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Chondrostei are primarily cartilaginous fish showing some ossification. The 52 living species are divided among two orders, the Acipenseriformes (sturgeons and paddlefishes), and the Polypteriformes (reedfishes and bichirs). The ancestors of the chondrosteans are thought to be bony fish, but that this characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame. Elderly chondrostean individuals show beginnings of ossification of the skeleton, which suggests this process is delayed rather than wholly lost in these fishes.〔(【引用サイトリンク】title =Chondrosteans: Sturgeon Relatives )〕 This group has at times been classified with the sharks: this because the chondrosteans, like the latter, mostly lack bone, and their structure of the jaw is more akin to that of sharks than of other bony fish; further, both groups lack scales (excluding the Polypteriforms). Additional shared features include spiracles and, in sturgeons, a heterocercal tail (the vertebrae extend into the larger lobe of the caudal fin). However the fossil record suggests these fish have more characters in common with the Teleostei than their external appearance might suggest.〔 Chondrostei is paraphyletic, meaning this subclass does not contain all the descendants of their common ancestor; reclassification of the Chondrostei is therefore not out of the question. The name comes from Greek ''chondros'' meaning cartilage and ''osteo'' meaning bone. ==Classification== *Acipenseriformes * *Acipenseridae — sturgeons * *Polyodontidae — paddlefishes * *Chondrosteidae(†) * *Errolichthyidae(†) *Cheirolepidiformes(†) *Guildayichthyiformes(†) *Luganoiiformes(†) *Palaeonisciformes(†) * *Acrolepidae(†) * *Birgeriidae(†) * *Palaeoniscidae(†) *Perleidiformes(†) *Phanerorhynchiformes(†) *Pholidopleuriformes(†) *Polypteriformes * *Polypteridae - bichirs and reedfish *Ptycholepiformes(†) *Saurichthyiformes(†) *Tarrasiiformes(†) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chondrostei」の詳細全文を読む スポンサード リンク
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