909. EARLY LIFE HISTORY OF TWO MIDDLE PLEISTOCENE SPECIES OF LIMOPSIS (ARCOIDA : BIVALVIA)

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  • 909. 中部更新世 Limopsis 属二枚貝 2 種の初期生活史
  • 中部更新世Limopsis属2枚貝2種の初期生活史〔英文〕
  • チュウブ コウシンセイ Limopsisゾク 2マイガイ 2シュ ノ ショキ

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Abstract

Early life history of two species of Limopsis, L. (Crenulilimopsis) oblonga (A. Adams) and L. (Limopsis) azumana Yokoyama is discussed on the basis of well-preserved Middle Pleistocene juvenile specimens from the southwestern Kanto region, central Japan. L. (C.) oblonga has a relatively large prodissoconch I (Pd I) (ca. 180 μm in length) and a well-marked prodissoconch II (Pd II), suggesting lecithotrophic development with a short pelagic stage. L. (L.) azumana, in contrast, probably underwent direct development or ovoviviparity because of its unusually large prodissoconch (ca. 400μm in length) without Pd II stage. The two species exhibit a similar pattern in hinge development : the hinge in the full-grown prodissoconchs is made of a long, uninterrupted series of ligamental pits (provinculum), and anterior and posterior cardinal teeth appear at the early dissoconch stage, increasing their number as the shell grows. Predominance of minute shells at the fossil locality strongly suggests an extremely high juvenile mortality in the two species.

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