The remarkable Middle Pliocene non-marine mollusc record from Ceresole d’Alba, Piedmont, north-west Italy: Biochronology, palaeobiogeography and palaeoecology supported by fossil plants

Abstract

A noteworthy, well-preserved non-marine mollusc fauna has been recorded from Middle Pliocene silty sediments at Ceresole d’Alba (Piedmont, NW Italy), a site described by F. Sacco in the 19th century. The fauna is characterised by rich assemblages of terrestrial and aquatic taxa showing a high degree of species diversity and the dominance of archaic land prosobranchs and pulmonates. A high rate of extinct taxa (at least 50%) and a good percentage of endemics are recognised. Many species are restricted to the Middle Pliocene and for some species, a Mio-Pliocene central-western European origin is noticed. The palaeoecological features of two distinct assemblages show dominant hygrophilous and thermophilous forest elements in the first one and prevailing freshwater taxa, testifying an environment of swamps or small water bodies, in the latter. A direct record of ancient vegetation has been provided by fruit and seed assemblages recovered from an organic layer and from two leaf assemblages sampled in the overlaying laminated muds. The fruit and seed assemblages clearly represent the record of a few azonal plant communities growing very close to the deposition site. On the ground of the ecological preferences of the modern plant analogues, both leaf and fruit assemblages indicate swamp communities poor in arboreal species. The percentage of extinct plant species is surely higher than 29% (nine species out of 31), and most probably close to 48% (15 species out of 31). The plant assemblages provide limited climatic indications, whereas warm-temperate and humid climate conditions are pointed out on the basis of modern preferences of several mollusc taxa. These climatic indications agree with those obtained from rich plant assemblages of neighbouring Pliocene sites. A correlation of the recorded palaeofauna and palaeoflora with the faunistic and floristic assemblages from the Middle Pliocene sediments of one of the most important non-marine palaeontological sites in the Piedmont Basin, the Villafranchian type-succession at Villafranca d’Asti (RDB quarry), is inferred.