Silurian vegetation stature and density inferred from fossil soils and plants in Pennsylvania, USA
Silurian fossil plants range from small vascular plants (rhyniophytes) to moderately large fungi (nematophytes), but give little idea of the stature, rooting depth and plant density of vegetation on land. Silurian to earliest Devonian palaeosols from the Bloomsburg Formation show unusually deep bioturbation of several distinct kinds. Surface ground-parallel rhizomes of vascular land plants (to 20 cm deep) are penetrated by burrows like those of millipedes (to 80 cm), but the deepest stratum (down to 2 m below the surface) has features interpreted as bioturbation by fungal hyphae and rhizines. Plant-like axes associated with palaeosols are evidence of vegetation with three distinct tiers above ground as inferred from diameters using allometric scaling equations. Nematophytes (Germanophyton psygmophylloides), up to 1.3 m tall, formed a tier above herbaceous vascular land plants (30 cm) and ground cover (<2 cm tall) of thallose organisms and litter. Drab haloed plant bases in the surface of palaeosols demonstrate that nematophytes grew densely (up to 51 m−2) with spacing (20 cm) that closed canopy in seasonally dry wetland palaeosols, comparable with modern marsh vegetation. Vascular land plants of well-drained soils in contrast were scattered, with bare earth between. Wetland ground cover was thus more extensive than cover of well-drained soils, and precursor lichens facilitated early evolution of vascular land plants.