Microspherules from the lowermost Ordovician in Scania, Sweden : affinity and taphonomy

Abstract: Microscopic spherules, 0.1-0.5 mm in diameter, from the lowermost Ordovician Rhabdinopora flabelliforme parabola graptolite Zone of the Alum Shale Formation in Södra Sandby, Scania, southern Sweden, have been investigated. The specimens have been studied by light and scanning electron microscopy in order to assess their affin-ity, taphonomic history, and possible relation to metazoan embryos. Different mor-phological types could be identified: specimens with a surface of pyrite crystals, black specimens partly enclosed in a pyrite crystal crust, black specimens with a smooth surface with lobes and grooves and also black specimens with a rougher surface. Ele-mental mapping confirms that the black specimens contain apatite, however, the smooth ones most likely have a surface layer consisting mainly of carbon. The condi-tions for phosphatisation through degradation of organic material and also pyritisation were favourable in the anaerobic sediments that formed the Alum Shale Formation, however, the formation of carbon films are more difficult to explain. The black spher-ules could be the result of a chemical process forming a phosphate peloid or grain, or by degradation and concomitant phosphatisation of fecal pellets or metazoan embryos. Especially those specimens with lobes and grooves resemble fossil embryos previously described from the Cambrian and Ordovician. Further studies of their internal structures are needed to confirm a metazoan affinity, however, their outer morphology strongly suggests such a relationship.