EXAMINING COMMUNITY CHANGE IN LATE ORDOVICIAN MARINE FAUNAS OF TENNESSEE AND OHIO FOLLOWING THE RICHMONDIAN INVASION

The Late Ordovician Richmondian Invasion was characterized by stark changes in community structure among marine fauna, resulting in new biofacies compositions over a geographical area encompassing the Cincinnati Arch and Nashville Dome. The structures of these marine communities, notably composed of articulate brachiopods, were compared using cluster analyses to evaluate similarities and differences in community structure prior to, during, and after the Richmondian Invasion.

For this analysis, samples were collected from several Cincinnati Arch outcrops spanning Ohio, Indiana, and Kentucky and a Nashville Dome outcrop located near Nashville, Tennessee. Bulk samples of fossiliferous limestones were collected from strata ranging from the Kope to the Whitewater formations (Edenian to Richmondian stages). Most taxa on bedding plane surfaces within the samples were identified to the genus level and then counted for abundance. The exceptions were Vinlandostrophia, which were identified to the species level, and bryozoans, which were classified based on morphology. Taxon counts were converted to percent abundances of the total and log-transformed to reduce bias associated with taxa abundance weight. Classical clustering analyses with the Dice similarity index were performed using the PAST (Paleontological Statistics) program to establish biofacies defining the C1 through C5 sequences. Dendrograms were then critically compared between the two regions.

The dendrogram produced for Nashville Dome taxa yielded the greatest similarity among taxa collected from the C2 and C3 sequences with the C4 and C5 sequences being the most disparate. Cincinnati Arch fauna exhibited a similar community structure with the Nashville fauna, and clearly shares the pattern of marked divergence of C4 and C5 faunas due to the influence by invasive species.