The first Pliocene albireonid (Cetacea, Delphinoidea) periotic from the western North Pacific and paleobiogeographic significance of fossil delphinoid ear bones of Na-arai Formation of Choshi, Chiba, central Japan

Abstract

A right periotic of Albireonidae (Cetacea, Delphinoidea), from the Pliocene Na-arai Formation of Chiba, central Japan, is described. The albireonid specimen is the second reported from the western North Pacific and the first recorded from the Pliocene. Thus, this discovery largely extends our knowledge of paleobiogeography of Albireonidae. The Na-arai specimen shares following features with the type species of Albireo: having the large aperture for the cochlear aqueduct; the trapezoidal cochlear portion and the large angle between the caudal tympanic process and the lateral edge of the cochlear portion in ventral view; the mediolaterally narrow posterior bullar facet; shallow grooves on the posterior bullar facet. However, the present specimen probably belongs to different species from the type species because of differences of morphology in the internal acoustic meatus and anterior process. The fossil ear bone assemblage of Delphinoidea from the Na-arai Formation provides important data for comparing Pliocene delphinoid faunas of the western North Pacific with those from other areas of the Pacific. The paleoclimate of the Na- arai Formation was probably more similar to those of delphinoid localities in the eastern regions of the North and South Pacific than to those of other fossil localities in the western North Pacific.