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

A partially preserved caudal vertebrae imprint of a tuna was discovered from the Duho Formation (Miocene) of South Korea. This specimen was assigned to the genus Auxis and represents the second record of fossil Auxis found in South Korea and in the world. We compared the vertebral morphology of the studied specimen to that of currently known species of Auxis, including extinct taxa. However, the specimen could not be assigned to any extant or new species of Auxis due to anatomical differences and a lack of comparability. The discovery of a new specimen of Auxis aligns with theories of high marine biodiversity in the East Sea (Sea of Japan) and its opening in the Early to Middle Miocene. A widely opened East Sea and upwelling activities might have increased the abundance and diversity of large oceanic fishes such as tunas during the deposition of the Duho Formation. The specimen supports paleoenvironmental interpretations of the Duho Formation as pelagic and subtropical. 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 would have undergone disintegration before being buried.