PATTERNS OF FAUNAL VARIABILITY THROUGHOUT THE EARLY EOCENE CLIMATIC OPTIMUM AT RAVEN RIDGE, UINTA BASIN, COLORADO AND UTAH

The Early Eocene Climatic Optimum (EECO; ~53-49 million years ago), was an interval of globally elevated temperatures that spanned the early/middle Eocene boundary and is represented in the mammalian fossil record by faunas from numerous basins in western North America. The most temporally expansive record of the EECO is captured at Raven Ridge on the northeastern edge of the Uinta Basin by interfingering strata of the underlying Colton and overlying Green River Formations. Raven Ridge has produced an abundant (~9,600 specimens) and diverse (>60 genera) fossil assemblage, composed primarily of small mammals, that spans an interval from the mid Wasatchian (Wa3-5) North American Land Mammal ‘Age’ through the mid-Bridgerian (Br2). The abundance and distribution of fossil localities throughout the well-exposed stratigraphic section (~1 vertical km) provides good biostratigraphic control, and makes Raven Ridge an excellent site for the observation of mammalian faunal variation throughout the EECO.

Contrary to patterns seen in nearby basins, the Raven Ridge fauna shows no increase in generic richness associated with warming climate across the Wasatchian-Bridgerian boundary. This steady taxonomic richness is unlikely to result from poor sampling, as rarefaction analyses are consistent with relatively complete sampling throughout the Raven Ridge section. While taxonomic richness remains relatively stable, other faunal metrics show considerable variability throughout the EECO at Raven Ridge. Long-lived taxa within the fauna, the most prominent being the condylarth Hyopsodus, show variability in estimated body size throughout the EECO interval, although this variability appears unrelated to fluctuating global temperatures. Relative abundance also fluctuates significantly, with rodents becoming more abundant, and hyopsodontid condylarths and microsyopid, notharctid, and omomyid primates (among other taxa) decreasing in abundance from the Wasatchian into the Bridgerian. Changes in rodent abundance at Raven Ridge appear to correlate roughly with shifts in global temperatures throughout the EECO.

While the observed patterns within the Raven Ridge fauna occurred in conjunction with global EECO climate trends, it has yet to be demonstrated that climate was a primary driver for this variation.