VARIABLE EVOLUTIONARY DYNAMICS UNDERLIE THE TREND OF INCREASING BODY MASS OF NORTH AMERICAN UNGULATES

Body mass is deeply integrated with an organisms’ ecology, physiology, morphology, and overall life history. Accordingly, the evolution of mammalian body mass has seen considerable attention, particularly with in the context of climate and environmental change throughout the Cenozoic. Specifically, prior analyses document persistent increases in body mass among numerous groups of mammals throughout the Cenozoic. To date, most studies have focused strictly on documenting the general pattern of these trends (e.g. Cope’s Rule) across various taxonomic scales and relate them to environmental changes. Revealing the drivers of these trends, and the precise mechanisms by which they drive body mass evolution requires more detailed quantitative analysis.

In this study, we use phylogenetic comparative methods to estimate the rates of body mass evolution of North American ungulate mammals (orders Artiodactyla and Perissodactyla). Abiotic drivers of body mass evolution are expected to place similar selective pressures on distantly related lineages (e.g., horses and camels). We therefore determined whether evolutionary rates shift across all ungulate clades throughout the Cenozoic (55 to 5 Ma). We estimated body mass of 678 species North American ungulates from measurements of their cheek teeth. Using a framework phylogenetic hypothesis, we fit Brownian motion models of increasing complexity to determine the number and timing of significant rate shifts.

Our results indicate the evolutionary history of ungulate body mass is characterized by multiple rate shifts throughout the Cenozoic. Median evolutionary rate initially is high during the early diversification of ungulates, and shows a sharp increase in the late Middle to latest Eocene (~42 - 34Ma), with highest rates near the Eocene/Oligocene boundary. Median rates then decline precipitously throughout the remainder of the Oligocene, and continue at low levels throughout the Miocene. These results indicate that the apparently gradual increase in median ungulate body mass throughout the Cenozoic is actually underlain by fluctuating evolutionary rates. Notably, the dramatic global cooling at the Eocene-Oligocene boundary seems to coincide with a substantial change in evolutionary dynamics.