Unaltered Nacre from the Pennsylvanian Buckhorn Asphalt, and Implications for the Arms Race Between Mollusks and Their Predators

Abstract

Nanoscale details of original aragonite crystals and organic inclusions are preserved in shells from the Pennsylvanian Buckhorn Asphalt of Oklahoma, USA. Exceptional preservation occurred because, either during or shortly after deposition, oil migrated along wrench faults generated during the simultaneous Ouachita Orogeny. The early sealing by oil (later converted into asphalt) prevented diagenetic alteration of shell material. Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Detection (EBSD), and Atomic Force Microscopy (AFM) reveal a striking high fidelity of preservation, including the oldest known unaltered nacre tablets in gastropods, bivalves, and cephalopods. These nacre tablets are indistinguishable from modern representatives in nanoscale morphology and crystallographic orientations. Fossils from the Buckhorn Asphalt show that by the Pennsylvanian Period, nacre and crossed lamellar were the dominant microstructures in the inner shell layer of the Mollusca. Calcitic microstructures and loosely organized horizontal bundles of aragonite fibers were common among Cambrian mollusks and problematic lophotrochozoans (e.g., hyoliths). Through the early to middle Paleozoic the dominance changed to more fracture-resistant textures nacre and crossed lamellar. This transition reflects the importance of these two types of shell microstructure in deterring predation, and it is clear that the ability to produce crossed lamellar and nacreous microstructures contributed to molluscan success during the Mesozoic and Cenozoic eras.