Quantitative Paleobiogeography of Maysvillian (Late Ordovician) Brachiopod Species of the Cincinnati Arch: a Test of Niche Modeling Methods for Paleobiogeographic Reconstruction

Numerous studies have examined the highly fossiliferous Late Ordovician strata in the Cincinnati, Ohio region including detailed analyses of biodiversity patterns, paleoecology, and sequence stratigraphy. No detailed analyses, however, have addressed the shifting paleobiogeographic patterns as a result of a regression during the Maysvillian (Late Ordovician). The intensive sampling, excellent preservation, and numerous prior paleoecological and sedimentological analyses within the tri-state region of Kentucky, Indiana, and Ohio form an excellent framework for detailed paleobiogeographic study. Relatively new quantitative biogeographic methods incorporating GIS (Geographic Information Systems) are utilized in order to assess and spatially analyze the geographic ranges of eight articulate brachiopod species of the Corryville and Mt. Auburn Formations of the C3 (latest Maysvillian) depositional sequence. This project employs the ecological niche modeling program GARP (Genetic Algorithm using Rule-set Prediction) to predict the geographic distribution of eight individual brachiopod species. This method predicts the geographic extent of a species range using environmental data, in this case inferred from sedimentologic proxies, coupled with known species occurrence data. Sites of known species occurrence are used to model the fundamental ecological niche for each species; the species is then predicted to occur wherever that set of environmental conditions occurs within the study region. 17 Distributional patterns are reconstructed for three time slices during the C3 sequence. Recovered range predictions were quantitatively analyzed for evidence of temporal range changes including habitat tracking. Results indicate that during the transition from the early to middle portions of the C3 depositional sequence, species track their preferred niche in response to a drop in relative sea level and as a result, average species ranges decreases. However, during the transition from the middle to later portions of the sequence, further lowering of relative sea level correlates to niche evolution of five of the eight brachiopod species modeled. The average species area increases during this interval, but there is a mixed response of species ranges increasing and decreasing. In general, the species that are adapting to new niches increase their geographic range size while those that continue to track their niche decrease in geographic range size. This research also demonstrates that ecological niche modeling methods coupled with GIS are useful tools in understanding paleobiogeographical patterns through time. Introduction The use of quantitative methods in paleobiogeography is relatively new, but the science of explaining the diversity and distribution of life dates back several centuries (See discussion in Lieberman, 2000; Lieberman, 2005). The goal of paleobiogeography is to understand what role geologic and climatic events play on the geographic distribution of a species (Mayr, 1982; Lieberman, 2005). Understanding previous events and how they shaped species distributions amongst regions and environments can help explain current phenomena, such as invasive species, and increase our knowledge of how life has 18 evolved through geologic time (Lieberman and Eldridge, 1996; Rode and Lieberman, 2004). The geographic distribution of each is the product of many variables including biotic, environmental, and historical factors (Lomolino et al., 2005). Some of these factors, particularly the environmental and biological factors, can also be used to study paleocommunities or ecological niches. An ecological niche is the set of environmental and spatial parameters necessary for an organism to survive and maintain viable populations (Grinnell, 1917; Hutchinson, 1957). This is also known as a fundamental niche because it includes areas where a species could potentially live based on the fundamental environmental characteristics required for a particular species to survive. In reality, species rarely occupy their entire fundamental niche since their distribution is further constrained by biotic interactions, such as competition for resources; this results in a smaller realized niche (Lomolino et al., 2005). Modeling the realized niche is typically not possible because data on interspecific interactions are rarely available for modern taxa and are typically unattainable from the fossil record. The fundamental niche, however, can be defined on the basis of environmental factors, which are often preserved by proxy in sedimentary rocks (Stigall Rode and Lieberman, 2005a). Niche modeling also provides a key tool for assessing the potential distribution of a species (Peterson, 2001). Environmental niche modeling (ENM) is a method that predicts species geographic distribution by estimating the geographic extent of the fundamental niche by using ecological parameters associated with known species occurrence sites (Stockwell 19 and Noble, 1992; Stockwell and Peters, 1999; Peterson, 2001). Species ranges reconstructed by direct mapping of known fossil occurrence sites (e.g., Rode and Lieberman, 2004) will necessarily result in the underestimation of a species actual geographic range due to biases of preservation and outcrop availability (Stigall Rode and Lieberman, 2005a). ENM ameliorates this problem since species ranges are constructed based on species ecological preferences, not simply sites of known occurrence. Ecological niche modeling is widely employed in modern biological studies to predict the geographic ranges of species for conservation purposes (e.g., Peterson et al., 1999; Wiley et al., 2003; Nunes et al., 2007). This study utilizes the GARP (Genetic Algorithm using Rule-Set Prediction) modeling system, which is a learning-based method in which the equation set used for determining a species’ ecological niche is modified until the maximum accuracy is achieved or the test reaches a pre-specified statistical threshold (Stockwell and Peters, 1999). There have been limited studies using GARP conducted for the fossil record, but they produced successful results (Stigall Rode and Lieberman 2005a; Maguire and Stigall, 2009) ENM analyses in the fossil record require three conditions: a densely sampled outcrop area, well-defined stratigraphy, and a large species occurrence dataset distributed throughout the outcrop belt (Stigall Rode and Lieberman, 2005a). The Late Ordovician (Cincinnatian) deposits exposed in the Cincinnati Arch of the Indiana, Kentucky, and Ohio tri-state region possesses all three of these parameters making it an attractive field area for a paleobiogeographical study. The entire Cincinnatian outcrop extends over an area of approximately 50,000 square kilometers. The rocks of this region have been 20 studied extensively for more than a century with recent stratigraphic revisions by Holland (1990, 1993) providing a detailed temporal framework of five third-order depositional sequences. This sequence stratigraphic framework allows analysis to understand graphic patterns among the numerous traditional lithostratigraphic formations (e.g., Pojeta, 1971; Sweet, 1979; Hay et al., 1981; Cuffey, 1998). In addition to a well defined stratigraphy and an expansive outcrop, abundant species occurrences are available from museum collections, published references, and new field collections (Fig. 1). This combination of features makes the Cincinnatian outcrop belt one of the best sites to employ ENM in the Paleozoic. In this study, ENM is used to estimate the realized ranges of eight articulate brachiopod species during three time intervals within the C3 sequence of the Maysvillian Stage (Fig. 2). During the C3 sequence, fluctuations in relative sea level altered environmental conditions and species shifted their ranges through time to accommodate the changing conditions. In this study, we test the hypothesis that brachiopod species tracked their ecological niche through time as a result of the reduction in relative seal level that occurred throughout the C3 sequence. We further hypothesize that the geographic range of the brachiopod species was reduced in the size as habitat areas diminished during the relative sea level fall. The broader goal of this research is to better quantify how changes in environment can affect species ranges through time. This research also demonstrates the usefulness of GIS and GARP as an EMN method, specifically for the fossil record. Testing these hypotheses can provide additional insight as to how species adapt to climate change.