The oldest reported fossil record of Delphinidae is from the late Miocene (11 Ma) of California. Reliable Miocene fossil delphinids, however, are few. Eodelphis kabatensis from the upper Miocene Mashike Formation (8.5-13.0 Ma), Hokkaido, northern Japan, is the oldest described Miocene delphinid including a skull. Therefore, this species is a significant clue to understanding the early evolutionary history of Delphinidae. The original taxonomic assignment of this species within the genus Stenella is questionable; thus, we propose a new combination for the species, Eodelphis kabatensis Horikawa, 1977. Eodelphis is a basal delphinid, and comprehensive morphological cladistic analysis, including molecular topological constraints, supported this taxonomic revision. Paleobiogeographic analyses based on the present morphological cladistic analysis and analysis under the molecular constraints suggest that the origin and early diversification of Delphinidae occurred in the middle Miocene Pacific Ocean or elsewhere, respectively.
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