Tethyan-"Northern Tethyan" transitional bivalve-fauna from the Kurosegawa Belt of Central Shikoku

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  • 四国中央部黒瀬川帯下部白亜系産の"テチス"-"テチス北方"の混在型群集
  • シコク チュウオウブ クロセガワ タイ カブ ハクアケイサン ノ テチス テチス ホッポウ ノ コンザイガタ グンシュウ

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The Early Cretaceous bivalve fauna from the Kurosegawa Belt in Central Shikoku was studied with respect to the faunal affinity. In the Kurosegawa Belt, two faunal-types of Hauterivian bivalves have been recognized (Kozai et al., 2002). The one, Tatsukawa-type from the Monobegawa Group is distributed in the Northern Kurosegawa Belt. Another one, Shobu-type from the Nankai and Takegatani groups is distributed in the southern Kurosegawa Belt. Based on the specific compositon, the former is regarded as "Tethyan fauna", and the later is regarded as "northern Tethyan fauna" of Tashiro (1994, 2000). The specific compositions of these two faunas are quite different except for the cosmopolitan species. A transitional fauna between the Tatsukawa-type and Shobu-type was newly discovered in Hauterivian sandstone bed of brackish facies in the north-south intermediate part of the Kurosegawa Belt. Among the six local species, Protocardia ibukii, Costocyrena otsukai, Pulsidis antiqua and Tetoria Sanchuensis are the Tatsukawa-type elements. Isodomella matsumotoi and Eomiodon matsumotoi are the Shobu-type elements. In addition to the finding of the marine Barremian "Tethyan"-"Northern Tethyan" transitional bivalve-fauna, new discovery of the Hauterivian brackish trasitional-fauna is significant for the analysis of paleobiogeography and tectonics during and after the Early Cretaceous. Especially, the existence of the transitional fauna indicates that the "Tethyan" and "Northern Tethyan" faunas are paleobiogeographicaly closer.

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