ANALYZING THE RELATIONSHIP BETWEEN SEA LEVEL, GEOGRAPHIC RANGE, AND INVASION EVENTS IN CINCINNATIAN (LATE ORDOVICIAN ) BRACHIOPODS USING GIS

The Late Ordovician was a time of active tectonism in the eastern United States, which greatly contributed to the development of intercontinental basins and arches that hosted invertebrate communities susceptible to frequent changes in the environment. This study focuses on the brachiopod fauna of the Cincinnati region (Ohio, Indiana, Kentucky area) which inhabited a shallow marine carbonate-siliciclastic ramp. This analysis investigates the relationship between changes in relative sea-level, the geographic extent of species ranges, and relationship between these parameters and the relative survival of native Cincinnatian (Maysvillian to Richmondian) brachiopod species during the Richmondian Invasion.

Species ranges were reconstructed both spatially and temporally (following the sequence stratigraphic framework of Holland [1993]) using a Geographic Information System to quantitatively derive the geographic area of species ranges and describe lateral migration patterns in response to regional sea-level fluctuation. Geographic extent of species' ranges across multiple time slices enabled analysis of habitat tracking patterns, which were examined through “time-slice” animations, as well as their close link to sea-level through statistical analyses.

Statistical tests support the link between species' geographic ranges and regional sea-level fluctuation; geographic range expands as water depth increases with relative sea-level rise. Species also track preferred habitats by lateral migration along the carbonate-siliciclastic ramp in response to sea-level change. Individual species, as well as much of the total population within each chronostratigraphic time-slice, display congruent patterns of movement throughout the Cincinnati Arch. The Richmondian Invasion, an influx of species into the Cincinnati Arch, resulted in regional extinction and the eventual replacement of many native species with immigrant taxa. Statistical analyses indicate that the native species which persisted into the Richmondian were only those with large geographic ranges. This suggests species with larger ranges, typically generalists, were best able to compete in the new invasive regime.