Exoskeletons : Tracing Molluscan Evolution(<Special Number>the 2nd International Chiton Symposium)

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  • 外骨格 : 軟体動物の進化を辿る(<特集号>第2回国際ヒザラガイシンポジウム)
  • Exoskeletons: Tracing molluscan evolution

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Abstract

A progression of modes of dorsal exoskeleton formation can be discerned from the Precambrian to the present in mollusc-like and molluscan forms. Examined here are the known modes in Polyplacophora, Aplacophora, Neopilina, Nautilus, Unionid bivalves and other Conchifera, and the probable modes in Kimberella, Halkieria, Wiwaxia, Maikhanella, Multiplacophorans, and Acaenoplax. The steps in the evolution of sclerites and shells seem to be from (1) mantle cuticle alone (Kimberella), to (2) development of sclerites that fuse within a cuticle matrix (Maikhanella), to (3) development of sclerites or sclerites and shells embedded in cuticle formed over the body, the sclerites formed within invaginations of individual cells and the shells by accretion onto a properiostracum (Halkieria, Wiwaxia, Polyplacophora including the Multiplacophora, Aplacophora, and Acaenoplax), to (4) loss of sclerites and development of periostracum at the mantle edge and shell formed by aragonite columns within cells that then become attached to periostracum at the mantle edge (Nautilus, probably Neopilina), to (5) crystallization in a vacuolated middle layer of periostracum (Unionid bivalves, Mytilus), to (6) crystallization directly onto periostracum from extrapallial fluid. In 4, 5, and 6 there is loss of sclerites, the shell is not embedded in mantle cuticle covering the dorsum, there is greater variation in morphology of the foot for locomotion, and there is a concomitant loss of iteration of the exoskeleton. Acaenoplax and Wiwaxia are considered not to belong to either the Mollusca or Annelida, but to the clade Spiculata that includes all the forms under discussion.

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