MAMMALIAN FAUNAL CHANGE DURING THE EARLY EOCENE CLIMATIC OPTIMUM AT RAVEN RIDGE IN THE UINTA BASIN, COLORADO AND UTAH

Recent studies based on faunas from the Wind River and Green River basins in Wyoming suggest a positive correlation between mammalian generic diversity and warmer climate during the Early Eocene Climatic Optimum (EECO; ~53-50mya, the warmest extended interval of the Paleogene). Raven Ridge, a 30km long ridge straddling the Colorado-Utah border on the northeastern margin of the Uinta Basin, provides an excellent mammalian fossil record with which to test this hypothesis. The ridge consists of fluvial mudstones and channel sandstones of the Colton Formation overlain by lacustrian carbonaceous shales and siltstones of the Green River Formation, and the area has proven to be extremely productive for the collection of fossil mammals, with >9,500 specimens collected by crews from the University of Colorado Museum of Natural History. This fossil mammal assemblage is consistent with a biostratigraphic age of late-early (Wasatchian) through early-middle (Bridgerian) Eocene (Wa3-5 through Br1b-Br2; ~53.5-49mya); an interval which coincides with the onset, peak, and decline of the EECO.

The Raven Ridge fossil mammal assemblage shows peak generic diversity during the middle Wasatchian (Wa3-5) with no significant change during the EECO until a notable decline during the Br1b biochron. The lack of congruence between the Raven Ridge and Wyoming faunas does not appear to be an artifact caused by sampling bias as rarefaction analyses are consistent with relatively complete sampling in all biostratigraphic zones. One possible explanation for the different patterns of generic diversity through time in adjacent basins is an increase in provinciality related to the onset of the EECO.

Unlike the stability observed in mammalian generic diversity, the taxonomic evenness and relative abundance of genera within the Raven Ridge fossil assemblage show considerable variation throughout the EECO. These changes are consistent with dynamic changes in habitat structure during Wa-Br transition. Changes in the relative abundance of taxa with inferred arboreal habits, such as omomyid primates, are consistent with an increase in arboreal habitat that roughly mirrors the increase in global temperatures during the EECO.