FRESHWATER PULMONATE GASTROPODS AS INDICATORS OF LITTORAL ZONATION AND CLIMATE CHANGE OVER EOCENE-OLIGOCENE TRANSITION

The White River Group of South Dakota and Nebraska records the climate change that occurred over the Eocene-Oligocene Transition. New collections of freshwater Pulmonate gastropods assemblages from Toadstool Geologic Park (Sioux County, NE) and Badlands National Park (Jackson County, SD) that straddle this climate transition reveal an overall decrease in average shell size and changes to the proportional abundance of represented gastropod families.

The Late Eocene age palustrine limestone lens within the Chadron Formation at Toadstool Geologic Park is gastropod fossil rich and comprised of large and small Lymnaeids and Planorbids; shell heights range from 3-40 mm. In contrast, the Oligocene age palustrine limestone lens within the Poleslide Formation at Badlands National Park contains abundant gastropod fossils of the families Lymnaeidae, Planorbidae, and Physidae. The specimens are on average smaller than their Toadstool counterparts, with shell heights ranging from 2-15 mm. In addition, preliminary results at both sites demonstrate spatial variation in the proportional abundance of taxa, which likely reflects differences tied to lacustrine facies and water depth. The physid Aplexa, for example, is generally found in temporary ponds or at very shallow depths (< 0.5 meters).

Together, these two sites indicate that average shell size of freshwater Pulmonate gastropods decreases over the Eocene-Oligocene Transition, which is likely a response to increased aridity and habitat instability. Large planorbids are lost and small physids are gained, and small planorbids become proportionally dominant. This study also demonstrates that spatial variation in gastropod communities can be preserved in lacustrine environments, which indicates that the proportional abundance of gastropod families can indicate faunal zonation within the littoral zone of lacustrine environments which can lead to paleoenvironmental reconstructions.

Finally, the lacustrine unit at Toadstool is laterally extensive enough to use as a local marker bed and suggests that the placement of the boundary of the Peanut Peak and Big Cottonwood Creek Members (BCCM) of the Chadron Formation at the Stratotype of the BCCM is in need of revision. Current work reveals that local pedogenic layers used previously as marker beds are discontinuous.