THE UPS AND DOWNS OF AN EOCENE LAKE—OSTRACODE FAUNAL RESPONSE TO LAKE LEVEL FLUCTUATIONS IN FOSSIL BASIN, GREEN RIVER FORMATION, WYOMING (USA)

The Eocene Green River Formation (USA) is home to one of the longest-lived, best preserved and diverse lacustrine faunas found within the fossil record and offers a unique opportunity to test hypotheses of lake level fluctuations and faunal response. Where most studies have focused on the fossil fish and plants from the greater Green River basin, few studies have examined the invertebrates, particularly the ostracodes of this remarkable fauna. To test hypotheses of lake level fluctuation and faunal response, we collected ostracodes throughout Fossil Basin at cm scale intervals. Three species of ostracodes, Candona pagei, Hemicyprinotus watsonensis, and Cypridea bisulcata, were recovered from 80 sampled intervals within a 350 meter section of the Road Hollow and Fossil Butte members and were preserved as internal molds or were replaced by various minerals. The three ostracode species represent various ecological niches that include both plant and mud dwelling lifestyles. These ecological tolerances were used to reconstruct environmental conditions, including lake levels through its depositional history via detrended correspondence as well as time series analyses. The benthic ostracode species typically occurred in micritic limestones and shales associated with deeper water conditions, while the plant dwelling C. pagei typically occurred in mud-cracked micrites deposited in shallower water. Ostracode and fish co-occurrences only existed in two intervals, both interpreted to be deeper water conditions. The ostracode response to the changing lake levels within the studied section demonstrates faunal tracking within this depositional sequence. Ostracodes with original shell material were recovered from the Angelo Member but were not part of this preliminary study.