Maurizio Magi

The Late Pliocene Site of Poggio Rosso (Central Italy): Taphonomy and Paleoenvironment

Abstract 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 assemblages 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.

The Plio-Pleistocene fluvio-lacustrine Upper Valdarno Basin (central Italy): Stratigraphy and basin fill evolution

The Upper Valdarno Basin stands out from the Neogene-Quaternary basins of the Northern Apennines given its outstanding fossil mammal record, good quality of natural and artificial outcrops and remarkable chronological control on the basin-fill succession. The present paper aims to summarize the stratigraphic and sedimentological studies focused on the Upper Valdarno Basin during the past decades, and integrate them with recent investigations. The Upper Valdarno Basin is located about 35 km SE of Florence between the Chianti Mountains and the Pratomagno Ridge. It consists of a main asymmetric tectonic depression filled with 550 m of Plio-Pleistocene fluvio-lacustrine deposits (Upper Valdarno Basin s.s.) and a minor basin known as the Palazzolo sub-basin. The Upper Valdarno Basinfill is made of three unconformity-bounded units, named Castelnuovo dei Sabbioni (CSB), Montevarchi (VRC), Torrente Ciuffenna (UFF) synthems, whereas the Palazzolo sub-basin fill consists of the Fosso Salceto (OLC) and Torrente Ciuffenna (UFF) synthems. The Upper Valdarno Basin formed during Late Pliocene because of the tectonic damming of a northeastward flowing drainage. The early phase of basin development is recorded by the accumulation of fluvial gravels in vallive settings, whereas the definitive of these streams damming caused the development of lacustrine conditions at about 3.1 Ma. The accumulation of deltaic sand fed from the SW margin caused the lake filling and stopped the deposition of the CSB Synthem.Before 2.58 Ma, a tectonic phase caused uplift of the basin and partial erosion of the CSB deposits. Deposition of the lower part of the VRC Synthem occurred during a marked basin broadening and accumulation of alluvial fan successions, which were capped by aeolian-reworked alluvial sand deposited at about 2.5 Ma. At about 2.3 Ma, a new deformative phase caused further basin widening, erosion along the SW margin and development of a small lake inthe central areas. Deposition of the upper part of the Montevarchi Synthem started just after this tectonic phase and was characterized by development of axial fluvial drainage and marginal alluvial fans.During the Early Pleistocene (Olduvai Subchron, 1.95-1.78 Ma) a subsidence pulse promoted development of floodplain lakes and swamps in the axial part of the basin, where thick organic-rich mud were accumulated. During late Early Pleistocene the capture of the paleo-Arno River, which started to flow into the basin, caused the development of a marked unconformity. This unconformity was covered by fluvial and alluvial fan deposit in the axial part and along the margin respectively.

Depositional environments of the Plio-Pleistocene Upper Valdarno Basin (Tuscany, Italy)

The Upper Valdarno Basin is located about 35 km SE of Florence between the Chianti Mountains and the Pratomagno Ridge. The basin fill is made of four synthems named as Castelnuovo dei Sabbioni, Montevarchi, Fosso Salceto and Torrente Ciuffenna synthems. The Castelnuovo dei Sabbioni Synthem (Late Pliocene) consists of coarse-grained, stream gravels grading upwards into sheet-like, alluvial sand. These sands are overlain by a muddy lacustrine unit bearing, at its base, two well-developed lignitiferous seams accumulated in a coastal marsh setting. The lacustrine mud grades upwards into deltaic sand accumulated in a shallow-water delta under repeated lake-level oscillations. The Montevarchi Synthem (Late Pliocene to Early Pleistocene) consists of two portions separated by an unconformity surface passing basinward into a correlative conformity. The lower portion of the Montevarchi Synthem is made of alluvial fan gravel and sand passing upwards into fluvio-aeolian sandsheet deposits, consisting of aeolian-reworked, alluvial sand bearing isolated channels. Fluvio-aeolian sandsheet deposits are covered by mollusc-rich, alluvial sand which makes lateral transition into lacustrine muddy deposits. The upper portion of the Montevarchi Synthem consists of fluvial and alluvial fan deposits. Fluvial deposits occupy the axial part of the basin and are referred to sandy channels wandering through a muddy floodplain hosting shallow lakes and swamps. Alluvial fan deposits occur along the basin margins and consist of proximal gravels grading downfan into gravelly sand and a variety of sandy facies. Floodplain lakes deposits are well-developed in the middle part of the upper Montevarchi Synthem and in the Palazzolo sub-basin (Fosso Salceto Synthem), where they are overlain by alluvial-fan gravels. The Torrente Ciuffenna Synthem (Early to Middle Pleistocene) consists of fluvial sediments in axial part of the basin and alluvial fans deposits along the basin margins. The axial fluvial deposits were accumulated by the paleoArno River and consist of gravel and overlying sand. The basal gravels were deposited by low-sinuosity channels, whereas sandy deposits were formed by moderate to high-sinuous channels. The alluvial fan deposits consist of proximal gravels passing downfan into gravelly sand and sandy facies.