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arowana
Arowanas are freshwater bony fish of the family Osteoglossidae, also known as bonytongues (the latter name is now often reserved for Arapaimidae). In this family of fish, the head is bony and the elongated body is covered by large, heavy scales, with a mosaic pattern of canals. The dorsal and anal fins have soft rays and are long based, while the pectoral and ventral fins are small. The name "bonytongues" is derived from a toothed bone on the floor of the mouth, the "tongue", equipped with teeth that bite against teeth on the roof of the mouth. The arowana is a facultative air breather and can obtain oxygen from air by sucking it into its swim bladder, which is lined with capillaries like lung tissue.
==Evolution==
Osteoglossids are basal (primitive) fish from the lower Tertiary and are placed in the actinopterygiid order Osteoglossiformes. As traditionally defined, the family includes several extant species from South America, one from Africa, several from Asia, and two from Australia.〔 Today Arapaimidae is often regarded as a separate family, which includes the arapaimas and the African arowana.〔 Consequently, the South American genus ''Osteoglossum'', and the Asian and Australian genus ''Scleropages'' are the only extant genera that remain in the osteoglossid family. Arapaimidae and Osteoglossidae split about 220 million years ago (Mya), during the Late Triassic.
Within Osteoglossidae, the South America ''Osteoglossum'' arowanas diverged from the Asian and Australian ''Scleropages'' arowanas about 170 Mya, during the Middle Jurassic.〔
The Osteoglossidae are the only exclusively freshwater fish family found on both sides of the Wallace Line. This may be explained by the theory that Asian arowanas (''S. formosus'') diverged from the Australian ''Scleropages'', ''S. jardinii'' and ''S. leichardti'', about 140 Mya, making it likely that Asian arowanas were carried to Asia on the Indian subcontinent.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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