A DETAILED VIEW OF THE RICHMONDIAN INVASION (LATE ORDOVICIAN, CINCINNATIAN): FACIES PARTITIONING OF INCUMBENTS AND INVADERS

The “Richmondian Invasion” was a coordinated species invasion of some 60 species of warm water marine invertebrates, such as brachiopods, bryozoans, and corals, that occurred in at least two major pulses. This invasion took place during the Late Ordovician, ~446-445 Ma, in the Cincinnati Arch region. Evidence of the first major influx of new species is recorded in the lower Richmondian Stage, mid Waynesville to Liberty formations (Sequences C5C to C6B). The overall biodiversity of the Cincinnatian rocks was strongly increased by the invasion; however, it remains poorly understood how this diversity was accommodated with respect to the partitioning of native (or incumbent) vs. invader species at the scale of individual paleocommunities. Determining the species richness, dominance, and species composition at the level of individual beds is critical to understanding interactions between local incumbent species and invaders at a fine scale. Multiple localities with exposures of the lower Richmondian interval are being sampled for corals, brachiopods, mollusks, and trilobites at the scale of individual bedding planes. The lithology, species presence, and relative abundance are being recorded for each bedding plane. Once the fossil assemblages of each bed are counted, the species richness (standardized to sample size) and proportion of invading fauna to incumbent fauna can be determined. Reconnaissance studies suggest that individual beds may show similar biodiversities to pre-Richmondian assemblages. Initial observations suggest the testable hypothesis that shale-rich highstand facies tend to be dominated by widespread incumbent taxa, whereas condensed skeletal limestone bundles have a dominance of invading taxa. The two types of assemblages therefore may be partitioned by microenvironments. This trend would signify that despite an increase in overall species richness post invasion, invading and incumbent faunas may have largely occupied separate environments and did not strongly interact. Understanding these interactions at a more detailed scale will elucidate the impact of coordinated species invasions on ecosystem structures and determine if invaders and incumbents are partitioned by facies.