The extinct baleen whale fauna from the Miocene-Pliocene of Belgium and the diagnostic cetacean ear bones

Significant advances of our understanding of the evolutionary history of baleen whales have been gained by the recent focus on the extinct baleen bearing groups. However, a historically important collection from the middle Miocene to Pliocene of Antwerp, Belgium, comprising a third of all described species of early baleen-bearing mysticetes, has proven difficult to include in modern studies. These species are surrounded by taxonomic confusion because they were based on non-comparable composite material. The Belgian species are revised here with a designation of comparable lectotypes. The best-represented comparable element is the periotic bone that surrounds the inner ear. The periotic is shown to have significant diagnostic value, like that of teeth in many extinct mammals. Of the Belgian mysticetes, the genera Amphicetus and Herpetocetus are retained. Plesiocetus is retained and referred to Balaenopteridae, while Idiocetus is tentatively referred to Balaenidae. Burtinopsis, Heterocetus, Isocetus and Mesocetus are considered nomen dubia. Interestingly, all the major extinct mysticete groups known from the west-North Atlantic are represented in the Belgian fauna, although with different relative diversities, while no species seem to fall outside of these families. Diorocetidae, Aglaocetidae and Pelocetidae were thus well established with an east-west North Atlantic distribution by the Middle Miocene, indicating that these families originated in the Oligocene to Early Miocene. Cetotheriidae (sensu stricto) is represented by only one species that is probably Pliocene in age, suggesting a later dispersal to the east-North Atlantic region.