The species richness of Miocene browsers, and implications for habitat type and primary productivity in the North American grassland biome

We have documented elsewhere, and briefly reviewed here, the anomalously high species richness of browsing ungulates (hoofed mammals) in the mid Miocene (∼18–12 Ma) woodland savanna habitats of North America. In the mid Miocene these habitats supported substantially more brachydont (browsing) species than do any present-day savanna habitats. We present some preliminary data to show that such species-rich browser communities are not observed after the mid Miocene. The data also suggest that these mid Miocene browser communities and their subsequent disappearance may have been a global phenomenon. We then focus on possible explanations for these observations. We discuss the possibility that the high species numbers are a preservational artifact, and conclude that taphonomic factors are unlikely to be responsible for the level of species richness observed. We then consider various possible explanations for high species richness, including the unique aspects of climatic change (cooling, drying, increased seasonality), unique biotic interactions (the effects of competition, keystone herbivores, changes in predator pressure), and consider that none of these are fully consistent with the available data. The most likely explanation for the observed species-rich browser palaeocommunities is an elevated level of primary productivity, relative to the present day, within (at least some) mid Miocene grassland habitats. Such an increase in productivity could possibly have been the result of higher-than-present levels of atmospheric carbon dioxide in the mid Miocene, but this conflicts with current interpretations of geochemical evidence, and a satisfactory mechanism for the inferred increased primary productivity is an unresolved issue.