Alumni Research

August 31, 2018 at 3:55 pm

Perera and Stigall Publish Study Detailing Ecological Structure of Local Fossil-Rich Limestone

a snapshot of a female with dark hair and glasses with dark skin wearing a red tank top and jeans examines a large rock against a backdrop of a lush green forest

Nilmani Perera MS ’17 examines the outcrop of the Ames Limestone on Witches Hill, Athens, Ohio

Generations of geology students at Ohio University have studied the Ames Limestone, the most fossiliferous rock layer in the Athens area, for class field trips and projects. This unit preserves skeletal remains of marine animals—corals, snails, brachiopods, trilobites, sharks— that inhabited a shallow sea that covered Athens about 300 million years ago. But the Ames Limestone is not only present in Athens, it is exposed throughout southeastern Ohio.

Dr. Alycia Stigall, Professor of Geological Sciences, and Nilmani Perera M.S. ’17 were interested in exploring whether the biological community preserved in the Ames Limestone changed over its outcrop belt and if community changes exhibited an organizational structure similar to modern marine communities. This research project formed the core of Perera’s master’s thesis and was recently published as “Identifying hierarchical spatial patterns within paleocommunities: An example from the Upper Pennsylvanian Ames Limestone of the Appalachian Basin” in the October 2018 issue of Paleogeography, Palaeoclimatology, Palaeoecology.

To address these questions, Perera counted the number of different fossil specimens that constituted Ames Limestone outcrops in seven localities throughout southeastern Ohio. Quantitative analysis demonstrated stark differences in northern and southern communities as well as lower level differences among more closely located outcrops. Thus, Ames paleocommunities showed hierarchical differences related to spatial distance. According to Perera and Stigall, the regional differentiation was due to the amount of mud washed into those areas of the erosion of the nearby ancestral Appalachian mountains was different; whereas differences between nearby locations were due to local topographic relief and biological processes.

Abstract: Identifying ecological mechanisms that produce hierarchically arrayed spatial variation in community structure can be difficult in the fossil record due to conflation of spatial and temporal patterns. However, this difficulty can be mediated by minimizing the temporal duration of deposition within the unit examined. In this study, the fauna of the Upper Pennsylvanian Ames Limestone (Conemaugh Group) was analyzed to explore whether Ames paleocommunities exhibited hierarchical structure in a spatial dimension. This widespread carbonate unit was deposited during the maximum flooding interval of a glacio-eustatically influenced fifth order sea level cycle, and preserved taxa are contemporaneous within only a few thousand years. Paleocommunity structure and variability was assessed at multiple spatial scales using samples collected from seven outcrops of the Ames Limestone throughout southeastern Ohio which form a northeast to southwest trending transect parallel to the paleoshoreline. Abundance data were collected using quadrat sampling for brachiopods, bivalves, gastropods, bryozoa, corals, crinoids, echinoids, trilobites and foraminifera. Paleocommunity structure was analyzed via cluster, ordination, guild, and abundance analyses at multiple spatial scales (within a single locality, among localities and within the total study area) to provide insight on geographic partitioning of paleocommunity variation. Multiple levels of paleocommunity organization were recovered within the Ames fauna. All levels exhibited spatial partitioning, but the inferred proximate controls shifted from abiotic environmental controls at higher hierarchical levels to biotic controls at the lowest level. At the highest level, differentiation into a northern and southern regional paleocommunity was controlled primarily by substrate consistency and habitat heterogeneity related to variation in fluvial input within the basin. Local paleocommunity differentiation reflects biotic responses to topographic and environmental conditions that were geographically distributed within the region; whereas within outcrop variation was due largely to biotic feedback mechanisms.

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