MacFadden 9528020 The rich 55 million year fossil record of horses demonstrates evolution in dietary adaptation through time, potentially in response to environmental change. Tooth and skull morphology changed many times and are associated with adaptation to new diets. Using a combination of new methods, this research will test, for the first time, the prevailing theory that the earliest horses were browsers in rain forests and then gradually evolved into savanna grazers. Evolution from horse browsers to grazers is assumed to be an adaptive response to climatic changes and the associated changes in the distribution of vegetation types. During the Miocene (approximately 20 million years ago), North American savannas and grasslands spread and replaced rain forests as global climate became cooler, dryer and more seasonal. In this study, the diet of fossil horses will be compared to modern ungulate herbivores by examining: (1) the microscopic scratches made by vegetation on the surface of the teeth as viewed with an electron microscope; (2) restoration of chewing muscles; (3) analysis of size and shape of snouts; and (4) carbon isotopic composition in tooth enamel (the isotopic signature of savanna grasses is different than forest leaves and the specific isotopic signature of the diet should be incorporated in tooth enamel). The project thus uses four methods to study the same dietary questions and the detailed interaction of morphologic change with the environment. The results may significantly alter the classic story of horse evolution as it is currently presented in books and museum exhibits. Furthermore, they will provide detailed insights into the process of evolutionary change in mammals in response to changing climates and vegetation.
