Paleobiogeography of Cincinnatian (Late Ordovician) Brachiopod Species Utilizing Ecological Niche Modeling Methods

The extensive Late Ordovician deposits in the Cincinnati, Ohio region provide an excellent framework against which to assess biogeographic response to sea level decline within shallow marine organisms. This study focuses on reconstructing the biogeographic ranges of articulate brachiopod species preserved in the Maysvillian (Late Ordovician) Corryville and Mt. Auburn Formations, an interval characterized by a relative fall in sea level. The strata of this region are among the best studied globally in terms of biodiversity, paleoecology, and sequence stratigraphy. However, no prior analyses have attempted to directly link biogeographic and environmental factors.

In this study, we employ ecological niche modeling, a quantitative method of analyzing ecological biogeographic patterns, which has been shown to be successful in studies of modern biota. Quantitative paleobiogeographic analyses require a large, densely sampled outcrop belt, well-defined stratigraphy, and large quantities of paleontological information on species occurrences throughout the outcrop belt. The Cincinnati Arch possesses these parameters making it an attractive field area for a paleobiogeographic study. The geographic distribution of several Cincinnatian (Late Ordovician) articulate brachiopod species ranges was modeled using GIS (Geographic Information Systems) and GARP (Genetic Algorithm using Rule-set Prediction). Data used to construct this model was obtained from field work, published literature and unpublished theses and dissertations. Environmental parameters obtained from this data, such as inferred water depth, percent silt, and biofacies, were utilized in the ecological niche model.

Niche modeling analyses accurately predicted several brachiopod species ranges in the Cincinnatian Series. Geographic range of those species contracted during the relative drop in sea level. Niche modeling analysis of species across multiple time slices demonstrate habitat tracking as a result of the regression. Results show that for shallow marine fauna a drastic decrease in geographic area caused by a relative sea level drop has negative effects on biogeographic diversity patterns.