COORDINATED FAUNAL TURNOVER IN THE MIDDLE DEVONIAN OF EASTERN NORTH AMERICA

The hypothesis of Coordinated Stasis, developed from observations of extended periods of ecological stability in the Middle Devonian of Appalachian Basin, has been tested in various depositional basins throughout the rock record. However, no tests of the hypothesis have been conducted in coeval sediments in areas outside of the Appalachian Basin. With the advent of a well-resolved stratigraphic framework based on biostratigraphic and sequence stratigraphic data, it is now possible to test for patterns of Coordinated Stasis in sediments coeval with the “type strata” where the hypothesis was developed. The Middle Devonian (late Eifelian through mid-Givetian) strata of the Michigan and Illinois basins, identified as separate biogeographic subprovinces, preserve a diverse, benthic fauna. High resolution stratigraphic correlations between the Appalachian, Michigan, and Illinois basins through this interval provide a stratigraphic framework in which to test fine-scale patterns of faunal turnover across a large portion of Eastern North America (ENA). Identified within the Appalachian Basin are a series of Evolutionary Ecological Subunits (E.E. Subunits) defined as extended periods of relatively stable faunal composition bounded by short intervals with high rates of faunal turnover. Examination of the patterns of faunal composition through the Middle Devonian of the Michigan and Illinois basins has resulted in the delineation of E.E. Subunits in these areas for the first time. The lowest E.E. Subunit recognized contains the unique Stony Hollow Fauna (SH), dominated by warm-water taxa, and is identifiable across all areas. This fauna is associated with a major flooding event in ENA. A major faunal turnover occurs between the SH faunal interval and the overlying Hamilton/Traverse (HT) E.E. Subunit. The boundary between these units is correlative between the basins, suggesting an extra-basinal cause of the faunal turnover, i.e. sea level fall. The HT interval is marked by a long-lasting, diverse fauna in all basins. The upper boundary of this interval is preserved only in the Appalachian Basin, having been removed by erosion in other areas. However, faunas above the HT interval in the Michigan and Illinois basins are dissimilar from the HT Fauna indicating a period of turnover in roughly the same interval.