OSTRACODE (ARTHROPODA, CRUSTACEA) DISTRIBUTION IN RESPONSE TO LAKE LEVEL FLUCTUATIONS IN THE EOCENE GREEN RIVER FORMATION, FOSSIL BASIN, WYOMING, USA

The Eocene Green River Formation is one of the best known Konservat lagerstätten, and comprises lacustrine strata that were deposited during the entire span of the Early Eocene Climatic Optimum (EECO). Two species of ostracodes, Pseudoeucypris pagei (Swain, 1949) and Hemicyprinotus watsonensis (Swain, 1964) (Family Cypridacea), were recovered from 16 intervals in three Green River Formation sites from the Fossil Basin, Wyoming, USA. Population density per sample was quantified as the number of ostracode valves per cm². Analysis of kerogen content shows a significant difference between preservation based upon lithology, with ostracodes being more frequently preserved in kerogen-poor micrites and more abundant when preserved in kerogen-rich micrites. The nature of preservation also was correlated to lithology with ostracodes being preserved as whole carapaces more frequently in kerogen-poor micrites and being more broken and disarticulated in kerogen-rich micrites. Species tracked lithology as well, with the epiphytic species P. pageioccurring exclusively in kerogen-poor micrites and dolomicrites, while the benthic species H. watsonenisoccurred in kerogen-rich micrites as well as some kerogen-poor micrites. The present study demonstrates the utility of ostracode species occurrences in tracking changing lake environments throughout the history of Fossil Lake, more specifically during the EECO, and has important implications and applications for studies of current climate change events.