ORSTEN-TYPE PRESERVATION: THE CASE OF THE MIDDLE-UPPER CAMBRIAN OF SWEDEN

The Alum Shale Formation of Sweden is dominated by uniform fine-grained blackish mudstone and shale deposits (Series 3 to Furongian Series).They enclose early diagenetic limestone concretions locally called ‘Orsten’. These were formed in a sulphur-rich, possibly originally dysoxic and stagnant epicontinental sea. Etching these ‘Orsten’ nodules yielded assemblages of three-dimensionally preserved, secondarily phosphatized microfossils of 0,1-2 mm in size. ‘Orsten’-type preservation results from an incrustation and also an impregnation of the surface of the organism by calcium phosphate during early diagenesis. Subsequent embedding in a fine limestone matrix prevented the fossil from any compaction. In the case of arthropods all body parts, such as appendages, and surface details, such as hairs and sensilla, are present in their original topology. Microbial processes might have played a role in the replication and authigenic mineralization of soft parts and several modes of phosphatization have been identified reflecting the degree of microbial influence on the mineralization process, such as microbial, substrate and intermediate microfabrics, but this is still unclear. The ‘Orsten’ material contains thousands of specimens of arthropods with preserved soft parts ranging from lobopods, pentastomids, and chelicerates to crustaceans of various evolutionary levels. Therefore it is not surprising that also larval stages have been found, besides miniaturized adults. Remains of other organisms indicate that, besides these, most likely, components of an early ‘meiofauna’ community, the Alum Shale Sea was inhabited by a rich benthic community including, e.g., trilobites, conodonts, cyanobacteria, sponges, echinoderms, conulariids and other small shelly-like forms. The biota was apparently highly diversified and has, at least, a comparable species richness as that of the other major Cambrian lagerstätten such as Chengjiang and Burgess Shale Biotas. We review the current but still limited knowledge about alum shales and ‘Orsten’ preservation and taphonomic aspects and discuss it against our observations on the large material at hand.