Présence d'Hélicelles (Helicellinae) dansle Pliocène du midi de la France nouvelles données sur la dispersion du groupe au Pléistocène inférieur implications paléoclimatiques

Abstract

Three Pliocene-Pleistocene sites of southern France recently yielded new elements concerned with the European0309 history of the Helicellids (Helicidae, Helicellinae) group. At Grenouillet (Alpes-de-Haute-Provence) the occurrence of Candidula unifasciata, in an upper Pliocene paleosoil, estimated at 3.2 Myr from the associated rodents, from the Valensole series, is the oldest known marker. This steppic species, which appears in moderate number at the bottom of the paleosoil, together with a traditional Neogene malacological assemblage, still of forest type, becomes abundant at the top. Such vertical distribution of the species is significant of the environment drying up and seems to correspond with a climatic aridification phase. This climatic occurrence at −3.2 Myr, already recognised by palynology, would combine with the general dynamics of xericity expanding towards the North of the Mediterranean Basin during late Neogene. The two lower Pleistocene malacological sites of La Viste (Bouches-du-Rhône) and Valensolette (Alpes de Haute-Provence) are separated by a long time gap. The former is referred to an early phase of lower Pleistocene; the latter, which also yielded a rodent fauna, is dated to about 0.7–1.0 Myr. Both malacofaunas are typically Quaternary; they no longer include any Neogene element, and however, at La Viste, one of the species (Trochoidea “trochoides”) shows archaic characteristics. These associations are related to clearly open environments and reveal conditions of climatic degradation, modulated on the two sites according to their location and age. They reflect two different episodes of the general cooling, which has occurred since −2,7 Myr with the succession of glacial-interglacial cycles.