Résumé
In northwestern Europe, continental records of Pleistocene interglacials are well preserved in fluvial sequences and particularly within calcareous tufas that formed at the top of the series. They are the sole deposits recording the full optima. Tufas contain rich malacological communities that allow the detailed palaeoenvironmental history of past interglacials to be reconstructed. Several tufas have been investigated in the study area, which have led to the recognition of the « Lyrodiscus fauna » as a characteristic biostratigraphical signature of tufa sequences of MIS 11 age in northwestern Europe. Among these, the site at La Celle in the Seine valley (France) is the only one where tufa has accumulated a substantial thickness (almost 9 m) in which the complete development of the Lyrodiscus fauna is represented, yielding a detailed record of the forest expansion during the MIS 11 interglacial. The La Celle succession therefore provides a regional framework into which other sequences can be set, shedding lights on various aspects of palaeoenvironmental dynamics, biostratigraphy, palaeogeography and palaeobiodiversity. Combined malacological and isotopic records from La Celle demonstrate a close correlation between increasing temperatures and the development of forest land snail communities. The palaeoe-cological framework of forest extension inferred from the malacological succession of La Celle allows the shorter successions from northern France and southeastern England to be located within the maximum development of the humid forest biotope during MIS 11c. The order of occurrence of forest snails indicates that Atlantic and Central European refugia are the original sources of recolonizations for northwestern European sites. Strict Mediterranean species reach the area only at the MIS 11c climatic optimum. Many species present during MIS 11c no longer live in this part of Europe, indicating a northward shift in the location of the modern alpine biodiversity hotspot.