First record of Cyanobacteria in Cambrian Orsten deposits of Sweden

Abstract

The Swedish Cambrian ‘Orsten’-type fossil sites have yielded diverse secondarily phosphatized three-dimensionally preserved microfossils, mainly of arthropod affinities. Similar material has also been recorded from Canada, the UK, Poland, Siberia, China and Australia. Only one other non-arthropod group, the Cycloneuralia, is commonly reported from any of these sites, leading to the general assumption that ‘Orsten’-type preservation is largely restricted to animals with a chitin-containing cuticle. We describe here secondarily phosphatized, originally unmineralized, thread-shaped fossils etched out of Cambrian ‘Orsten’-type deposits from the Agnostus pisiformis Biozone of the Alum Shale Formation in Sweden. These fossils strikingly resemble specimens previously described from Precambrian deposits, with at least two different morphotaxa identified (Siphonophycus kestron Schopf and Oscillatoriopsis longa Timofeev & Hermann) as well as the modern Oscillatoria. This leads us to interpret the new fossils as unbranched, uniseriate filamentous cyanobacteria. Our morphological investigations, combined with morphometrics, allow grouping the specimens assigned to O. longa into two size classes, suggesting an even higher diversity within the ‘Orsten’ assemblages. The lack of cyanobacterial material in any sample younger than the A. pisiformis Biozone indicates that rather drastic changes occurred in the environment, that is, substrate conditions on the Alum Shale seafloor. This coincided with a significant change in the composition of the trilobite communities and onset of the globally recognized Steptoean Positive Isotope Carbon Excursion (SPICE) in Scandinavia.