Pharyngeal teeth of cyprinid fishes from Plio-Pleistocene Koitsuma Formation in Oita Prefecture, central Kyushu, Japan

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  • 大分県大山町の鮮新-更新統小五馬層産のコイ科魚類咽頭歯化石
  • オオイタケン オオヤママチ ノ センシン コウシントウ コイツマソウサン ノ コイカギョルイ イントウ ハカセキ

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Among fossil remains collected from the Plio-Pleistocene Koitsuma Formation at Nosebu, Oyama Town, Oita Prefecture, Central Kyushu, were 163 pharyngeal teeth of cyprinid fishes. This is the first finding of cyprinid fossils in the Koitsuma Formation, and representatives of five subfamilies were included: Leuciscinae, Cultrinae, Xenocypridinae, Acheilognathinae, and Cyprininae. The single leuciscine tooth found was similar to those of species of Tribolodon. The single cultrine tooth was of the genus Anabarilius. The eight xenocypridine teeth could be referred to two genera, Xenocyprinus and Distoechodon. The three acheilognathine teeth were of the genus Acheilognathus. The remaining, cyprinine teeth belonged to two genera, Carassius and Cyprinus, those of Carassius being the most abundant in the lot. The two fossil A2 teeth assigned to Cyprinus were distinguishable from those of extant species, and two taxa were recognized between them on the basis of the number of grooves on the grinding surface. The A2 tooth of Cyprinus sp. a, with two grooves, is close to those occurring abundantly in the Pliocene Ueno and Gamo Formations of the Kobiwako Group. That of Cyprinus sp. b, with three grooves, is close to those found in the Pliocene Ayama and Koka Formations of the Kobiwako Group; such teeth do not occur in the Ueno Formation. The fossil cyprinid assemblage occurring in the Koitsuma Formation at Nosebu is representative of the typical Plio-Pleistocene cyprinid fauna of southwestern Japan and indicates that the sedimentary environment there was an open lake with a broad littoral zone.

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