Isolated sponge spicules from the late Cambrian Alum Shale Formation (‘Orsten’ nodules) of Sweden

The late Cambrian (Furongian) Alum Shale Formation of Sweden is famous for its diverse macrofauna (mostly trilobites and brachiopods) and microfauna (‘Orsten’ fauna). Until now, no remains of sponges were known from the Alum Shale biota. We report here the presence of numerous isolated sponge spicules from several limestone nodules found across the late Cambrian Alum Shale Formation. The collection mostly comprises unusual obese pentactine and hexactine spicules that occur in close association with modified acanthose pentactines and hexactines, as well as relatively regular ones. The spicules exhibit a wide range of variation, but their diversity appears rather low, belonging to at least three hexactinellid and one possible stem sponge taxon. One of the hexactinellid species seems closely related to the sponge Rigbykia ruttneri. The occurrence of sponges in the Alum Shale biota, associated with other sessile organisms, suggests that the substrate of the Alum Shale Sea might have been firm enough to allow sessile organisms to invade and colonize the sea floor when the oxygen supply was sufficient. These conditions in the Alum Shale Sea appear to have been spatially and temporally restricted, thus limiting the geographic distribution of sessile organisms. However, until a more complete overview of the Alum Shale biotas can be drawn, uncertainties remain regarding the substrate properties of the Alum Shale Sea.