Taphonomy, palaeoecology and taxonomy of an ophiuroid-stylophoran obrution deposit from the Lower Devonian Bokkeveld Group, South Africa Town Cape of by Mhairi Reid Supervisors: Dr Wendy Taylor and Dr Emese M. Bordy DissertationUniversity presented for the degree of Master of Science in Geology in the Department of Geological Sciences, University of Cape Town January 2017 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town DECLARATION I, Mhairi Reid, hereby declare that the work on which this dissertation/thesis is based is my original work (except where acknowledgements indicate otherwise) and that neither the whole work nor any part of it has been, is being, or is to be submitted for another degree in this or any other university. Signature removed 18 January 2017 i ACKNOWLEDGEMENTS First of all, I would like to express my deepest gratitude to my supervisors for this project, Dr Wendy Taylor and Dr Emese Bordy, whose unwavering guidance, advice, meticulous feedback, support and time, has been invaluable. This research project would not have been possible without funding from the NRF/DST Centre of Excellence in Palaeosciences (CoE- Pal), thank you for the generous funding. My gratitude is extended to Dr. Aaron Hunter for being so kind as to let me be a guest in his home in Australia and for teaching me the intricacies of Paleozoic ophiuroid taxonomy. As well as, Bertrand Lefebvre for his detailed discussion, thoughts and help on the stylophorans. I would like to thank Claire Browning form the Council of Geoscience for helping me whenever I visited and viewed the fossil collections housed there. Further, I would like to thank Dr. Roger Smith for allowing me to use the fossil preparation tools from the Iziko South African Museum. Thank you to the Robles family for use of their farm Karbonaatjies during fieldwork. I would like to thank Cameron Penn-Clarke for his many years of support, helpful discussions and his never ending passion for the fossils of the Bokkeveld Group. I would like to thank my fantastic team of excavators Bianca Harrison, Claire Browning and Tanya Deyer for all their support. And finally, Michael for wielding the pickaxe like a champion and for being so caring and patient during this time. ii ABSTRACT The Lower Devonian Voorstehoek Formation is a fossil-rich, siliciclastic unit (Ceres Subgroup, Bokkeveld Group, Cape Supergroup) in South Africa. This Emsian unit contains a highly endemic benthic fossil biota characteristic of the cool to cold water Malvinokaffric Realm of southwestern Gondwana. The palaeontological and sedimentological investigations 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. A relatively diverse, ophiuroid–stylophoran assemblage, well-preserved in the Karbonaatjies obrution bed, was excavated at the study site in the Hex River Pass, Western Cape. In this study the taphonomy, taxonomy and the palaeoautecology of Palaeozoic ophiuroids and stylophorans was investigated using micro CT scans. Over 60 samples were scanned, manually segmented and stitched together to create a virtual 3D model of a portion of the Karbonaatjies obrution bed. This method allowed for the determination of the degree of fossil articulation, fossil orientation and faunal counts, without damaging the delicate echinoderm fossils. Furthermore, the ability to digitally analyse the fossil-rich bed has revealed an echinoderm assemblage composed of over 700 articulated ophiuroids dominated by a proposed new genus and species Gamiroaster tempestatis, over 145 articulated mitrate stylophorans Paranacystis cf. petrii Caster, 1954 and eight Placocystella africana (Reed, 1925). Taphonomic analysis of this ophiuroid–stylophoran assemblage indicates this obrution deposit formed due to rapid burial that smothered a potentially gregarious community during a single storm event. Additionally, the admixture of skeletal debris and intact echinoderms present in the Karbonaatjies obrution bed reflects a complex history with significant time-averaging. This unique assemblage provides a taphonomic window into the marine ecosystems of the Early Devonian, including the structure of an unusual, echinoderm-dominated benthic community that forms part of a much wider fossil biota from the Falkland Islands and Precordillera of Argentina, which formed part of SW Gondwana. iii TAXONOMIC DISCLAIMER This thesis is not deemed nor intended to be a valid publication for the naming of new taxa as stipulated in the International Code of Zoological Nomenclature, Article 8.2. iv Table of Contents Declaration ............................................................................................................................................... i Acknowledgments ................................................................................................................................... ii Abstract .................................................................................................................................................. iii 1. Introduction ......................................................................................................................................... 1 1.1 Geology of the Bokkeveld Group ................................................................................................ 6 1.1.1 Geological context ................................................................................................................. 6 1.1.2 Depositional environment ................................................................................................... 10 1.1.3 Previous study on the depositional environment of the Karbonaatjies obrution bed .......... 11 1.2 Palaeontology of the Bokkeveld Group ..................................................................................... 12 1.2.1 Malvinokaffric Realm Biota ................................................................................................ 14 1.2.2 Paleoecology of the Voorstehoek Formation ....................................................................... 15 2. Materials and methods ...................................................................................................................... 17 2.1 Excavation and preparation ........................................................................................................ 17 2.3 Digital analysis ........................................................................................................................... 19 1.2.1 Malvinokaffric Realm Biota ................................................................................................ 14 1.2.2 Paleoecology of the Voorstehoek Formation ....................................................................... 15 2.4 Taphonomic analysis .................................................................................................................. 20 2.4.1 Echinoderms ........................................................................................................................ 20 2.4.2 Shells .................................................................................................................................... 24 2.5 Taxonomic analysis ..................................................................................................................... 26 3. Taphonomy and palaeoenvironment ................................................................................................. 27 3.1 Observations ............................................................................................................................... 27 3.2 Echinoderm taphonomy results ................................................................................................... 29 3.3 Population structure and dynamics ............................................................................................. 41 3.3.1 Density ................................................................................................................................. 41 3.3.2 Size distribution ................................................................................................................... 43 3.4 Interpretation of echinoderm taphonomy .................................................................................... 43 3.4.1 Obrution by storm deposition .............................................................................................. 43 3.4.2 Autecology of ophiuroids and stylophorans ........................................................................ 45 Gregarious behavior of ophiuroids and stylophorans ............................................................. 45 Loss of arms by autotomy vs. in storms ................................................................................... 48 Escaping ophiuroids and distressed stylophorans ................................................................... 50 Mode of life .............................................................................................................................. 51 Population structure ...............................................................................................................