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.

The glassy appearance of the scales of this spotted gar is due to ganoine.
The mineral texture of alligator gar scales consists of bundles of cross-plied hydroxyapatite minerals oriented towards the scale's surface.

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]

References
  1. Zylberberg, L.; Sire, J. -Y.; Nanci, A. (1997). "Immunodetection of amelogenin-like proteins in the ganoine of experimentally regenerating scales of Calamoichthys calabaricus, a primitive actinopterygian fish". The Anatomical Record. 249 (1): 86–95. doi:10.1002/(SICI)1097-0185(199709)249:1<86::AID-AR11>3.0.CO;2-X. PMID 9294653.
  2. Sire, Jean-Yves; Donoghue, Philip C. J.; Vickaryous, Matthews K. (2009). "Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates". Journal of Anatomy. 214 (4): 409–440. doi:10.1111/j.1469-7580.2009.01046.x. ISSN 0021-8782. PMC 2736117. PMID 19422423.
  3. Bruet, B. J. F.; Song, J.; Boyce, M. C.; Ortiz, C. (2008). "Materials design principles of ancient fish armour". Nature Materials. 7 (9): 748–756. Bibcode:2008NatMa...7..748B. doi:10.1038/nmat2231. PMID 18660814. S2CID 17288901.
  4. Richter, M. (1995). "A microstructural study of the ganoine tissue of selected lower vertebrates". Zoological Journal of the Linnean Society. 114 (2): 173–212. doi:10.1006/zjls.1995.0023.
  5. Ørvig, T. (1967). "Phylogeny of tooth tissues: Evolution of some calcified tissues in early vertebrates.". Structural and Chemical Organization of Teeth. New York: Academic Press. pp. 45–110.
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