HERBIVORE BODY SIZE LIMITS AND COMMUNITY STRUCTURE DURING AND AFTER THE MIDDLE MIOCENE CLIMATIC OPTIMUM

During climatic shifts, we expect changes in the overall limits on mammalian herbivore body size within clades as a consequence of metabolic demands and shifts in vegetation structure and distribution. The Middle Miocene Climatic Optimum is well-established as an interval with exceptionally high taxonomic diversity of browsing ungulates in North America, with considerably higher taxic diversity than is seen in any modern fauna. Following the onset of cooling, species richness of low-crowned (browsing) ungulates declined rapidly, while richness of higher-crowned (open-area feeding) taxa did not. These changes in richness reflect the loss of smaller-bodied hindgut fermenting herbivores, but are not indicative of the shifts in limits on size.

Maximum body size distributions of clades indicate that the maximum size of most artiodactyls and perissodactyls during this stable interval remained unchanged, but North American camelids, and equids did increase in maximum size. The increase in camelid size was relatively modest, but the increase in equid maximum size was fourfold, resulting in the largest equids known in the North American fossil record, and decreased again rapidly following the MMCO. During this time interval, minimum size of equids increased somewhat, a shift which was not affected by the subsequent cooling and drying climate.

Although the radiation of grazing ungulates had begun, none of them occupied the largest body size classes for their respective clades, even as the climated cooled and became more arid following the climatic optimum. While grazing taxa began to increase in taxic diversity and in size, browsing taxa remained the largest herbivores in North American ecosystems until the Plio-Pleistocene.