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.

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.

[3] Ganoine indeed contains amelogenin-like proteins[1] and has a mineral content similar to that of tetrapod tooth enamel.