INSIGHTS FROM A MONOSPECIFIC LINGULID BRACHIOPOD BED IN THE LATE DEVONIAN OF SOUTH AFRICA

Abstract: A formerly unknown fossil-bearing locality in the lower part of the Witpoort Formation (Witteberg Group, Cape Supergroup) is described from the Eastern Cape (South Africa). Uniquely for these strata, it provides evidence for a back-barrier lagoon hosting a monospecific lingulid brachiopod fauna. This represents the youngest record of marine invertebrates within the Cape Supergroup. The occurrence extends the age range of the genus Dignomia Hall into the Famennian Stage, contrary to previous estimates of an Ordovician to mid-Devonian range. Abundant lingulid brachiopod valves are associated with plant and fish remains within a laterally extensive, up to nine-meter-thick mudstone. Shell compressions are concentrated in patches within the upper portion of the horizon and are preserved parallel to bedding as well as perpendicular and oblique to bedding (apparently in situ). Plant fossils exhibit a greater degree of transport than in other studied Witpoort Formation mudrock layers (Coombs Hill and Waterloo Farm). The monospecifity of the shelly invertebrate fauna derives from environmental stress and lingulid brachiopods' unusual ability to tolerate fluctuating environmental conditions, including salinity, oxygen levels, and temperature. Ichnofauna within the deposit include, among others, Nereites and ‘Spirophyton’, suggesting opportunistic colonization of the substrate by deposit feeding invertebrates in a marine-dominated backshore setting. The Late Devonian was a time of global environmental disequilibrium, characterized by periodic flooding of continental margins and massive biotic overturn. Ongoing research into eustatic and environmental changes recorded within the Witpoort Formation uniquely provide insights into high latitude conditions during the Famennian.