Reconstructing the evolution of cave systems as a key to understanding the taphonomy of fossil accumulations: The case of Urşilor Cave (Western Carpathians, Romania)

The Urşilor Cave (NW Romania) is a famous cave bear paleontological site hosting an important Late Pleistocene faunal assemblage and subject to systematic excavation works. To better understand the origin of fossil assemblages, the sedimentary history of the cave must be reconstructed. We conducted a series of investigations on various cave deposits which included sedimentology and grain-size analyses, U-series dating of speleothems, OSL dating of sediments, and AMS radiocarbon dating of fossil remains. The results allowed for the identification of several major chronological controls for the evolution of the cave during the last 300,000 years. Five evolutionary stages or key-moments were dated, and a tentative speleogenetic scenario is presented. The combined numerical dates and sedimentological study show that the evolution of the cave was more complex than previously thought. In particular, the fossil accumulation was related to a succession of rapid flooding events at ∼47–40 ka. Alternating depositional and erosional phases have occurred since at least 210 ka complicating the sediment structures. The results suggest that the upper and lower levels of the cave may have been functioned occasionally as hydrologically separated karst systems and that the animal populations from what is now a single cave system may not necessarily be synchronous. This case study shows that reconstructing sedimentary history of a given cave is crucial for the correct understanding of its thanatocenosis.