| Summary: | The genus Uria includes volant wing-propelled diving birds widely distributed in the Arctic and boreal waters in the Northern Hemisphere. It includes two Recent species, U. lomvia and U. aalge, which represent the largest living members of Alcidae, all of which are volant. Compared to other parts of its current distribution, the fossil record of the genus is scarce in the western North Pacific. In this report, two species of Uria, extant U. lomvia and extinct U. onoi, sp. nov., are reported from the middle–upper Pleistocene of Shiriya, northeast Japan. U. onoi is represented by several isolated fossil bones including nearly complete humeri, whose length and stoutness greatly exceed those of Recent species. Body mass estimation based on a scaling relationship of body mass and humerus length in volant alcids resulted in an estimate of ~1.5 kg for the body mass of U. onoi. This value, as well as those previously estimated for several extinct Alca spp. from the Atlantic, exceeds a commonly supposed "upper limit" of body mass attainable by volant wing-propelled diving birds, ~1 kg. These examples indicate that independent evolutions and extinctions of large-bodied species took place in two different lineages of Alcidae and in two major oceans, Alca in the Atlantic and Uria in the Pacific, and demonstrate the importance of incorporating extinct taxa in analyses of evolutionary patterns and morphological diversities. ウミガラス属Uriaはウミスズメ科に属する翼推進性潜水鳥類の一群で，北半球の高緯度海域に分布 する2種の現生種を含む．本論では青森県北東部尻屋地域の中部–上部更新統より産出した2種のウミガラス類，現生種U. lomviaおよび絶滅種U. onoi（新種）について報告する．U. onoiはほぼ完全な上腕骨を含む複数の単離した骨化石によりその存在が認められる大型種で，その大きさは現生のウミガラス類を大きく上回る．飛翔性のウミスズメ科鳥類における体重と上腕骨長のスケーリング式より推定されるU. onoiの体重は約1.5キログラムで，先行研究による化石オオハシウミガラス類Alcaの推定体重と同様，一般に知られる飛翔性の翼推進性潜水鳥類の体重の「上限」約1キログラムを上回っている．これらの例はウミスズメ科の2つの系統で独立に大型種の進化と絶滅が起こったことを示している．
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