Evolutionary trends in the Plio-Pleistocene ichthyofauna of the Mediterranean Basin : nature, timing and magnitude of the extinction events

Abstract Trends in standing diversity and turnover rates of Italian fish communities from the Late Pliocene to the Middle Pleistocene are analyzed on the base of a list of 197 taxa (represented by both ichthyoliths and otoliths) obtained through the study of 16 Italian fossil deposits with good chronostratigraphic resolution. Standing diversity shows a regular and positive trend, maintained by intense episodes of turnover, which have been recognized in Late Pliocene, Plio/Pleistocene boundary and Early/Middle Pleistocene boundary. On the contrary, an absolute stasis corresponds to the Early Pleistocene and an episode of extinction not balanced by originations characterizes the Middle Pleistocene. None of the observed episodes appear to be substantially selective: all environments are vulnerable, even if with different intensities, and the mechanisms of recharge seem to compensate the extinction. Evolutionary trends related to the biogeographic structure of the association are identified. In particular the most meaningful are: (1) the progressive loss of Indo-Pacific taxa; (2) the relative increase of endemic taxa; (3) the increase of Atlantic taxa; (4) the decline of cosmopolitan taxa. Among the causes responsible for these events, the influence of the Plio-Pleistocene climatic oscillations is important, even if the substantial non-selectivity of the events suggests the involvement of other mechanisms such as change of the circulation models and mechanisms of biotic interchange.