemical record of changes to climate and diet during that time, and the evolution of mammal teeth during this period. Basically many have argued that early evolution of hypsodonty in South America was driven by development of grasslands, in turn driven by climate change. Southern Argentina is important in this scientific debate because faunas there indeed exhibit hypsodonty long before anywhere else in the world, and the area has been proposed as the site of Earth’s earliest grasslands based on sparse plant microfossil data. Our primary results show, surprisingly, that grasses were relatively sparse throughout much of the 40 to 20 Ma period, and only begin to become abundant at about 20 Ma. Even then, they are not the dominant plant form. Instead, palms and other indicators of forests predominate. This implies that teeth evolved not because of grasses specifically, but in response to some other factor. The area is well known for its abundant volcanic deposits and volcanic ash, and we hypothesize that abrasive dust, rather than grasses, are responsible for increasing hypsodonty through time. Our main paleobotanical results are currently in review for publication. A second main result relates to changes to climate through records of stable isotopes (Isotopes are different masses of the same atom, for example, uranium-235 and uranium-238 are different masses of the same element, uranium. Stable isotopes simply refer to atoms that do not undergo radioactive decay, that is, they are stable.) We examined stable isotopes of tooth enamel to investigate precipitation patterns (using oxygen-18 and oxygen-16), and diet and precipitation (using carbon-13 and carbon-12). Like the plant record, which shows relatively small changes through the section, oxygen and carbon isotopes show little change from 40 to 20 Ma. These data are consistent with our hypothesis that volcanic ash, not climate, drove hypsodonty. One perplexing question is how estimates of precipitation, derived from carbon isotopes, compare with the plant record. In general palms require abundant water, and are normally assumed to reflect high precipitation. However, our carbon isotope results imply quite dry conditions. We are currently investigating whether carbon isotope compositions in teeth might be affected after burial. Every project has ancillary research endeavors and outcomes, mainly directed towards understanding what controls our signals, and towards providing further context for interpretations. In this study, we engaged in several activities designed to illuminate what controls the chemistry of fossils and their use in paleodietary and paleoclimate studies. We are quite excited about our study, published in 2010 in the Proceedings of the National Academy of Sciences, which lays out how carbon isotopes may be used to infer paleoprecipitation amounts. This work transcends analysis of tooth enamel to include other carbon records, and received attention from a diverse range of scientists including researchers interested in the modern carbon cycle. Eight other studies, published or accepted for publication, explore additional climatic and trace element controls on the chemistry of modern and fossil bones and teeth. Two of these studies, recently accepted for publication in the Journal of Archaeological Sciences and the Proceedings of the National Academy of Sciences, have broad implications for the diets of human ancestors and for the uptake and release of radioactive elements in humans. This grant directly supported the research and educational activities of 4 undergraduate researchers and 2 technicians, and has been publicized through numerous public and institutional outreach activities, both in the US and Argentina. Published papers referred to above: Kohn, MJ and Moses, R. Trace element diffusivities in bone rule out simple diffusive uptake during fossilization but explain in vivo uptake and release. Proceedings of the National Academy of Science, in press. Kohn, MJ, Morris, J and Olin, P. Trace element concentrations in teeth – a modern Idaho baseline with implications for archaeometry, forensics and palaeontology. Journal of Archaeological Science, in press. Kohn, MJ (2010). Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate. Proceedings of the National Academy of Sciences, 107, 19691-19695.
