Internal morphology and taphonomic history of the Neoproterozoic vase-shaped microfossils from the Visings Group, Sweden

The morphology and mineral composition of the vase-shaped microfossils (VSMs) from phosphatic nodules of the upper Visingsö Group in the Lake Vättern area have been studied using confocal and transmitted light microscopy along with the EDX analytical technique. The taphonomic history of the microfossils and the internal anatomy of the original organism were reconstructed. An early diagenetic precipitation of phosphate accounts for the formation of phosphatic surfaces replicating the external and internal morphology of the vase-shaped organism. The vase-shaped vesicle had an internal compartment containing a reproductive cell or cyst. The presence of these structures indicates that the vesicle does not in itself represent an encystment stage of a protist as previously suggested by some authors. Hypotheses on the biological affinities of the VSMs are reviewed. The VSMs global record is considered in the palaeogeographic and palaeoenvironmetal contexts, suggesting that the vase-shaped microfossils were produced by stenothermal planktonic organisms, thriving in shallow marine seas in tropical to temperate climatic zones.