Upper Miocene molluscs of Monti Livornesi (Tuscany, Italy): Biotic changes across environmental gradients

The upper Miocene mollusc assemblages of Monti Livornesi, used as a means to study the nature of Mediterranean benthic communities at the edge of the Messinian salinity crisis, are framed in a high-resolution stratigraphic scheme and quantitatively approached by the study of historical museum collections and modern samples. Instead of a single assemblage from either Tortonian or Messinian age, as previously thought, this fauna comes from three consecutive shallowing-upward depositional sequences bounded by regional unconformities, the Luppiano (upper Tortonian-lowermost Messinian), Rosignano and Raquese units (early Messinian). Facies analysis, taphonomy and quantitative paleoecology show that the Luppiano assemblage is characterised by aragonitic species from a eutrophic brackish-water shallow marine muddy bottom, the Rosignano assemblage by calcitic species from an oligotrophic coarse-grained seafloor close to a coral reef and the Raquese assemblage from an open-marine muddy bottom. Published comparisons between Miocene and Pliocene faunal lists should be considered only crude estimates of faunal change until more is known about the distribution of species along paleoenvironmental gradients. The analysis of Miocene and Pliocene abundance data allows us to frame the Monti Livornesi molluscs along carbonate-siliciclastic and water depth gradients and to reach a better understanding of the effects of salinity crisis on the Mediterranean biota.