Ecological Response of Ostracodes (Arthropoda, Crustacea) to Lake-Level Fluctuations in the Eocene Green River Formation, Fossil Basin, Wyoming, USA

Abstract

The Eocene Green River Formation is one of the best-known Konservat lagerstatten and comprises lacustrine strata that were deposited during the Early Eocene Climatic Optimum (EECO). Two species of ostracodes, Pseudoeucypris pagei (Swain, Journal of Paleontology, 23:172–181, 1949) and Hemicyprinotus watsonensis (Swain, Journal of Paleontology, 23:172–181, 1949), were recovered from 16 intervals at three sites from 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 commonly preserved in kerogen-poor micrites but 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. pagei occurring exclusively in kerogen-poor micrites and dolomicrites, while the benthic species H. watsonenis occurred in kerogen-rich micrites as well as some kerogen-poor micrites. The present study demonstrates how ostracodes are preserved differently within the basin and the utility of species occurrences in tracking changing lake environments, as similarly reported in other Green River Formation basins. Here, we interpret that the presence of P. pagei and the rocks it is preserved in indicate shallower lake conditions, while shifts to H. watsonensis are indicative of deepening lake conditions.