九州芦屋層群産のヒゲ鯨類新種とその意義

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  • A new mysticete from the upper Oligocene Ashiya Group, Kyushu, Japan and its significance to mysticete evolution

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<p>The Ashiya Group (Late Oligocene) is distributed in the northern part of the Kyushu Island, West Japan, yields many cetacean fossils. Among them, an archaic mysticete is reported here. The fossil occurred at the Wakamatsu Coast of Kitakyushu City (Tominohana (cape), 33°56′09″N, 130°41′08″E), from siltstone of the Jinnobaru Formation, Ashiya Group. The material is composed of an almost complete skull with both dentaries, cervical vertebrae, anterior thoracic vertebrae, several ribs, and fore limb elements. Deformation of bones is limited. The specimen is described as Yamatocetus canaliculatus, new genus and new species in the Eomysticetidae SANDERS and BARNES, 2002. Yamatocetus canaliculatus has following characters: the rostrum is wide and long, indicating the presence of functional baleen: there are several alveoli along the grooves of lateral margin of upper jaws and dorsal margin of dentaries. The external naris opens anteriorly as seen in Eomysticetus whitmorei SANDERS and BARNES 2002, but different from E. whitmorei at the uplift of nasals and other features. Yamatocetus canaliculatus has advanced features in the anterior limb for swimming, the lengths of bones are different from extant species of mysticetes. The ribs are mostly two-headed, indicating a less flexible nature of the rib cage. It indicates that the whale was not adapted enough for deep diving, compared to extant baleen whales which have a limited number of two-headed ribs. The occurrence of Yamatocetus canaliculatus is important for discussion of early stage of the mysticetes, especially at the stage of loss of teeth. There are several species of such evolutionary stage in west coast of North Atlantic Ocean and New Zealand. They have migrated rapidly in the Late Oligocene.</p>

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