MAMMAL BODY SIZE EVOLUTION IN NORTH AMERICA AND EUROPE OVER 20 MILLION YEARS: SIMILAR TRENDS GENERATED BY DIFFERENT PROCESSES

Because body size connects with many other aspects of a species, broad-scale patterns of body size variation can shed light onto the underlying processes that have shaped the world’s biodiversity. In this study, we combine the macroevolutionary and biogeographic perspectives to investigate whether body size variation is associated with differential origination and/or extinction rates in different faunas, thus directly linked to the regional dynamics of biodiversity. We took advantage of the fossil record, serving as a direct window to the evolutionary history, and applied a Bayesian analytical approach to an extensive body size dataset of Neogene large herbivores (971 species in the orders Artiodactyla and Perissodactyla) in Europe and North America. We estimated species-specific regional origination and extinction times from distributions of fossil occurrences, and reconstructed the temporal patterns of body size in each order on each continent independently. We found increases of body size through time in all four focal assemblages, and three (except European perissodactyls) showed significant increases in minimum size so that their clade-level patterns are most likely driven by selection for large bodies, rather than a bounded diffusive process. We then assessed whether variation in body mass are linked to clade-specific diversification dynamics by fitting trait-correlated birth-death models. The common trend of body size increase seems to have been generated by different processes in different clades and regions. In Artiodactyla, we found a significant correlation of large body size with higher species origination rates and lower extinction rates in North America, but only with higher originate rates in Europe, while in Perissodactyla, we found larger-sized species to have lower extinction rate in North America. These results further support the idea of a driven evolution towards larger body sizes. Notably, further sampling effort is unlikely to alter the patterns, as we did not find larger bodies associated with higher preservation rates in any of our groups. Collectively, our results highlight the value of investigating macroevolutionary dynamics in a biogeographic context, as evolution takes place in environmental templates provided by the different regions.