TAPHONOMY AND PALEOECOLOGY OF AN OPHIUROID-STYLOPHORAN OBRUTION DEPOSIT FROM THE LOWER DEVONIAN BOKKEVELD GROUP, SOUTH AFRICA

The Lower Devonian (Emsian) Voorstehoek Formation is a siliciclastic unit within the Ceres Subgroup of the Bokkeveld Group in South Africa that comprises mudstones and siltstones. This fossiliferous unit contains a highly endemic marine benthic biota of the cool to cold water Malvinokaffric Realm of southwestern Gondwana. The paleontological and sedimentological features of the Voorstehoek Formation suggest that deposition took place in a shallow marine environment within the storm-influenced, proximal part of an offshore transition zone. In 2014, a 5cm thick “obrution bed” or rapid burial deposit composed of siltstone/fine grained sandstone, was excavated at the study site in the Hex River Pass, Western Cape. Through the use of micro CT scans, it was found to contain an assemblage of complete ophiuroids (brittle stars) associated with rare stylophoran (free-living benthic) echinoderms. Taphonomic analysis of this ophiuroid–stylophoran assemblage indicates this obrution deposit formed due to rapid burial that smothered both attached and vagrant taxa of the benthic community during a high-energy storm event. The occurrence of mobile ophiuroids throughout the bed suggests failed escape attempts by mobile organisms. This assemblage provides a taphonomic window into the marine ecosystems of the Early Devonian, including the structure of an unusual, echinoderm dominated benthic community from SW Gondwana.

The current project uses micro CT scanning on a larger scale to model individual samples of the bed extracted in the field. Over 60 samples were scanned, manually segmented and then stitched together to create a virtual 3D model of a portion of the obrution bed. This method allowed the detailed taphonomic analysis of the bed, including the determination of the degree of fossil articulation, fossil orientation and faunal counts, without damaging the delicate echinoderm fossils and primary sedimentary features. Furthermore, the ability to digitally dissect the fossil-rich bed has revealed extremely small complete stylophorans that otherwise would have been missed or destroyed using traditional, mechanical fossil preparation techniques.