The diagenetic fate of collagen as revealed by analytical pyrolysis of fossil fish scales from deep time

The mechanism of protein degradation has remained a topic of debate (specifically concerning their preservation in deep time), which has recently been invigorated due to multiple published reports of preservation ranging from Miocene to the Triassic that potentially challenge the convention that protein preservation beyond the Cenozoic is extremely uncommon or is expected to be absent altogether, and thus have attracted skepticism. In this paper, we analyze fossil fish scales from the Cretaceous, Jurassic, and Triassic using comprehensive pyrolysis gas chromatography coupled with time-of-flight mass spectrometry and compare the pyrolytic products so obtained with a well-preserved fish scale from Late Pliocene, in an attempt to better understand the effects of diagenesis on protein degradation at the molecular level through deep time. We find that the Pliocene fish scale displays a large number of N-bearing pyrolytic products, including abundant substituted cyclic 2,5-diketopiperazines (2,5-DKPs) which are diagnostic products of peptide and amino acid pyrolysis. We identify N-bearing compounds in the Mesozoic fish scales—however, among the 2,5-DKPs that were identified in the Pliocene scale, only diketodipyrrole (or cyclo (Pyr-Pyr)) is present in the Mesozoic scales. We discuss the implications of N-bearing pyrolytic products with emphasis on 2,5-DKPs in geological samples and conclude that the discrepancy in abundance and variety of N-bearing products between Pliocene and Mesozoic scales indicates that the protein component in the latter has been extensively diagenetically altered, while a suite of DKPs such as in the former would imply stronger evidence to indicate preservation of protein. We conclude that analytical pyrolysis is an effective tool for detecting preservation of intact proteins, as well as for providing insights into their degradation mechanisms, and can potentially be utilized to assign proteinaceous origin to a fossil sample of unknown affinity.