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Early Silurian (Llandoverian) macrofossils from the lower Massanutten Sandstone at Passage Creek in Virginia represent the oldest known terrestrial wetland communities. Fossils are preserved as compressions in overbank deposits of a braided fluvial system. Specimens with entire margins and specimens forming extensive crusts provide evidence for in situ preservation, whereas pre-burial cracks in the fossils demonstrate subaerial exposure. Developed in river flood plains that provided the wettest available environments on land at the time, these communities occupied settings similar to present-day riverine wetlands. Compared with the latter, which are continuously wet by virtue of the moisture retention capabilities of soils and vegetation, Early Silurian flood-plain wetlands were principally abiotically wet, depending on climate and fluctuations of the rivers for moisture supply. Varying in size from <1 cm to >10 cm, fossils exhibit predominantly thalloid morphologies but some are strap-shaped or form crusts. Their abundance indicates that a well-developed terrestrial groundcover was present by the Early Silurian. Morphological and anatomical diversity of specimens suggests that this groundcover consisted of several types of organisms and organismal associations, some characterized by complex internal organization. Earlier microfossil finds at Passage Creek corroborate an image of systematically diverse but structurally simple communities, consisting only of primary producers and decomposers. Ten to fifteen million years older than the oldest previously known complex terrestrial organisms (e.g., Cooksonia), they provide a new perspective on the early stages of land colonization by complex organisms, whereby the earliest terrestrial communities were built by a guild of thalloid organisms and associations of organisms comparable to extant biological soil crusts.

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