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.

Deep-sea Ordovician lingulide brachiopods and their associated burrows suggest an early colonization of proximal turbidite systems

“…However, the study area was in a high-latitude position 76 and at a time of global cooling 121 , 122 , suggesting that lingulides were at once able to adapt to low water temperatures. Similarly, lingulide body and trace fossils were found in Upper Devonian lagoonal deposits 47 and Late Carboniferous deltaic environments 41 at high paleogeographic latitudes, supporting the idea of a greater ability to tolerate low temperatures and inhabit high latitudes earlier in their evolutionary history.…”

“…Lingulide burrows are known as Lingulichnus 27 – 29 , a trace fossil reflecting dwelling, equilibrium, and escape behaviours depending on the sedimentation rate and hydrodynamic energy of depositional events 30 – 32 . Lingulichnus is a trace fossil typical of ancient tidal flat, deltaic, estuarine, and shoreface successions, with some specimens recorded in offshore and lagoonal deposits 14 , 19 , 30 , 33 – 47 . Until now, Lingulichnus has never been found in ancient deep-marine successions 11 .…”