Ganoine
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Ganoine or ganoin is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some basal ray-finned fishes.[1] It is composed of rod-like, pseudoprismatic apatite crystallites, with less than 5% of organic matter.[2] Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa.[3] It is a characteristic component of ganoid scales.
Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis.[3] While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii.[3]
It has been suggested that ganoine is homologous to tooth enamel in vertebrates[1] or even considered a type of enamel.[2] Ganoine indeed contains amelogenin-like proteins[1] and has a mineral content similar to that of tetrapod tooth enamel.[4]
References
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