The oldest fossil record of the extant genus (Odontoceti, Ziphiidae) from the Middle to Late Miocene boundary of the western North Pacific.

A new species of a beaked whale that belongs to the extant genus is described from the Middle to Late Miocene boundary age Tsurushi Formation ( 12.3-11.5 Ma) on the Sado Island, Niigata Prefecture, Japan. The new species, sp. nov. represents the oldest record of this genus and provides a minimum age for the emergence of this extant genus. sp. nov. has the following generic characters: the ratio between the width of the premaxillary crests and the width of the premaxillary sac fossae is 1.0-1.25, nodular frontals make isolated protuberance on the posterior part of the vertex. Among the species within the genus, sp. nov. shares a unique character with : the apices of the left and right hamular processes of the pterygoids contact medially, forming together a posteriorly directed medial point. In addition, sp. nov. displays a unique combination of the following characters: it is extremely small in size, and the nasals are short, the ratio between the length of the medial suture of nasals on the vertex and the maximum width of nasals is less than 0.4. sp. nov. fills the gap between the origin of the genus and later diversifications of the extant species. This discovery is also key to elucidate the process of the emergence and dispersal of the genus during the Middle to Late Miocene. Based on the distributional patterns of the fossil and extant species of the genus, the western North Pacific including the Sea of Japan may have been one of the areas for the evolution and radiation of this genus at the time before 11 Ma.