A Fossil Humerus of Pliocene Alcidae (Aves: Charadriiformes) from the Fukagawa Group in Hokkaido, Japan

Abstract Alcidae (auks) is a family of wing-propelled diving Charadriiformes, which includes 24 extant species distributed in the Holarctic region. Here, we describe a fossil humerus of Pliocene Alcidae (approximately 4.5–3.5 Ma) from the Fukagawa Group in Hokkaido, Japan. Although this specimen is incomplete and the diagnostic characters of the specific genus are lacking, it shares similar characteristics with Uria, such as the size, dorsoventrally compressed humeral shaft, low and smooth crista deltopectoralis, and a nearly rectangular supracoracoideus scar. While several Alcidae fossils have been reported from the Miocene–Pleistocene deposits (23.0–0.0117 Ma) of the Eastern Pacific (i.e., Mexico and California, USA), little is known about fossils from the Western Pacific (i.e., Asia), most of which have been found from Pleistocene deposits (approximately 0.7–0.12 Ma). This study, reporting the first Pliocene Alcidae from the Western Pacific, implies that the Alcidae were already distributed in the Holarctic region during the early Pliocene.