Taphonomic analysis of microfossil bonebeds from Western Gondwana: a case study from the late Permian Corumbataí Formation (Paraná Basin, Brazil)

Storm deposits are common in geological records and encompass a variety of fossil assemblages affected by taphonomic processes that can biased fossil preservation. Here, we describe a microfossil bonebed to demonstrate the role played by certain filters in the final composition of storm-generated fossil accumulations. These layers are recurrent in the Paraná Basin and possibly in other localities of Gondwana, outcropping over 1000 km in the Permian units of the basin. Thirty samples were systematically collected from the state of São Paulo and their components were analysed. These samples consisted of clay matrix, coprolites, scales, and teeth of lower actinopterygian (Palaeonisciformes) and Xenacanthiformes fish. The taphonomic signatures demonstrated that the layer formed as a lag deposit in a distal aquatic environment. Two major taphonomic filters were recognised. The first filter selected bioclasts according to size and density, whereas the second filter preserved only the original phosphatic elements. The third and minor filter were related to breakage, which occured during reworking and diagenesis, respectively. All of these processes contributed to the generation of an assemblage with limited composition and species richness. Thus, distal tempestites might not be useful for estimating palaeodiversity, but may still offer some valuable information about intraspecies variation.