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An Avian Sternum (Aves: Procellariidae) from the Pleistocene Ichijiku Formation in Chiba, Japan

Abstract. We report a fossil record of Procellariidae from the Ichijiku Formation (Middle Pleistocene), Kimitsu City in Chiba Prefecture, Japan. Although this specimen is a partial sternum, we assign it to Procellariidae based on several of its osteological characteristics, such as the size, shape of the linea intermuscularis, and aspect ratio of the incisurae costales. A few marine birds that are uncommon in the area today, such as an Atlantic auk and an extinct flightless auk (i.e., Alle and Mancalla) have been reported from the Ichuijiku Formation, while the procellariid species are still common in the extant seabird fauna of the area. This new discovery suggests that not only did various marine birds exist around the Kanto area (now part of the Boso Peninsula) in the Middle Pleistocene, but also that it is important to consider about an avian faunal transition between the Pleistocene and the present.