Tracing the origins of the modern Mediterranean baleen whale fauna: hints from the fossil record

The current cetacean fauna of the Mediterranean Sea substantially represents a sub-set of the North Atlantic one. In particular, a single mysticete species (the fin whale Balaenoptera physalus) is commonly observed in the Mediterranean Sea, where it forms a well-distinct residential sub-population, whereas a few other forms of baleen whales are known from the same basin only for a small number of occasional and often dubious sightings. On the contrary, the Italian record of fossil mysticetes from the Pliocene epoch (ca. 5.3–2.6 millions of years ago) depicts a strongly different scenario, characterized by a great number of species, belonging to at least three families (Balaenidae, Balaenopteridae, and Eschrichtiidae), exploring a wide spectrum of body sizes and ecotrophic habits. The transition from the highly diversified Pliocene Mediterranean mysticete assemblage to the strongly simplified modern fauna has been substantially unexplored for a long time, mainly because of the scarcity of fossil-bearing marine deposits of Quaternary age (from ca. 2.6 millions of years ago onward), a time span which features the final establishment of permanent glacial conditions at the high latitudes of the Northern Hemisphere and consequent low mean values of sea level. Now however, investigations focusing on the few Quaternary fossil specimens of baleen whales and on the more abundant fossil record of whale barnacles (Coronula and allied forms) are highlighting a surprisingly complex Quaternary history for the Mediterranean mysticetes, featuring time intervals characterized by the occurrence of balaenid and balaenopterid communal breeding grounds, as well as periods in which the pygmy right whale Caperea occurred in the waters off South Italy. Here we present a synthesis of the ongoing research on the Quaternary history of Mediterranean baleen whales and try to trace the origin of the current monotypic Mediterranean mysticete fauna in the light of major climatic, biooceanographic, and evolutionary trends.