Meeting Abstract
Horses (family Equidae) are unique among mammals in being monodactyl, having a single toe. Classically, the evolution of this trait, along with increased body size and tooth height, has been explained as an adaptation to the spread of grasslands. To test the hypothesis that monodactyly arose in grassland environments while tridactyl horses lived in more wooded areas, we used EcoSim to analyze the niche overlap of fossil horse species between different paleoenvironments in the deposits in which they were found. To enable this analysis, we have compiled a dataset of nearly 4000 published occurrences of horse fossils, trait data for each species, and habitat type (forest, savanna, grassland, etc.) of the deposits where fossils were found. This information allows us to place trait evolution in its environmental context, as well as to look for correlation between toe number, body size, and hypsodonty index in incipient grassland species. Our results show low niche overlap in a matrix of all occurrences in habitat categories against number of toes, suggesting that habitat partitioning did occur. Further analysis will subdivide by North American Land Mammal Ages when monodactyl and tridactyl horses coexisted, which may continue to support this hypothesis or, if monodactyl and tridactyl horses are not found in significantly different environments, call into question the interpretation of monodactyly arising due to an adaptive advantage in grasslands. Our analysis integrates occurrences, fossil traits, and past vegetation patterns in North America and will shed light on an important relationship between adaptive trends in horse evolution and environmental changes.
