Anomalous facies and ancient faeces in the latest middle Cambrian of Sweden

Abstract

The middle Cambrian–Furongian transitional interval was a time of significant biotic and environmental changes. Strata of this age in Scania, southern Sweden, contain two interlayered biofacies, a normal one dominated by trilobites and an anomalous one dominated by phosphatocopines (small bivalved arthropods). In places these biofacies are separated by intervals barren of fossils. In a phosphatocopine facies without trilobites in the upper Agnostus pisiformis Zone at Andrarum we recovered scattered fossil aggregates with a homogeneous composition of tightly packed and stacked phosphatocopines. These aggregates are interpreted as coprolites produced by an undetermined predator, possibly the chaetognath-like protoconodont animal or some other soft-bodied metazoan. The so-called barren intervals of Scania are not necessarily barren of fossils, but only trilobites, brachiopods, and other skeletal elements with a calcium-carbonate composition. The phosphatocopine facies with coprolites in the uppermost part of the A. pisiformis Zone correlates with the trilobite mass extinction at the top of the Marjumiid Biomere in Laurentia and immediately prior to the onset of the Steptoean Positive Carbon Isotope Excursion (SPICE), inferring a global shift in the oceanic chemistry that in Scania favoured phosphatocopines over the more common, trilobite-dominated faunas.