Late Ordovician-Early Silurian terrestrial biotas of Virginia, Ohio, and Pennsylvania: An investigation into the early colonization of land

An early phase in the colonization of land is documented by investigation of three fossil compression biotas from Passage Creek (Silurian, Llandoverian, Virginia), Kiser Lake (Silurian, Llandoverian, Ohio), and Conococheague Mountain (Ordovician, Ashgillian, Pennsylvania). A framework for investigation of the colonization of land is constructed by (1) a review of hypotheses on the origin of land plants; (2) a summary of the fossil record of terrestrial biotas; (3) an assessment of the potential of different continental depositional environments to preserve plant remains; (4) a reevaluation of Ordovician-Silurian fluvial styles based on published data; and (5) a review of pertinent data on biological soil crusts, which are considered the closest modern analogues of early terrestrial communities. The three studied biotas are non-paradigmatic biotas, defined here as biotas whose components escape unequivocal taxonomic assignment because their features preclude assignment to any known group, or compare them to several known groups without allowing further distinction.; The Passage Creek biota consists of rich, morphologically and anatomically diverse fossil assemblages. They demonstrate that a well-developed groundcover was already present at the beginning of the Silurian, occupying river floodplains. This groundcover consisted principally of thalloid organisms and the diversity of terrestrial communities included cyanobacteria, as well as organisms characterized by complex internal organization, the affinities of which remain unresolved. The Kiser Lake fossils consist of carbonaceous compressions, one of which compares to liverwort leaves and lichen rhizines. The same sediments yield scraps of cellular tissue and the earliest spores assignable to ascomycetes. These are preserved in peritidal carbonate sediments. The Conococheague Mountain biota includes rich compression assemblages from transitional shallow marine and deltaic facies. Such deposits usually harbor mixtures of marine and continental fossils, rendering interpretation of the ecology of original organisms difficult. These three biotas contain the oldest macrofossils of terrestrial (Passage Creek) and potentially terrestrial (Conococheague Mountain, Kiser Lake) organisms and demonstrate that diverse communities including complex forms were present much earlier than previously thought. The thalloid morphology represents a transitional stage in the evolution of complexity among terrestrial primary producers from exclusively microbial photosynthesizers to complex embryophytes with axial morphology.