THE DISTRIBUTION OF HEMPHILLIAN RHINOCEROTIDS AND EQUIDS: TAPHONOMY OR ECOLOGY?

Localities from the Hemphillian North American Land Mammal Age have yielded copious amounts of rhinocerotid and equid fossils. However, it has been suggested that many localities do not yield large amounts of both taxa, instead preserving plentiful fossils of one and relatively few of the other. We tested the hypotheses that a) the distribution of Hemphillian rhinoceratids and equids was, in fact, nonrandom, and that b) the relative abundance of rhino specimens would be greater in fluvial rather than lacustrine faunas due to winnowing. Using abundance and distribution data from museum databases – including Hemphillian sites from Canada, the USA, and Mexico – we performed a chi-squared test, which showed that the observed distribution of equid and rhinoceratid fossils differed significantly from a random distribution. Additionally, a Kruskal-Wallis test compared equid and rhinoceratid abundance in different paleoenvironments, demonstrating that rhinoceratids had a much greater relative abundance in lacustrine sites than fluvial ones. These results, while not conclusively refuting the preservation bias explanation for Hemphillian perissodactyl distribution, do suggest an ecological rather than taphonomic cause. Interestingly, a t-test of equid abundance showed a north-south gradient, where horses appear to be more abundant in southern localities than northern ones. This could possibly represent latitudinal niche partitioning between horses and rhinos. If these various lines of evidence do represent niche partitioning between Hemphillian perissodactyls, it would provide valuable insights into the coevolution of such interactions with the evolution and expansion of grasslands in the Miocene.