Taphonomic, paleobiological, and paleoecological analyses have been conducted on a rich bone accumulation on Poggio Rosso in the Upper Valdarno basin (Tuscany, central Italy). Fossil mammalian remains from the Upper Valdarno, including the type specimens of important European Villafranchian taxa, have been collected from the Renaissance through the present, mostly with little concern for stratigraphy. Because the specimens, elements, skeletal parts, and even entire skeletons generally were found in isolation, a great deal of information on taxonomic co-existence, population dynamics, proportions of limb segments, and other metrics, was precluded. The discovery of the latest Pliocene Poggio Rosso site, where bones are preserved in a sandy floodplain bed, has provided opportunity to rectify these omissions. The bone accumulation largely consists of limb bones (mostly articulated) and skulls, many of which are associated with mandibles. Axial skeletal elements are strongly underrepresented. Several bones were bimodally oriented. The specimens are slightly weathered, and have common bite and gnaw marks. Some coprolites also occur. Because of its complex genesis, Poggio Rosso does not match conventional taphonomic categories. The bimodal arrangement of the elements and sedimentologic evidence indicate that an unconfined flood flow contributed to the assemblage's final arrangement. The role of carnivores, however, was far more substantial. The occurrence of skulls, limb bones, bite and gnaw marks, and coprolites and the comparative rarity of axial skeletal parts, attest to the presence and activity of carnivores—hyenas in particular, represented in the Upper Valdarno by Pachycrocuta brevirostris. Poggio Rosso thus is a sort of den with characteristics of a kill site. The proportionally high amount of carnivore remains, especially skulls, intimates that the hyaenids might have had cubs at the time. General wastage and incompletely consumed carcasses indicate surplus killing as well as actively transported prey elements; all of which, along with specific paleobiological speculations, suggest that the bone accumulation was formed by a cooperative group of hyenas preying on debilitated game populations in a moment of great environmental stress (possibly a period of drought). Sedimentological and palynological analyses confirm arid environmental conditions and reveal that the bones accumulated over a short time. Moreover, these same data indicate that the changes in the floodplain occurred during a major climatic fluctuation, with an increasingly arid phase that ended close to the fossil-bearing layer, which was then followed by wetter conditions.