THE RICHMONDIAN INVASION (LATE ORDOVICIAN, CINCINNATIAN): PATTERNS OF INCUMBENT AND INVADER DISTRIBUTION AT A REFINED SCALE

The “Richmondian Invasion” was a coordinated species invasion of some 60 species of warm water marine invertebrates, including brachiopods, bryozoans, and corals, during the Late Ordovician, ~446-445 Ma, into the Cincinnati Arch region. Evidence of a first major influx of new species is recorded in the lower Richmondian Stage, mid Waynesville to Liberty formations. Overall biodiversity greatly increased; however, details of the invasion remain poorly understood. For example, how does the proportion of invading species vary among taxa, what proportion of taxa were successful invaders, and how do the ecologies of incumbents and invaders differ in paleocommunities?

Determining species richness, ubiquity, and dominance at the level of submembers and beds is critical to understanding interactions between local incumbent species and invaders. Presence/absence data of all species, compiled by W.H. Shideler were used to determine species distribution throughout the Richmondian Stage. Brachiopods were the primary focus as they are well preserved and include a high proportion of invading taxa. To document relative abundance patterns, exposures of the lower Richmondian interval were sampled at the scale of individual bedding planes. Species richness (standardized to sample size) and relative abundance was used to determine proportions of invading to incumbent taxa.

Brachiopod species and generic richness peaked (19 genera) in the upper Waynesville Fm. During the main phase of the invasion and included almost equal numbers of incumbent and invading genera (10, 9). However, several of the invaders (7) only appeared ephemerally in a few widespread horizons, as epiboles. The remainder (5) persisted through the Richmondian Stage as single species. Ubiquitous incumbent species also persisted and underwent no diversification, but less common genera show appearances of new species at several levels. Initial results suggest that shale-rich facies are dominated by incumbent taxa, whereas condensed skeletal limestone bundles have a dominance of invading taxa. This trend would signify that invading and incumbent faunas may have largely occupied separate habitats and did not strongly interact. These results will help elucidate the long-term impact of species invasions on ecosystem structure.