Taphonomy of a Panopea Ménard de la Groye, 1807 shell bed from the Pisco Formation (Miocene, Peru)

Invertebrate taphonomy can provide significant information about the post-mortem processes that affected the fossil record. In the East Pisco Basin of southern Peru, a storm-generated Panopea-rich shell bed was found in the upper Miocene strata of the Pisco Formation, hinting at a peculiar biostratinomic and diagenetic history. This bed is abundant in internal molds of invertebrates. The specimens of the deep infaunal Panopea sp. occur besides representatives of shallow infaunal species (Trachycardium sp. and Dosinia ponderosa) and balanid barnacles, which are sessile encrusters. Furthermore, Panopea specimens host compound molds witnessing an abundant encrusting fauna that includes serpulids, ?foraminifera, bryozoans, and barnacles that colonized the inner side of the valves before their final burial. Biostratinomic data provide compelling evidence that short-term, storm-related processes played a role in initially concentrating shells by scouring the seafloor through sediment winnowing and exhumation of live animals, resulting in a sedimentologic concentration of well-preserved shells. After the exhumation and death of the bivalves, the inner surfaces of the articulated Panopea shells, representing hard-substrate, sheltered environments in an otherwise unstable sandy seafloor (i.e., “benthic islands”), were colonized by different encrusting organisms. Following the final burial, dolomite precipitated, cementing the sediment infill of the valves. Lastly, a decrease of pH occurred at the sulfate reduction-methanogenesis boundary, inducing the dissolution of the shell carbonate.