NONMARINE OSTRACODA FROM THE EOCENE FLORISSANT FORMATION, COLORADO, USA

The Florissant Fossil Beds National Monument in central Colorado preserves a formation of lacustrine shales that contain a diverse collection of fossil plants and insects. The formation has a reported radiometric age of 34.07 Ma that corresponds to the cusp of the Eocene-Oligocene boundary.The nonmarine ostracode fauna were first reported and described in 1910 without illustration, and since then no significant study has been performed to further confirm the taxa present in the formation.

Samples of fossiliferous mudstone were excavated out of the upper shale unit of the Florissant Formation in relative proximity to the type locality of the reported ostracodes. Exceptional specimens were chosen from the processed residue and evaluated on a Hitachi 3400S Variable-Pressure Scanning Electron Microscope. Our evaluation of carapace size, shape, adductor muscle scars, and the nature of the duplicature confirms the species as valid. On the basis of external and internal characteristics, we recommend Cypris florissantensis COCKERELL be reassigned to a more appropriate genus within the Family Cyprididae. We further report the presence of previously undocumented species Candona artesensis SWAIN. Closely related taxa, including those of genera Heterocypris and Candona, have been reported from the coeval deposits of the early Eocene Green River Formation which spans across Colorado, Utah, and Wyoming, and the Middle Eocene Brian Head Formation in Utah.

The occurrence of this ostracod fauna in the upper shale unit suggests the paleoecology of the later generation of Lake Florissant recovered after the prior deposition of volcanic lahars from the nearby Guffey Volcano complex. Moreover, the abundance of cypridid instars suggests an in-situ biocoenosis; the comparative lack of Candona instars indicates the cypridid assemblage would be the most suitable paleoecological indicator for future studies on Lake Florissant.