Stratigraphic Distribution and Paleoecology of Pennsylvanian Conchostracans (Crustacea: Branchiopoda) on the East Side of the Illinois Basin

Abstract

ABSTRACT- Conchostracans are known to occur at nine stratigraphic levels in Pennsylvanian rocks of the eastern side of the Illinois Basin. Two species, Pemphilimnadiopsis ortoni and Leaia tricarinata, have been identified in these rocks. Except for two occurrences, one of which represents transported material, the two species are mutually exclusive. The one in situ co-occurrence suggests that differences in salinity, alkalinity, or both influenced where the conchostracans lived. Other occurrences are generally characterized by having great numbers of specimens that nearly cover bedding surfaces of shale that is closely associated with a coal bed. The fact that these shales generally lack other invertebrate fossils and are usually rich in organic matter suggests that Pennsylvanian conchostracans of the Illinois Basin tolerated narrow salinities in standing bodies of water that were associated with a swampy, deltaic setting. Conchostracans are apparently limited to the upper part of the Pennsylvanian System within the field area, but they are not well correlated with large-scale, climatic cycles