EXCEPTIONAL PRESERVATION OF GLYCINE-RICH PROTEINS AND SHELL ULTRASTRUCTURE IN PINNID BIVALVES

PhotoEmission Electron spectroMicroscopy (PEEM) was used to observe exceptional preservation in organic matrix components and shell ultrastructure in 66Ma bivalve shell. PEEM is a novel method to detect, in situ, preservation quality, and provides a non-invasive, non-destructive, and spatially explicit map of prismatic and nacre table ultrastructure, organics, minerals and their orientations. This technique was used to compare Cretaceous and modern bivalves in the genus Pinna; results demonstrate that 66Ma shells: (1) preserve original aragonite and calcite crystals in nacre and prismatic layers, respectively, (2) maintain original nacre tablet and prism ultrastructure and crystal orientations, and (3) preserve interprismatic organic proteins. Interprismatic proteins are glycine-rich and preserved with intact peptide bonds. In both modern and Cretaceous Pinna shells, glycine is a major component of the interprismatic proteins, due to its importance for protein folding and mechanical flexibility. However, as the smallest amino acid, glycine is also the first to break down during diagenesis. Preservation of glycine chains in 66Ma shells supports the exceptional quality of protein preservation documented here. Notably, this quality of preservation may not be uncommon amongst fossil shells with nacre, because shell minerals entrap and protect organic compounds. Thus, PEEM analysis provides a new understanding the taphonomic processes in shell and molecular fossils, including the effects of molluscan diagenesis, physiology, biomineralization and evolution on fossil preservation.