INTRODUCTION
The highly fossiliferous Upper Ordovician units exposed along the Cincinnati Arch in southwestern Ohio, northern Kentucky, and southeastern Indiana comprise the type strata for the Cincinnatian Series (Figure 1;
Davis and Cuffey 1998). These strata are well known for their tremendous abundance of fossils, faunal diversity, and high quality of preservation. The fossils of the Cincinnati region have been studied for over 150 years, and major patterns of faunal turnover, community structure, and paleoecological gradients have been well characterized. Large-scale paleobiogeographic patterns, particularly as they pertain to either origination of the Cincinnatian fauna or paleoecological gradients, have been examined. However, no quantitative analyses of paleobiogeographic patterns at the species level have yet been published. In this paper, the paleobiogeographic ranges of rhynchonelliform brachiopod species and genera of the type Cincinnatian strata are reconstructed and quantified.
Quantitative paleobiogeographic analyses have the potential to provide new insights into patterns of faunal turnover by facilitating statistical analysis and hypothesis testing (Stigall Rode 2005). Biogeographic patterns are controlled by a combination of both evolutionary and ecological factors (Lomolino et al. 2006). Consequently, biogeographic assessments can provide detailed information about the impact of environmental changes (both biotic and abiotic) on faunal dynamics (Stigall 2008). Within the Cincinnatian strata, a significant interval of faunal immigration, termed the Richmondian Invasion, is associated with the transition between the Maysvillian and Richmondian Stages (Holland 1997). In this study, the biogeographic impact of the introduction of the invader taxa on native brachiopods is analyzed. Specifically, the long-term effects of community restructuring previously identified during this interval (Holland and Patzkowsky 2007) are assessed as they pertain to biogeographic patterns and niche breadth.
Modern invasive species have been known to significantly disrupt ecosystems into which they are introduced. Most modern invasive species occur in new ecosystems due to human activity (Davis 2009), either through intentional introductions (such as song birds or cultivated plants that become naturalized) or inadvertently (such as stowaways on cargo ships). The term invasion has, however, also been used to describe any process of dispersal and establishment beyond a former range for modern species (Reise et al. 2006). Species invasions in the fossil record occur due to natural causes, for example a relative rise in sea level that leads to breaching of geographic barriers or removal of oceanographic barriers (Vermeij 2005). Therefore, for a fossil species to be considered analogous to a modern invader, it must have evolved in one geographic region or tectonic basin and then subsequently immigrated into a second region in which it did not evolve (Stigall 2010). These species have similar impacts to modern invasive species because they are newly introduced to ecosystems with which they did not evolve in concert.
The impact of these invaders (fossil or modern) is expected to include increased competition for resources and increased predation on native taxa (Lockwood et al. 2007;
Davis 2009). The effect of increased predation is well characterized for modern invasive species: the introduction of novel predators often results in local extinction of prey species (Davis 2009). For example, the introduction of the Brown Tree Snake has decimated the endemic species of ground-nesting birds in Guam (Fritts and Rodda 1998). The long-term effects of increased competition, however, are unclear. To date, no studies of modern invasive taxa have successfully demonstrated complete extinction of native species due to competition from invaders (Davis 2003). Studies of invasion biology, however, are limited in temporal duration. Field studies typically are conducted over several years, and even the best-studied historical invasions have an observational record of only decades. Longer term patterns inferred from invasion events in the fossil record, therefore, have the potential to provide insight into the long-term impacts of species introductions.
Cincinnatian strata that preserve the geographic distribution of species before, during, and after the Richmondian Invasion provide a natural laboratory to test patterns of species level and ecosystem response to species invasions over geologic time. This paper expands on previous studies of community organization across the invasion interval (e.g.,
Holland and Patzkowsky 2007) by analyzing patterns of biogeographic distribution at the species level. In particular, the impact of the Richmondian invasion on native taxa is assessed by examining temporal shifts in geographic ranges of species within species groups. The differential response of native species that adapted successfully to the invasive regime versus those that did not can thus be ascertained. Furthermore, the timing of invasion impact is investigated by analyzing biogeographic patterns in each of six depositional sequences separately. This study, thereby, provides insight into the long-term effects of invasive species in modern ecosystems. This is particularly significant because the longer term effects of biotic invasions on these issues cannot be studied over ecological timescales.
