EVIDENCE FOR PRE-TRACHEOPHYTE TERRESTRIAL COMMUNITIES FROM THE EARLY SILURIAN OF VIRGINIA

We describe the earliest terrestrial megafossil assemblages from the Early Silurian (Llandoverian) Tuscarora Formation of Virginia. The Tuscarora represents the fluvial member of the Massanutten Sandstone, a sequence that records fluvial to marine transition in the Early Silurian. Extensive sampling of the layers representing floodplain deposition within the fluvial sequence at Blue Hole (Shenandoah Co.) has produced rich fossil assemblages. These include principally thalloid organisms displaying diverse morphology and excellent preservation of anatomy to the ultrastructural level. The 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.

Situated temporally between the oldest dispersed microfossils of putative land plant origin (from Llandeilian marine strata), and the earliest unequivocal land plant megafossils (from late Wenlockian marine strata), the Blue Hole fossils represent the best megafossil evidence for pre-tracheophyte terrestrial ground cover. The diversity of the fossil assemblages suggests that diverse communities were already present on land by the Early Silurian. The Blue Hole 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.