Collaborative Research: Paleoecology of Fossil Horses (Family Equidae) over the past 55 Million Years

MacFadden 9528020丰富的5500万年马化石记录表明，随着时间的推移，饮食适应的进化，可能是对环境变化的反应。牙齿和头骨的形态发生了多次变化，与适应新的饮食有关。利用新方法的组合，这项研究将首次验证流行的理论，即最早的马是雨林中的食草动物，然后逐渐进化为稀树草原食草动物。从马的食草动物到食草动物的进化被认为是对气候变化和相关植被类型分布变化的适应性反应。在中新世(大约2000万年前)，随着全球气候变得更凉爽、更干燥和更具季节性，北美大草原和草原扩张并取代了热带雨林。在这项研究中，化石马的食物将通过以下几个方面与现代有蹄类食草动物进行比较：(1)电子显微镜下植被在牙齿表面留下的微观划痕；(2)咀嚼肌肉的恢复；(3)口鼻的大小和形状分析；以及(4)牙釉质中的碳同位素组成(稀树草原草的同位素特征不同于森林树叶，饮食的特定同位素特征应纳入牙釉质)。因此，该项目使用四种方法来研究相同的饮食问题以及形态变化与环境的详细相互作用。这一结果可能会显著改变马进化的经典故事，因为它目前出现在书籍和博物馆展品中。此外，他们还将为哺乳动物应对气候和植被变化的进化变化过程提供详细的见解。

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.