Exceptional Preservation of Organic Matrix and Shell Ultrastructure in a Cretaceous Pinna Fossil*

PhotoEmission Electron spectroMicroscopy (PEEM) was used to observe exceptional preservation in organic matrix components and shell ultrastructure in 66 Ma bivalve shell. PEEM is a novel method to detect, in situ, preservation quality, and provides a noninvasive, nondestructive, and spatially explicit map of prismatic and nacre tablet ultrastructure, mineralogy, crystal orientations, and organic compounds. This technique was used to compare Cretaceous and modern bivalves in the genus Pinna; results demonstrate that 66 Ma shells: (1) preserve original aragonite and calcite crystals in nacre and prismatic layers, respectively, (2) maintain nearly identical nacre tablet and prism ultrastructure and crystal orientations, and (3) preserve components of interprismatic proteins Remarkably, interprismatic proteins are preserved with intact peptide bonds and show an abundance of glycine, an important amino acid for protein folding and mechanical flexibility. Preservation of glycine chains in 66 Ma shells supports the exceptional quality of protein preservation documented here. Notably, this quality of preservation may not be uncommon among fossil shells with nacre, reflecting the entrapment and subsequent preservation of the molecular matrix by shell minerals. Thus, PEEM analysis provides new insight into the taphonomic processes influencing shell and molecular fossils, including the effects of molluscan diagenesis, physiology, biomineralization, and evolution.