S/G RATIOS AS AN ASSESSMENT OF MAMMALIAN COMMUNITY RESPONSE TO CLIMATE CHANGE DURING THE PALEOGENE OF THE SOUTHERN ROCKY MOUNTAINS

The S/G ratio is the average number of species within each genus of a contemporaneous community of species. It is assumed that S/G ratios would be low following perturbations in communities due to changes in the climate. Furthermore it is assumed that during periods of prolonged climatic stability, S/G ratios would be higher due to increased niche partitioning of closely related species over time. Thus, during abrupt climatic events S/G ratios are predicted to be low, while periods of prolonged climate stability S/G ratios are predicted to be high. Utilizing the exceptional fossil record of 825 species of mammals from Colorado, Wyoming and eastern Utah during the late Paleocene, Middle Tiffanian (58.0 Ma; Ti-5 biozone) until the end of the Eocene, Late Chadronian (33.5 Ma; Ch-4 biozone), S/G ratios were calculated for 500,000 year bins of time. Ratios were the lowest following the Paleocene Eocene Thermal Maximum global warming event (55.5 to 55.0 Ma) that marks the boundary between the Paleocene and Eocene (S/G = 1.1), this is despite high levels of species richness (87 species). It took until 53.5 Ma (Wa-5 biozone), until the S/G ratios were at the same level as seen in the late Paleocene (S/G = 1.4). S/G ratios were the highest (S/G = 1.51) during the Middle Bridgerian; 49.0 to 47.0 Ma (Br-2 biozone). This high S/G ratio corresponds to the gradual warming event known as the Early Eocene Climate Optimum and the subsequent Bridgerian Expansion, in which mammalian species richness (115 species) was even higher than today’s level of mammalian species richness (80 to 110). The rarity of fossils from the Duchesnean (40.0 to 37.5 Ma) prevent assessing the S/G ratio during this time, while the period from 37.0 to 33.5 Ma reflects S/G ratios around 1.3. The decrease in S/G ratios in the late Eocene likely reflects the gradual cooling of the climate that preceded the Eocene-Oligocene boundary. The S/G ratio of paleontological communities is another index to use in accessing the long term effects of climate change in the geological record.