BODY MASS EVOLUTION AND DIVERSIFICATION WITHIN THE EQUIDAE FAMILY

Within large taxonomic groups, extant species often exhibit a broad and right-skewed body mass distribution. This pattern can be explained as the result of macroevolutionary “diffusion” between a minimum physiological size and extinction risks that increase with size. This explanation has previously been shown to accurately predict the extant distribution of mammal sizes as well as their size diversification over the past 80 million years. It remains unknown if this explanation can also explain species size dynamics within subclades. We investigate this question using a novel database of Equidae fossil species over the past 56 million years. Importantly, Equidae exhibits a dramatic increase in both maximum size and taxonomic diversity during the Miocene. Using the time-dependent solution of the constrained convection-diffusion-reaction model introduced by Clauset and Redner, we show that the Equidae family exhibits a consistent expansion away from a 20kg minimum boundary in the form predicted by the model. This strong agreement between theory and data supports the hypothesis of universal macroevolutionary dynamics for terrestrial mammal sizes. Furthermore, we estimate that 90% of the 10-fold increase in the sizes of the largest horses during the early Miocene can be attributed to diffusion processes rather than the simultaneous 5-fold increase in taxonomic diversity. This difference suggests that morphological disparity and species diversity were not coupled during this period. Finally, the decline of Equidae's diversity in the late Miocene appears concentrated among the smallest species, in contrast to the typical extinction pattern where large species disappear first, suggesting large-scale competitive effects or ecological turnover.