Ganoine

Ganoine or ganoin is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some non-teleost ray-finned fishes,^[1] such as gars and bichirs, as well as lobe-finned coelacanths.^[2] It is composed of rod-like, pseudoprismatic apatite crystallites, with less than 5% of organic matter.^[3] Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa.^[4] 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.^[4] While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii.^[4]

It has been suggested that ganoine is homologous to tooth enamel in vertebrates^[1] or even considered a type of enamel.^[3] Ganoine indeed contains amelogenin-like proteins^[1] and has a mineral content similar to that of tetrapod tooth enamel.^[5]