FUNGAL EVOLUTION IS NOT RELEVANT TO THE CARBONIFEROUS PEAK IN COAL DEPOSITION

The fact that much of the Earth’s coal was deposited during the Carboniferous has often been attributed to a lag between the evolution of lignin-producing vascular land plants and of fungi capable of metabolizing this newly available resource, resulting in the accumulation of vast amounts of decay resistant plant matter. This idea has received recent support from molecular clock studies looking at the distribution of different metabolisms across the fungal phylogeny. This hypothesis requires that the biochemical inputs to coal formation have always been dominated by lignin and have not changed significantly through time, a problematic assumption given the repeated complete biotic turnovers in wetland vegetation. Lignin production is ancestral to all vascular plants, but its abundance is highly variable across the different organ and tissue types of any particular plant. The greatest proportion of biomass input to post-Paleozoic coals will be lignified wood, but the tree ferns that dominate latest Carboniferous coals in North America are supported by roots, not wood, and the greatest biomass input to earlier Carboniferous coals is typically the periderm (or “bark”) of tree lycopsids. Comparative organic geochemistry of the lycopsid periderm indicates that it was not lignified and that the original biochemistry was likely more similar to the suberin found in the analogous bark of seed plants. Thus, subsumed in the Paleozoic peak in coal production is a turnover from non-lignin to lignin-dominated biochemical inputs, due to the replacement of lycopsids with tree ferns, without any obvious manifestation in the abundance of coal deposition—negating a significant role for the evolution of fungal metabolisms. Phanerozoic patterns of coal deposition are more consistent with combined forcing from climate and tectonics.