Manouria oyamai, an extinct, endemic testudinid turtle known from the upper Pleistocene of the Ryukyu Archipelago, southwestern Japan, has been reviewed on the basis of new specimens. The new specimens consist of a skull, as well as the first records of the epiplastron and the pelvis, all of which were found in a late Pleistocene fissure-filling deposit on Okinawajima Island, the Central Ryukyus, Japan. The present material provided four new informative morphological characters for M. oyamai: the anterior epiplastral projection, the epiplastra lacking extensive overhanging lip, the wide and fan-shaped iliac blade, and the wide ischiadic plate. The results of morphological comparisons and phylogenetic analysis indicated that M. oyamai is sister to M. emys (occurring in Bangladesh, eastern India, Myanmar, Thailand, the Malay Peninsula, Sumatra, and Borneo), and that M. impressa (in southern China, Laos, Malaysia, Myanmar, Thailand, and Vietnam) is the basal-most taxon within Manouria. In addition, the extremely thin carapace observed in M. oyamai, which had been cited as one of three diagnostic characters of this species as well as a feature thought indicative of insular adaptation, is also present in M. impressa within this genus. The present observation therefore indicates that this character is not always attributable to insular adaptation. The results of this study along with the current paleogeographic hypothesis of the Ryukyu Archipelago suggest that M. oyamai had differentiated from an extinct ancestral species shared with M. emys on the eastern margin of Asian continent not later than the early Pleistocene. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Takahashi, A., R. Hirayama, and H. Otsuka. 2018. Systematic revision of Manouria oyamai (Testudines, Testudinidae), based on new material from the Upper Pleistocene of Okinawajima Island, the Ryukyu Archipelago, Japan, and its paleogeographic implications. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1427594.
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