Biota preserved within cavities in Cambrian Epiphyton mounds, upper Shady Dolomite, southwestern Virginia

Microcavities within allochthonous Cambrian Epiphyton algal mounds in the upper Shady Dolomite (Shady Formation) of southwestern Virginia contain a preserved coelobiontic (cavity-dwelling) biota of probable late Early Cambrian age. The microcavities (mean, 2.0 mm) are constructed by overgrowth of Epiphyton fronds. Epiphyton dominates the coelobiontic biota, although other algae occur on the cavity roof and walls. Globular Foraminifera (?) or foraminifer- like microorganisms, possible brachiopods, and cylindrical borings also occur on or in the roofs and walls. Disarticulated, small inarticulate brachiopod valves, sponge spicules, and grains bored by endolithic microphytes are in the sediment flooring many microcavities. This indicates that microboring algae or fungi, sponges, and inarticulate brachiopods, probably inhabited the microcavities. Traces produced on the surface of the sediment in some microcavities and small spines show that vagrant organisms were moving within or through cavities. Localized areas of Epiphyton have been planed flat, pointing to the presence of an unidentified grazing organism in the microcavities. These mounds occur stratigraphically above archaeocyathid patch reefs in the Shady (Dolomite) Formation, but the mounds contain no archaeocyathids. The presence of coelobiontic organisms in Lower Cambrian algal, mounds lacking a skeletal framework suggests that coelobiontic organisms were not restricted to skeletal reefs but that at least by the end of the Lower Cambrian they had exploited the cavity habitat in algal mounds.