EXAMINING MORPHOLOGICAL VARIATION AS A RESULT OF ENCRUSTATION ON PARASPIRIFER BRACHIOPODS

Biotic interactions are well documented in the fossil record and are critical to study how communities and ecosystems existed and changed through time. The Silica Shale Formation (Middle Devonian of Ohio and Michigan, USA) has an extensively studied fauna with an abundance of epizoans documented. Paraspirifer, a spiriferid brachiopod, was a common component in the Silica Shale fauna. Most specimens have between one to six other organisms encrusting on their shells, creating micro-communities of epibionts on Paraspirifer. To examine the degree of surface modification produced by epibionts, we employed 3D geometric morphometrics.

Fifty specimens were chosen from the University of Michigan Museum of Paleontology, selected from a larger sample for the types of epibionts present on the shells. Digital 3D specimens were generated using photogrammetric techniques. The Paraspirifer specimens were imaged with a Nikon D810 camera with a 60mm lens; images were stitched together, and 3D meshes were processed in MeshLab and Blender. We used nine fixed landmarks and surface semilandmarks, analyzed in the R package ‘geomorph’, to capture shell surface variations. A Principal Component Analysis (PCA) was conducted in ‘geomorph’ with visualization tools from R package ‘borealis’ to explore morphospace occupation and shifts from shells lacking epibionts.

The resulting PCA suggests a strong morphological shift with Paraspirifer individuals that have Aulopora encrusting compared to the individuals lacking epibionts. Additionally, we note differences in topographically low encrusters, depending on their texture (e.g. Hederella, Petrocrania). 3D geometric morphometrics provides a comprehensive approach to exploring morphological patterns and ecological interactions through time. Future work includes addressing how the life position of the brachiopods may have been impacted by epibionts, considering challenges with feeding and other life functions.