Orbisiana linearis from the early Ediacaran Lantian Formation of South China and its taphonomic and ecological implications.

Orbisiana is an Ediacaran fossil characterized by linearly arranged circular or cylindrical units. The genus was first described from the late Ediacaran Redkino Series in the Moscow Syncline of Russia where it is preserved as pyritic molds in siltstone. Here we report Orbisiana linearis , originally described as Seirisphaera lineare , from black shale of the early Ediacaran Lantian Formation in South China. Unlike the type species ( O. simplex ), O. linearis is preserved as carbonaceous compressions on the bedding surface, with rare specimens preserved three-dimensionally, allowing morphological characterization using X-ray microCT techniques. Morphological, paleoecological and taphonomic analysis suggests that O. linearis was a chain-like modular organism characterized by uniserially arranged, millimeter-sized, and tangentially contacting circular or cylindrical units. Ontogenetic growth of O. linearis was likely achieved through both accretionary addition of new modules and inflational expansion of existing units, and it appears that inflational growth was determinate. Although its phylogenetic affinity remains uncertain, O. linearis was likely a procumbent epibenthic or semi-endobenthic organism that lived on the seawater-sediment interface or partly buried in sediments. O. linearis in the Lantian Formation was preserved in situ, although some specimens are disarticulated and many are tectonically deformed. Orbisiana joins a growing list of Ediacaran genera that have a remarkable range of stratigraphic, environmental, ecological, and taphonomic distribution. [ABSTRACT FROM AUTHOR]