Pleistocene debris-flow deposition of the hippopotamus-bearing Collecurti bonebed (Macerata, Central Italy): Taphonomic and paleoenvironmental analysis

An integrated taphonomic-sedimentological study provides insight into the origin, preservation biases and paleobiological significance of the latest Early/earliest Middle Pleistocene bonebed of Collecurti, near Colfiorito (Macerata, central Italy), which represents an important benchmark in the European biochronological scale. Sedimentological evidence indicates that the bonebed is hosted in a debris-flow deposit which was emplaced in a shallow lacustrine setting. Geologic and geomorphic controls on sedimentation in the intermontane Colfiorito Basin provided an ideal setting for the development of cohesive mass flows, due to high-relief and poorly integrated catchments along the basin margin, and the abundance of detrital fines yielded by Mesozoic sedimentary rocks of the local basement. The fossil assemblage includes 496 bones, with a minimum of 24 individuals representing 9 species and 8 genera, dominated by Hippopotamus antiquus. The hippopotamus remains include partially articulated carcass parts as well as closely associated elements. Few disarticulated elements, which are bones that can be more easily transported hydraulically, were involved in pre-debris-flow winnowing and sorting, and then entrained by the debris flows in nearby uplands, transported a short distance down into a shallow pond or lake margin, where the flow buried hippopotamus carcasses that had accumulated in the pond/lake. The hippopotamus carcasses were in an advanced state of decomposition when the debris flow entrained them. Their bones show no evidence of weathering, trampling, scavenging, sorting or transportation prior to debris-flow reworking, which indicates that the animals were probably lying on the bottom of the pond where they lived, or were already partially buried in mud. Previous palynological investigations have shown that the strata of interest at Collecurti were deposited during a climatic trend toward ever increasing cooler conditions. The low richness of the fauna confirms long exposure to harsh climatic and environmental conditions. Two are the plausible causes of the death of the Collecurti animals, i.e., severe cold and/or drought. Taphonomic attributes rule out drought, indicating that the assemblage accumulated in a short time as the result of a mass mortality event, possibly due to a cold snap.There are no other reported examples of Quaternary debris-flow-hosted bonebeds in Europe, probably because this agent of taphonomic accumulation is rarely investigated in high-energy paleogeomorphic settings. It is likely that more such sedimentary events took place in the area, and that similar fossil sites await to be discovered.