No support for the emergence of lichens prior to the evolution of vascular plants.

Abstract

The early-successional status of lichens in modern terrestrial ecosystems, together with the role lichen-mediated weathering plays in the carbon cycle, have contributed to the long and widely held assumption that lichens occupied early terrestrial ecosystems prior to the evolution of vascular plants and drove global change during this time. Their poor preservation potential and the classification of ambiguous fossils as lichens or other fungal-algal associations have further reinforced this view. As unambiguous fossil data are lacking to demonstrate the presence of lichens prior to vascular plants, we utilize an alternate approach to assess their historic presence in early terrestrial ecosystems. Here, we analyze new time-calibrated phylogenies of ascomycete fungi and chlorophytan algae, that intensively sample lineages with lichen symbionts. Age estimates for several interacting clades show broad congruence and demonstrate that fungal origins of lichenization postdate the earliest tracheophytes. Coupled with the absence of unambiguous fossil data, our work finds no support for lichens having mediated global change during the Neoproterozoic-early Paleozoic prior to vascular plants. We conclude by discussing our findings in the context of Neoproterozoic-Paleozoic terrestrial ecosystem evolution and the paleoecological context in which vascular plants evolved.