大分県津房川層より得られたマチカネワニの尾椎について

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タイトル別名
  • Note on the fossil caudal vertebra of Japanese crocodile, <i>Toyotamaphimeia machikanensis</i> (Crocodylydae, Crocodylinae) from the Tsubusagawa Formation (Pliocene) of Japan
  • オオイタケン ツ ボウガワソウ ヨリ エラレタ マチカネワニ ノ ビシイ ニ ツイテ

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The crocodilian caudal vertebra is obtained from the Tsubusagawa Formation. Its size is same and its gross shape is resemblance to the 20th caudal vertebra of Crocodylus porosus, NTM R16036, total length for 513 cm. However, in comparison with NTM R16036, the direction of neural spine is more dorsocaudal, and the M. longissimus caudae and M. semispinalis caudae might be more developed in the Tsubusagawa specimen. With respects to the hitherto regarding crocodilian fauna, the Tsubusagawa specimen is identified to the crocodyline crocodylid, Toyotamaphimeia machikanensis. The caudal vertebrae are preserved only three, 1st to 3rd , in the type specimen of T. machikanense, and thus direct comparison with the Tsubusagawa specimen is impossible. According to Chinese classic dictionary by Lu (1174, the 12th Century), the tail of crocodile which may be identified as T. machikanensis is prehensile as if elephant trunk. The posterior condyle of this vertebra is well developed, and thus there is some possibility to support the evidence of prehensile tail. At the early Miocene, T. machikanensis might be derived from crocodyline crocodylid Maomingosuchus petrolicus. T. machikanensis is differ from Maomingosuchus petrolicus in having the largest 7th maxillary teeth and much larger body. The large canini-form development of 7th maxillary teeth is an adaptation for the relatively larger prey. As a larger prey, the presence of oxygasterine cyprinid Hypophthalmichthys (Silver Asian Carp), total length for 1 metre or more, is seemed to be involved with the secondary caniniform development of 7th maxillary teeth and gigantism of Toyotamaphimeia. The oldest fossil record of Toyotamaphimeia is the early Miocene and that of Hypophthalmichthys is too. Hypophthalmichthys is famous for its frequently jumping when disturbed. Although this jumping is hitherto claimed as enigmatic, it could be interpreted as anti-predatory behavior against Toyotamaphimeia.

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