Middle Pleistocene molluscan fauna from the Valle Giumentina (Abruzzo, Central Italy): Palaeoenvironmental, biostratigraphical and biogeographical implications
Résumé
Palaeoenvironmental reconstructions through the Middle Pleistocene sequence in the Valle Giumentina, located in the Abruzzo region of Central Italy, are discussed. The sampled sequence is 16 m thick and includes nine levels with Lower Palaeolithic industries. The lithostratigraphy shows fluvio-glacial sediments with interbedded colluvial deposits and volcanic tephras. Fine sediments are composed of calcareous sands and silts, most of them yielding well-preserved mollusc shells. Forty-five samples, at 10 cm resolution, yielded 45 taxa, which could be assigned to four biozones. The oldest (biozone VGM1) indicate an open environment, which becomes a closed forest landscape developing under temperate conditions (VGM2). After a decline of thermophilous species (VGM3), mollusc assemblages indicate a dry
open environment typical of a glacial period (VGM4). All prehistoric occupation horizons occur during stable environmental episodes but under both temperate and cold climatic conditions. The molluscan succession is allocated to the Middle Pleistocene on the basis of the occurrence of Jaminia malatestae, a well-known Italian species, now extinct, as well as the similarity of the molluscan record to that of Case Picconetto, a well-dated site nearby. This attribution is supported by three tephra layers dated by 40Ar/39Ar at 556 ± 6, 531 ± 5 and 456 ± 2 ka. The sequence is correlated with marine isotopic stages 14, 13 and 12. Four species of land snail (Azeca goodalli, Ruthenica filograna, Pagodulina pagodula, Nesovitrea hammonis) occur beyond their modern range and are therefore of biogeographical interest. These snails are western and central European in origin and their presence within the Valle Giumentina deposits highlight a north-south gradient of colonization during Pleistocene interglacial periods.