THE ROLE OF POLYSACCHARIDES IN PRESERVING THREE-DIMENSIONAL CELL WALL STRUCTURES IN PLANT FOSSIL LAGERSTäTTEN

Plant remains from fossil lagerstätten have played a significant role in paleobotanical and paleoenvironmental studies, partly due to their remarkable preservation. While the mechanism for these exceptional preservations is still poorly understood, polysaccharides have been found more abundant in anatomically well-preserved fossils. We investigate decay of modern Metasequoia, an important taxon in Cenozoic plant fossil lagerstätten, both a natural decay (from fresh leaves to pond decayed leaves) and a laboratory decay (using polysaccharide-decomposing Geotrichum candidum and a consortium containing Trichoderma reesei) to mimic the early stages of diagenesis in leaf fossilization. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), 3-methyl-2-benzothiazolinone hydrazine hydrochloride (MBTH) assay, and nuclear magnetic resonance (NMR) analyses captured different patterns of polysaccharide concentrations during natural and lab decay, whereas morphological degradation observed using SEM (scanning electron microscopy) shows progressive destruction of cell walls. The lignin-strengthened tracheid cell walls in the xylem are almost intact in laboratory decay, whereas the cellulose-dominant phloem cell walls are gradually collapsed, suggesting the critical role of polysaccharides in maintaining particular leaf cell structures. Bulk carbon isotope ratios measured by IRMS (isotope ratio-mass spectrometry) remain constant throughout the laboratory decay, although the natural decay shows a negative shift in samples buried in pond sediment, suggesting that decomposition in the natural aqueous environment led to more carbon isotopic change after partial removal of polysaccharides. Our morphological, molecular, and isotopic data from the natural and laboratory decay series provide benchmarks for estimating the degree and the quality of preservation in plant fossils lagerstätten.