現生ザルガイ科二枚貝Fulvia mutica(トリガイ)の祖先種,鮮新統穴内層 産Fulvia sp.の殻形態と生息環境

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  • Shell morphology and habitat of Fulvia sp., a candidate for ancestral species of extant F. mutica (Bivalvia: Cardiidae) from the Pliocene Ananai Formation
  • ゲンセイ ザルガイカ ニマイガイ Fulvia mutica(トリガイ)ノ ソセンシュ,センシントウ ケツ ナイソウサン Fulvia sp.ノ カクケイタイ ト セイソク カンキョウ

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Fulvia sp., a candidate species for the direct ancestor of Fulvia mutica (Cardiidae) that usually lives in muddy substrates of sheltered bays around Japan, was found from the Pliocene Ananai Formation in Kochi. Fulvia sp. has been misleadingly assigned to the extant species F. mutica, because both morphotypes are very similar in shell form. However, Fulvia sp. has a characteristic radial groove between shell surface inflations corresponding to inhalant and exhalent siphons, while it is absent in recent F. mutica. The average shell size of Fulvia sp. is about one third of that of F. mutica. In addition, observation of the daily growth lines in cross section indicates that the average life span of Fulvia sp. is much shorter than F. mutica. Fulvia sp. occurs mostly from sandy storm deposits formed in lower shoreface and inner shelf, while extant F. mutica usually lives in muddy substrates in embayed environments. Moreover, all the fossil records of F. mutica are younger than 2.8‒2.0 Ma, whereas the appearance datum of Fulvia sp. is ascended to 3.1 Ma. F. mutica might have been evolved from Fulvia sp. and adapted to cooler climate during Late Pliocene/Early Pleistocene period acquiring larger shell-size and a new habitat in muddy sheltered bay.

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  • 化石

    化石 98 (0), 5-15, 2015-09-30

    日本古生物学会

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