DISSERTATION RESEARCH: Macroevolutionary drivers of digit reduction in fossil horses

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This study uses the rich fossil record of the horse family, combined with information about changing habitats and anatomy in horse evolution, to test the causes and consequences of digit reduction. Domestic horses and their relatives (zebras and donkeys) are large, grazing animals with slender limbs that end in a single digit (the hoof). The evolution of today?s horses is traditionally explained as simple ongoing adaptation to grasslands: as grasslands gradually took over from forests, the early dog-sized, three-toed horses evolved into large, single-toed grazers. Their teeth also became longer to deal with abrasive grass, while their digits reduced so they could run faster or more efficiently across open spaces. However, this textbook explanation is oversimplified and remains largely untested. This study will shed light on a significant evolutionary transition that culminated in one of humans? most culturally and economically important species. Results will be featured in the Harvard Museum of Natural History (HMNH), whose fossil horse exhibit currently shows the oversimplified pattern of evolution. They will also be shared via lectures designed to engage with members of the public, as well as teacher education events at the HMNH that will bring this project into the classroom. A variety of hypotheses have been proposed for the selective pressures driving digit reduction in the horse lineage: speed for predator escape, economy of movement over open grasslands with scarce resources, and compensation for increasing body size. However, none have been explicitly tested with modern methods. By integrating information on fossil occurrences, trait change, phylogenetic relationships, ecology, and environment, this project aims to directly test potential evolutionary causes of digit reduction in horses. Specifically, this research will use ecological and phylogenetic comparative methods to assess the following questions: 1) Did tridactyl and monodactyl horses overlap in space and time or did they occupy separate niches? 2) Are changes in digit morphology, body mass, tooth height, or habitat correlated across the horse phylogeny? 3) Do changes in digit morphology or environment influence speciation and extinction rates in the fossil record? The results of this project will provide quantitative support (or lack thereof) for the cause of digit reduction in fossil horses, filling in a key gap in our understanding of why this major evolutionary transition happened.