A new specimen of Paleoparadoxia found from a marine lower Miocene deposit in the Chikubetsu area, Hokkaido, Japan, is described. The material consists of a distal part of the scapula, proximal end of the humerus from the right side, as well as a fragmentary rib, preserved in a float of calcareous fine sandstone. The specimen is referred to the order Desmostylia and subsequently to the genus Paleoparadoxia sp. The well preserved shoulder girdle of this specimen provides the first detailed morphology of this anatomical region in Paleoparadoxia. We compared the specimen with a wide range of desmostylid samples to reveal new diagnostic characters for the genus, such as the greater tubercle extending toward the proximal side above the head and the distinct lesser tubercle located on the medial side, projected medially. The lower Miocene Sankebetsu Formation outcrops in the area where the float was found, and the lithology and associated fossil fauna of the float indicate that it was derived from there. The fossil pollen assemblages and molluscs found in the formation indicate that it was deposited under cool to temperate conditions. The new specimen thus suggests that Paleoparadoxia had inhabited a cool environment in addition to warm areas, as suggested in previous studies. The published age estimate of the formation places the present specimen between 23.8±1.5 and 20.6±1.0 Ma. This represents the oldest record of the genus Paleoparadoxia, exceeding the previous record of ca. 19 Ma for a specimen found from the Chichibu Basin, and nearly matches the oldest record of Paleoparadoxiinae in the Northwestern Pacific. This indicates that the ages of the oldest occurrences of basal and derived paleoparadoxiines overlapped (i.e., Archaeoparadoxia from the Northeastern Pacific Region and Paleoparadoxia from the Northwestern Pacific, respectively). It is also likely that the geographic range of Paleoparadoxiinae had already expanded from the Northeast Pacific to the Northwest Pacific in the early stage of their evolution.
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Vol. 19 • No. 3