Mammalian Faunal Change during the Early Eocene Climatic Optimum (Wasatchian and Bridgerian) at Raven Ridge in the northeastern Uinta Basin, Colorado and Utah

摘要

This project investigated patterns of mammalian faunal change at Raven Ridge, which straddles the Colorado-Utah border on the northeastern edge of the Uinta Basin and consists of intertonguing units of the fluvial Colton and lacustrine Green River Formations. Fossil vertebrate localities comprising u3e9,000 fossil mammal specimens from 62 genera in 34 families were identified and described. Included in this fauna are the index taxa Smilodectes, Omomys, Heptodon, and Lambdotherium, among others, which were used to biostratigraphically constrain the Raven Ridge strata as mid-Wasatchian (Wa3-5, ~53.5mya) through mid-Bridgerian (Br1b, ~48.5mya) in age. This time interval coincides with the Early Eocene Climatic Optimum (EECO), an extended interval of globally warm temperatures that is coincident with a large negative δ13C excursion.The onset, peak, and decline of the EECO at Raven Ridge were constrained by analyzing 197 sediment samples, collected from measured sections at Raven Ridge, for Total Organic Carbon (TOC) content and then chemostratigraphically correlating this data with established marine isotope curves. The Raven Ridge TOC data show a large negative carbon excursion that starts during the Wa6 biochron, peaks during the Wa7 biochron, and is followed by a positive excursion near the Wa-Br boundary. Results of the chemostratigraphic correlation are consistent with onset of the EECO at Raven Ridge occurring during the Wa6 biochron, the peak of the warm interval near the Wa-Br boundary, and the decline of the EECO during the Br1a and Br1b biochrons.Generic diversity of the mammalian fauna from Raven Ridge is relatively stable during the onset and peak of the EECO, with a sharp decline during the Br1b biochron. The relative abundance of arboreal taxa increases significantly during the EECO interval, which is consistent with the appearance of dense tropical forests in central North America during this period. This change in habitat structure is marked by an increase in abundance of omomyids primates and a decrease in abundance of terrestrial taxa such as Diacodexis and hyopsodontid ‘condylarths’. The results of this study show equivocal support of the hypothesized direct link between higher global temperatures and increased mammalian generic diversity during the Wa-Br transition, but are consistent with ecological restructuring associated with climate-driven habitat change.