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,...

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