PALEOECOLOGICAL TRENDS IN EPIBIONT PLACEMENT ON SILICA SHALE BRACHIOPODS

The preservation of symbiotic relationships in the fossil record can provide a wealth of hypotheses to test. Some of these questions can be answered by examining the fossils of the Middle Devonian Silica Shale Formation of Ohio, USA, a sedimentary rock layer of alternating shale and limestone that is known for abundant coral, mollusk, echinoderm, bryozoan, brachiopod, and trilobite fossils, representing what once was a shallow, tropical marine environment. The Silica Shale provides a unique window into studying paleoecological interactions.

This study examined hundreds of specimens of the spiriferid brachiopod Paraspirifer bownockeri of the Silica Shale in order to identify all encrusting organisms and to evaluate the relationships between the encrusting organisms, their host, and their environmental conditions. Trends in location and orientation of epibionts on the brachiopod valve could relate to feeding patterns of the encrusting organism. Location could also provide information on whether the encrusting event occurred during life or post-mortem and if alive, what type of symbiotic relationships the epibiont has with the host, whether it be commensal, mutual, or parasitic. Placement of the organisms may also be related to optimal positioning in the water column for access to resources.

Cluster analyses, PCA tests, and statistical analysis of trends were conducted on hundreds of Paraspirifer samples, utilizing both hand samples and 3D models. Evidence indicates likely preferential placement from specific organisms, including tiering on the host and on each valve. Other organisms show preferential cohabitation or preferred orientations on the brachiopod shell. In the case of certain annelid taxa (e.g., Cornulites), preferred orientations on the shells deviate from what has been documented in other formations.These data can be used to build a better understanding of niche partitioning of epibionts on a single host and how these epibionts may have interacted with the surrounding seawater. Future work will explore whether these epibiont trends are consistent with those of other times and areas throughout the geologic record.