クジラ類における摂餌機構の進化的刷新(<特集>生物イベントとしての哺乳類の海生適応)
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概要
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Filter feeding is one of the most characteristic features of the extant mysticete cetaceans. Each living mysticete family has been acquired unique filter feeding strategies : i. e., the skim feeding in balaenids and neobalaenids, the mud scooping in eschrichtiids, and the engulfment feeding in balaenopterids. In this paper, the author reviews the evolution of feeding strategies in mysticetes, especially engulfment feeding. Some "cetotheres" had already acquired a primitive engulfment feeding mechanism by the end of the Early Miocene. Apomorphic characters of the early Miocene cetotheres (a high and elongated dorsal mandibular ridge and a ventrally well-projected mandibular angle) suggest robust development of the musculature of the mandible in contrast to the weak development of the musculature in the balaenopterids. Their feeding mechanism required more active recruitment of jaw musculature than in the Balaenopteridae. During their evolutionary history, balaenopterids acquired highly elastic elements in their ventral pouch and this may have enabled passive movement of the mandible in the feeding. Another cetothere specimens from the early Middle Miocene (Diorocetus chichibuensis) and early Pliocene (Piscobalaena sp.) show a different morphology of the lower jaw (a rounded ventral margin of the cross section in the middle part of the mandible) that suggests a feeding strategy other than engulfment feeding. Some Pliocene cetotheres (Piscobalaena and Herpetocetus) show a unique morphology of the mandible (a posteriorly wellprojected mandibular angle). Although the precise feeding mechanism is unclear, by use of the mandible during feeding suggests that at least some "cetotheres" evolved a different feeding strategy and likely occupied a niche different from that of the other contemporaneous mysticetes.
- 2005-03-25