Halitheriine sirenia from the Itahana Formation (late Middle Miocene early Late Miocene) in Annaka, Gunma Prefecture

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  • 群馬県安中市の板鼻層(中期中新世後期〜後期中新世前期)産のハリテリウム亜科の海牛目化石
  • グンマケン アンナカシ ノ イタハナソウ チュウキ チュウシンセイ コウキ コウキ チュウシンセイ ゼンキ サン ノ ハリテリウム アカ ノ カイギュウモク カセキ

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Abstract

A sirenian fossil from the Itahana Formation (late Middle Miocene to early Late Miocene) in Annaka, Gunma, is described as a Halitheriinae, based on the right scapula, sternum, ribs and vertebrae. The fossil is called "Shimoakima specimen" in this paper after the name of the locality. Size and shape of those materials of Shimoakima specimen are similar to those of the genus Metaxytherium and Halitherium schinzi. Particularly, the specimens resemble the genus Metaxytherium in having plump ribs from the first to the fourth. A trapezoid scapula, a rectangular sternum and compressed thorax of the specimen are similar to those of Metaxytherium crataegense, M. subapenninum, M. serresii and M. floridanum. However, the Shimoakima specimen is more derived than Halitherium schinzi in having a large scapula with the broad dorsal margin, strong coracoid process. The Shimoakima specimen is presumed to be one of the earliest Dugongidae in Japan, and very important for researching the migration of the halitheriine dugongid from Europe to Japan in the Miocene.

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