Ordovician-Devonian lichen canopies before evolution of woody trees

Abstract

Devonian evolution of woodlands has been envisaged as a protracted increase in size of vascular plants, which can be reconstructed from fossil stumps and trunks. However, Late Silurian and Early Devonian nematophytes such as Prototaxites would have towered over land plants, including vascular plant trees, in the same fossil plant assemblage, until finally overtaken by vascular land plants during the Early Carboniferous. Nematophytes lack tissues of vascular plants, and some have spherical photobionts encircled and indented by hyphae, as in lichens. Nematophytes were not monolithic poles, but branched, and trunk spacing in paleosols is evidence that they formed closed canopies. Depths of root and hyphal bioturbation and pedogenic calcite precipitation in paleosols though time increase with greater height of trees. In addition to large non-vascular trunks and early land plants in Ordovician to Devonian paleosols, there also were extensive, nutrient-gathering, networks of glomeromycotan mycorrhizae. An Ordovician-Silurian “age of lichens”, when nematophytes were the tallest elements of terrestrial vegetation and soils were riddled with mycorrhizae, may have nurtured and sheltered Ordovician-Silurian land plants and then Devonian woody plants. Fungi preceded and facilitated the evolution of early land plants.