PALEOECOLOGICAL FRAMEWORK

Cincinnatian strata preserve a rich and diverse fauna. Even though graptolites, conodonts, and nautiloids occur, the preserved fauna is primarily benthic and is dominated by brachiopods and bryozoa with associated echinoderms, trilobites, gastropods, bivalves, and corals (Feldmann 1996; Davis 1998; Meyer and Davis 2009). Numerous recent analyses have elucidated aspects of Cincinnatian paleontology including studies of diversity structure (Novack-Gotshall and Miller 2003), autecology of specific taxa (Leighton 1998; Datillo 2004), biofacies (Anstey et al. 1987; Barbour 2001; Holland et al. 2001; Holland and Patzkowsky 2007; Patzkowsky and Holland 2007), characterization of ecological and faunal gradients (Miller et al. 2001; Holland et al. 2001; Holland 2005; Holland and Patzkowsky 2007), microevolutionary patterns (Webber and Hunda 2007), and taphonomy (Meyer 1990; Brandt-Velbel 1985).

The most dramatic shift in paleoecological patterns occurs across the Maysvillian-Richmondian (C3-C4) boundary. The onset of the Richmondian Invasion occurs in the C4 sequence when a suite of taxa from the paleoequatorial region immigrated into the Cincinnati region (Foerste 1917; Holland 1997). Significant faunal reorganization occurred during the C4 sequence. Detailed gradient ecology and biofacies analyses of genus-level community composition by Holland and Patzkowsky (2007) indicated that the C1 to C3 pre-invasion communities exhibited high levels of similarity in community structure. The C4 sequence preserves a fundamental breakdown/restructuring of biofacies. Clearly differentiated biofacies are reestablished during the C5 sequence, although the C5 community structure differs significantly from those of the Maysvillian due to the ecological dominance of many invader taxa (Holland and Patzkowsky 2007).

The Richmondian Invasion was a cross-faunal immigration event. Taxa new to the Cincinnati region included species from all trophic levels including tabulate and rugose corals, nautiloid cephalopods, gastropods, bivalves, trilobites, and brachiopods (see Holland 1997 for a complete list of invader taxa). The influx of invaders appears to have been largely unidirectional from the paleoequatorial region of the western United States and Canada (Figure 3) and thus was originally referred to as an invasion of the "Arctic Fauna" (Foerste 1917). Significantly, the influx of invaders did not result in an appreciable increase in extinction rate. Instead, many native species persisted across the invasion interval, resulting in higher diversity levels in Richmondian units compared to Maysvillian strata of the Cincinnati region (Patzkowsky and Holland 2007). Particularly notable were the introductions of taxa such as rugose corals and rhynchonellid brachiopods. Member of these orders—and in some cases the same genus (i.e., Rhynchotrema)—were present in the Cincinnati Arch during the Mohawkian Stage, but were absent for at least five million years prior to their reintroduction (Elias 1983; Holland 1997). During the interval of their absence, members of the invader genera occurred in paleoequatorial waters and are preserved in strata in Canada and the Western United States. For example, Grewingkia, Streptelesma, and Rhynchotrema occur in the Steamboat Point Member of the Bighorn Dolomite of Wyoming, which is correlative with the C2 sequence (Holland and Patzkowsky 2009). Furthermore, certain invader species, such as Thaerodonta clarksvillensis and Hiscobeccus capax, occur in the Maysvillian portions of the Red River Formation of Southern Manitoba (Jin and Zhan 2001). Holland and Patzkowsky (1996) have linked the invasion with paleo-oceanographic changes that resulted in warm, low nutrient waters replacing the former nutrient-rich temperate conditions in the Cincinnati region, thereby facilitating the migration of paleoequatorial taxa into the area.