The Lower Silurian (Llandoverian) Tuscarora Formation of Virginia has yielded the oldest megafossil evidence for pre-tracheophyte terrestrial ground cover. Extensive sampling of this fluvial sequence at Blue Hole (Shenandoah Co.) has produced a rich association of principally thalloid organisms displaying diverse morphology and excellent preservation of anatomy to the ultrastructural level. Fossils consist of carbonaceous crusts up to 8 cm in greatest dimension that often cover a large percentage of individual bedding planes. Specimens typically have predominantly rounded margins, indicating that they have been buried in situ (autochtonous). Other bedding planes bear a combination of autochtonous and hypoautochtonous remains. Electron- and light-microscopy shows three-dimensionally preserved, complex multi-layered anatomy. Two main types of internal anatomy have been identified among the specimens: one consisting of an anastomosing network of probable hyphae, and the other involving two dense and variably stratified layers of organic matter separated by a median space containing scattered, light organic material. Morphology and internal anatomy are reminiscent of mutualistic symbioses such as extant lichens and also the looser biotic associations known as microbiotic crusts. These fossils expand the search for the earliest land plants from fragments of embryophytes preserved in marine strata to terrestrial biotic communities preserved in situ, and they provide crucial new data for understanding early phases of the colonization of land.

Key words: fossil, lichen, Llandoverian, Lower Silurian, terrestrial, Tuscarora