| Summary: | Several aspects of the fascinating evolutionary history of toothed and baleen whales (Cetacea) are still to be clarified due to the fragmentation and discontinuity (in space and time) of the fossil record. Here we open a window on the past, describing a part of the extraordinary cetacean fossil assemblage deposited in a restricted interval of time (19−18 Ma) in the Chilcatay Formation (Peru). All the fossils here examined belong to the Platanistoidea clade as here redefined, a toothed whale group nowadays represented only by the Asian river dolphin <i>Platanista gangetica</i>. Two new genera and species, the hyper-longirostrine <i>Ensidelphis riveroi</i> and the squalodelphinid <i>Furcacetus flexirostrum</i>, are described together with new material referred to the squalodelphinid <i>Notocetus vanbenedeni</i> and fragmentary remains showing affinities with the platanistid <i>Araeodelphis</i>. Our cladistic analysis defines the new clade Platanidelphidi, sister-group to Allodelphinidae and including <i>E. riveroi</i> and the clade Squalodelphinidae + Platanistidae. The fossils here examined further confirm the high diversity and disparity of platanistoids during the early Miocene. Finally, morphofunctional considerations on the entire platanistoid assemblage of the Chilcatay Formation suggest a high trophic partitioning of this peculiar cetacean paleocommunity.
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