TRACKING THE WATER DEPTH HABITAT RANGE OF MEGALOGRAPTUS THROUGH THE KATIAN FROM THE APPALACHIAN BASIN TO THE CINCINNATI REGION

Megalograptus, the best-known Ordovician eurypterid, occurs in multiple localities and horizons in the Cincinnati region and Appalachian Basin. Previous discussions of the habitat of eurypterids have focused on the question of whether they lived in open marine, restricted-marine, or even non-marine environments. Given that they are consistently associated with normal marine faunas, early studies in the 1960s concluded that megalograptids occupied open marine environments. Since the 1960s more effort has been made to subdivide marine lithofacies and biofacies. Recent revisions of Cincinnatian stratigraphy utilized sedimentological and paleontological criteria to divide the ramp into peritidal, lagoonal, barrier/biohermal, shoreface, shallow subtidal, and deep subtidal facies, and has resulted in the generation of multiple thin time slices throughout the Cincinnati Ordovician. This allows finer environmental and temporal placement of Megalograptus occurrences. Original localities and stratigraphic descriptions are precise and most occurrences in the Cincinnatian can thus be located to the nearest few meters stratigraphically. Megalograptid occurrences projected onto this high-resolution sequence stratigraphy show a consistent association between megalograptids and the lagoon-barrier-to shoreface facies. Appalachian Basin occurrences in the Martinsburg Formation appear to be of comparably shallow depths based on fossil evidence. The megalograptid lineage apparently invaded the Cincinnati region from the Appalachian region. Martinsburg occurrences are dated Edenian or Maysvillian, whereas the earliest Cincinnatian occurrence is in the later part of the Maysvillian. Both the Cincinnati Region and the Appalachian basin received sediments from the Taconic Orogen and are the same sedimentary system. During the Edenian the Cincinnatian sea was far too deep for megalograptids, but the Appalachian Basin region was shallow. These suitably shallow water habitats prograded from the Appalachians to the Cincinnati basin –and occurrences of megalograptids track the facies northward and westward through the Cincinnati region, with the older being further SE, and the younger being further NW.