Effects of Oligo-Miocene global climate changes on mammalian species richness in the northwestern quarter of the USA

Abstract

We derived species richness curves using three different methods for mammal species recorded in fossil deposits between 30 million and 9 million years old (late Oligocene through late Miocene) for three geographic regions in the USA: the Northwest, northern Rocky Mountains and northern Great Plains. The data were used to examine the relationship between global climate change and species richness at the regional scale. Our goal was to test the hypothesis that geographic scaling issues account for the lack of correlation that has been observed between continental species richness and the oxygen–isotope curve. The results of all three methods used in this study suggest that species richness in the three regions analysed did not change in response to the global temperature signal, supporting the inferences drawn from continentalscale analyses. The most prominent signal is a peak in species richness in the Rocky Mountain region about 15 million years ago, possibly due to increased beta diversity within the mountains, although many of these species were shared between all three biogeographic provinces. This peak coincides with the Mid-Miocene Climatic Optimum, but it appears unlikely that global temperature change was the direct cause because no response in species richness characterized the even greater Late Oligocene Warming. The Mid-Miocene richness peak also closely follows the onset of major tectonic events in the Rocky Mountain region, which may have led to increased within-province endemism through a combination of physiographic and related climatic effects not recorded in the global temperature signal, a hypothesis that deserves detailed testing.