Biomolecular preservation of Tertiary Metasequoia Fossil Lagerstätten revealed by comparative pyrolysis analysis

Abstract

We compared structural biopolymers from morphologically well-preserved Metasequoia tissues from three Tertiary deposits and detected a continuum of biochemical preservation seen in this evolutionarily conserved conifer. Pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) was applied to solvent-extracted residues from both fossil leaf and wood remains in comparison with tissues from their living counterparts. The late Paleocene–early Eocene leaves from Ellesmere Island, Canadian Arctic Archipelago, exhibit the best quality of biochemical preservation and show pyrolysis products derived from labile biomolecules characterized by large amounts of polysaccharides. These labile biomolecules are the oldest record of these kinds so far characterized by the pyrolysis technology. The middle Eocene leaf tissues from Axel Heiberg Island, Canadian Arctic Archipelago, yielded slightly lesser amounts of polysaccharide moieties, but the lignin products are similar to those identified from the Ellesmere Island fossils. Compared with these Arctic materials, the Metasequoia leaves from the Miocene Clarkia site in Idaho, USA, show the lowest quality of molecular preservation, characterized by a dramatic reduction of polysaccharides. This continuum of relative quality of biomolecular preservation is further confirmed by SEM observations of transverse sections of these fossil leaves, illustrating a case example where the extraordinary morphological preservation seen in the fossils parallels that detected at the biochemical level. The investigation revealed that both original molecular components and tissue-specific degradation control the final pyrolysis products from fossil material, thus suggesting that comparative studies of molecular preservation are best performed on an individual tissue basis within an evolutionarily conserved taxonomic lineage. Our data support the in situ polymerization hypothesis for the origin of long-chain homologous pairs of aliphatic n-alk-1-enes/ n-alkanes as leaf alteration products.