Tissue decay tested in modern Metasequoia leaves: Implications for early diagenesis of leaves in fossil Lagerstätten

Abstract

Sedimentary deposits yielding extraordinarily-preserved fossils (known as Lagerstätten) may provide significant insights into the physiology and environments of ancient plants, particularly when the fossils represent their original characteristics with limited diagenetic modifications. To better understand molecular, isotopic, and morphological changes during the early stages of diagenesis, degradation experiments were conducted in two time series: 1) a laboratory decay series using fungi on leaves over the course of a month and 2) a natural decay series with leaves collected from different stages of leaf senescence and early diagenesis. Both experiments used modern leaves of the dawn redwood Metasequoia glyptostroboides, referred to as a “living fossil” due to the morphological stability of the genus Metasequoia over the past 100 million years. Both decay series demonstrate that microbial degradation of polysaccharides occurs on extremely short timescales and results in cell collapse, first in the exclusively cellulose-based primary cell walls and then much later in the lignin-strengthened secondary cell walls. Despite morphological and molecular changes, the stable carbon isotopic composition of bulk leaves and n-alkanes remained virtually unchanged. Together, these findings suggest that: 1) rapid burial and tissue stabilization is essential in the formation of Lagerstätte fossils, 2) polysaccharides play a key role in maintaining three-dimensional fossil leaf structures and thus polysaccharide preservation implies rapid burial and minimal microbial degradation, and 3) carbon isotope signals, including at the molecular level, altered little during diagenesis. Thus, interpretations of physiological and environmental signals from conifer leaves in Lagerstätten are not likely impacted by early diagenesis.