比較発生学的にみたオウムガイ類の原始性

概要

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As compared with extant coleoids, species of Nautilus retain plesiomorphic conditions in various anatomical characters, such as the pin-hole type eyes without lenses, numerous tentacles without suckers, and two pairs of gills. Their reproductive biology is fairly different from that of extant coleoids by having much longer life span and embryonic period, and a smaller number of offspring. The embryo of early organogenetic stage possesses a cup-shaped, Monoplacophora-like primary shell (cicatrix), and its overall morphology is similar to the cephalopod prototype supposed by previous authors. Relatively large embryonic shells starting with the cicatrix have been widely recognized in fossil nautiloids of various geologic ages and are distinguished from the small embryonic shells of coeloids and ammonoids starting with a spherical initial chamber ("protoconch"). Oxygen isotopic analysis of aquarium-reared specimens strongly suggests that in the wild, Nautilus migrates into a deeper water soon after hatching in relatively shallower and warmer environments. This phenomenon can be interpreted as one of the adaptive stratetgies to escape increasing predatory pressure in shallower environments since the Cenozoic.
日本古生物学会の論文
1994-06-30