MICRO-INVERTEBRATE AND CONODONT PALEOECOLOGY OF THE DEPAUPERATE ZONE IN THE UPPER ORDOVICIAN MAQUOKETA FORMATION OF LUANA AND POSTVILLE QUARRIES, NORTHEAST IOWA

Preliminary work for this study focused on the unusual diminutive fauna of the Depauperate Zone within the lower Maquoketa Formation in northeast Iowa. This region was part of an upwelling system that brought phosphate rich waters to the surface and produced dysoxic and anoxic water conditions. The fauna within this zone are unusually small in size and could either represent a seasonal die off so that only juveniles are present, or are diminutive due to harsh water conditions. To better understand the living conditions, a population study was done.

Fossils from bulk shale samples from the Postville and Luana quarries were picked, identified and counted. Resulting data was used to investigate the local population dynamics. The Postville location had a much higher diversity and abundance of fauna than the Luana locality. Postville samples contained 8 different Phyla, where as Luana had 6. Postville samples contained 2752 Mollusca, 941 Brachiopoda, 454 Arthopoda, 348 Echinodermata, 48 Protochordata, 30 Hyolitha, 8 Cnidaria, and 7 Bryozoa. Luana was found to have 3808 Mollusca, 25 Protochordata, 7 Arthopoda, 1 Brachipoda, and 1 Cnidaria. Only one of the phyla, the Mollusca, in the Luana sample had a higher abundance than that of the Postville.

The Luana is interpreted to have experienced lesser bottom water oxygenation than the Postville locality on the basis of lower diversity and overall faunal composition. Work is currently being conducted on the conodonts at both locations to evaluate age equivalence and whether the conodont fauna shows differences in diversity/abundance that parallels or departs from the patterns seen in the invertebrates. Conodonts also hold interest in this study due to their nektonic nature, as they may record environmental conditions different from those represented by the benthic invertebrates.