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Extant Ganges and Indus river dolphins are endangered species, but their relatives were more diverse in the past. The family Squalodelphinidae is a group of Miocene relatives of Ganges and Indus river dolphins. Our knowledge of squalodelphinids increased slowly in the nineteenth century and has expanded dramatically since the 2010s. Italy, Switzerland, eastern USA, Argentina, and Peru are fossil localities of named and well-preserved squalodelphinid fossils. Squalodelphinids are also known from New Zealand, Germany and Venezuela. However, only two specimens of this family have been reported from the North Pacific, in Washington State, USA, and Japan. Here, a new fossil dolphin (including the skull, right and left periotics and bullae, malleus, incus, hyoid bones and ribs) from the Haze Formation, Ichishi Group, Early Miocene (18.7–18.5 Ma) of Mie Prefecture, Japan is named as the new genus and species Miodelphinus miensis. Phylogenetic analysis places Miodelphinus miensis among squalodelphinids. The periotic of Miodelphinus miensis shows a large, posteriorly widened, ventrally opening, funnel-like articular depression between the posterior and articular processes of the periotic. Miodelphinus miensis contributes to expanding our knowledge of squalodelphinid diversity. Squalodelphinids were distributed widely not only in the Atlantic but also the South and North Pacific by the Early Miocene. This suggests that the family had a chronologically deeper origin such as the beginning of the Early Miocene or older. https://zoobank.org/urn:lsid:zoobank.org:pub:DB7DDDE2-1095-4815-975B-8E1686559C2E
Tanaka et al. (Sun,) studied this question.