A mixed ophiuroid–stylophoran assemblage (Echinodermata) from the Middle Ordovician (Llandeilian) of western Brittany, France
Published:January 01, 2007
Aaron W. Hunter, Bertrand Lefebvre, Serge Régnault, Philippe Roussel, Roland Claverie, 2007. "A mixed ophiuroid–stylophoran assemblage (Echinodermata) from the Middle Ordovician (Llandeilian) of western Brittany, France", Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls, J. Javier Álvaro, Markus Aretz, Frédéric Boulvain, Axel Munnecke, Daniel Vachard, Emmanuelle Vennin
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In the abandoned slate quarry of Guernanic, Gourin (Morbihan, France), a single horizon (Upper Member of the Schistes de Postolonnec Formation) has yielded an exquisitely preserved Llandeilian (Middle Ordovician) echinoderm assemblage composed of the ophiuroid Taeniaster armoricanus sp. nov. and the mitrate Mitrocystella incipiens. These two groups of echinoderms represent the first fossils formally described from the Middle Ordovician of the Gourin area. The brittlestar T. armoricanus sp. nov. is the third and oldest ophiuroid reported so far in the Palaeozoic of the Armorican Massif. The mitrate Mitrocystella is also described for the first time from western Brittany. Taphonomic features of this ophiuroid–stylophoran aggregation suggest that it probably corresponds to the rapid burial of a life assemblage in an otherwise quiet and moderately deep setting (shelf) below, but close to, storm wave base. This echinoderm association represents the oldest evidence for a gregarious mode of life for ophiuroids, as well as the oldest indisputable example of a mixed ophiuroid–stylophoran meadow.
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Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls
The geological record contains a fascinating diversity of reefs and shell accumulations. As my other biosedimentary structures, their facies characterization requires careful observation at outcrop and sample scale, and in thin-section to provide information about the global geometries, fabrics and textures respectively.
This collection of papers encompasses the breadth of sedimentary geometries and facies displayed by Palaeozoic reefs, shell accumulations, and transitional composite deposits. The definition of reefs and shell concentrations has given rise to variations in nomenclature. The papers in this volume cover specific problems regarding the nomenclature and facies characterization of reefs, shell accumulations and transitional composite deposits. However, rather than attempt a complete revision of terms, the authors have touched on some of the important issues at this stage of development in the field: the main climatic, environmental and evolutionary factors that controlled the Palaeozoic development of shell accumulation and reefs.