EARLIEST COMMUNITIES OF COMPLEX TERRESTRIAL ORGANISMS: STRUCTURALLY SIMPLE AND SYSTEMATICALLY DIVERSE

Evidence from the Appalachian Basin suggests that well developed communities of complex terrestrial (subaerial) organisms were present at the beginning of the Silurian and probably in the Late Ordovician. The oldest macrofossils of complex terrestrial organisms (Llandoverian) occur in overbank deposits of braided rivers in the lower Massanutten Sandstone of Virginia. The compression fossils represent discrete thalloid individuals or more extensive crusts, exhibiting morphological and anatomical diversity. Rounded margins and cracked surfaces indicate autochthony and desiccation due to subaerial exposure, suggesting in situ preservation.

Macrofossil compressions also occur in the Ashgillian Oswego Sandstone of Pennsylvania, a succession that contains marine fossils only at the base and has been interpreted as shallow marine and close to a deltaic source. Morphologically similar to the Massanutten fossils, the Oswego assemblages are currently under investigation and probably represent the same type of communities.

The two biotas can help understand the composition and structure of early terrestrial communities of which the Massanutten biota, best characterized of the two, renders a comprehensive picture. Abundance, diversity, and complexity of the fossils denote a well developed and diverse groundcover of complex organisms. Presence of fungi and embryophytes indicates that although simple structurally, the communities included at least primary producers and decomposers. The emerging image differs from that corroborated by younger communities of axial polysporangiophytes and may hold the key to the colonization of land and the advent of embryophytes. Systematic diversity, thalloid morphology, and structural simplicity of these pre-tracheophytic terrestrial communities are similar to those of extant biological soil crusts. As is the case in these crusts, apart from a role as decomposers, fungi may have been involved also in associations with photoautotrophic organisms, an evolutionary pathway suggested by hypotheses proposing colonization of land by fungal-algal associations. Trilete spores indicate that embryophytes were present in these thalloid communities, lending support to the hypothesized archetypal embryophyte morphology: thalloid gametophytes with sessile sporangia.