cosmine

Cosmine is a histological tissue found in the scales and dermal plates of certain fishes, including lungfishes and various extinct species. It is a dentine-like layer that forms part of the scale's internal structure, typically situated beneath the outer enamel-like tissue in cosmoid scales. Through light and electron microscopy, cosmine reveals a complex, porous network that accommodates a vascular system and, in life, nerves. This tissue gives the scale a distinctive, reticulate appearance.

Anatomy and development of cosmine involve a dense arrangement of lamellae and a labyrinth of dentine tubules. The tissue is produced by odontogenic cells during growth and adds to the scale’s thickness as the animal matures. Compared with true dentine, cosmine is more porous and richly vascularized, reflecting its formation within a protective dermal skeleton rather than a tooth. In life, the canalicular network within cosmine may have supported growth, sensory function, or rapid repair of the scale.

Distribution and significance of cosmine span both living and fossil fishes. It is well documented in lungfishes and appears in many extinct sarcopterygian and some early actinopterygian lineages, particularly in Paleozoic deposits. The presence and structure of cosmine aid paleontologists in diagnosing scale types and inferring evolutionary relationships among early vertebrates. Cosmine thus contributes to our understanding of dermal skeleton evolution and scale development in ancient fishes.