UPPER PALEOZOIC PALEOECOLOGY OF MIDCONTINENT NORTH AMERICA: TAXONOMIC AND ECOLOGIC STABILITY OF BENTHIC MARINE FAUNAS, SOUTHEASTERN NEBRASKA

The Late Carboniferous Willard and Hughes Creek Shale Formations were sampled at road cut locations in Pawnee and Richardson Counties in Southeastern Nebraska. Both are “core” shales representing a transgressive phase of offshore marine paleoenvironments in the cyclothems of the North American Midcontinent. As such, they are ideal for testing the null hypothesis of taxonomic and ecologic stability over time (=coordinated stasis). Literature review and preliminary field observations suggested that the two formations contained broadly similar faunas dominated by familiar brachiopod, bryozoan and crinoid taxa. Bulk samples of shales were supplemented with surface collecting of fossil material. All identifiable skeletal material was identified, sorted, counted and then subjected to multiple paleoecologic and taphonomic analyses for hypothesis testing. Specific paleoecologic comparisons were made for: taxonomic composition, relative abundance, multiple diversity indices, rarefaction, guilds, feeding type and life habit composition and brachiopod attachment and hinge type. Taphonomic analyses performed included: brachiopod articulation ratio, dorsal/ventral valve ratio, skeletal assessment of corrasion, epibiont coverage and fracture type and size-frequency histograms. Preliminary results reveal significant similarity in the broad paleoecologic categories (e.g., feeding type, life habit) which breaks down and is lost once taxonomic composition and diversity are seen. Therefore, ecologic stability was minimal and coordinated stasis is falsified. Taphonomic comparisons do show that the two formations have different skeletal destruction profiles; however, an original pattern of ecologic stability might be obscured by differences in preservation. More data are needed before coordinated stasis can be fully falsified for this paleoecosystem.