A new tuna specimen (Genus Auxis ) from the Duho Formation (middle Miocene) of South Korea

The partially articulated caudal vertebrae of a tuna were discovered from the Duho Formation (middle Miocene) of South Korea. This specimen was assigned to the genus Auxis and represents the second record of fossil Auxis found in South Korea. We compared the vertebral morphology of the studied specimen to that of currently known species of Auxis , including extinct taxa, but the specimen is not assigned to a new or existing species of Auxis due to poor preservation and a lack of diagnostic features. The discovery of a new specimen of Auxis supports theories of high marine biodiversity in the East Sea (Sea of Japan) and the opening of the East Sea in the early to middle Miocene. A widely opened East Sea might have increased the abundance and diversity of large oceanic fishes such as tunas during the deposition of the Duho Formation. A taphonomic scenario of the specimen was inferred based on the lack of anal pterygiophores and the leaf imprint on the matrix. The specimen would have been exposed for at least a month in a low-energy sedimentary environment at the deep-sea bottom and undergone disintegration before being buried.