cosmoid

Cosmoid refers to a primitive type of dermal scale found in certain fishes, most notably lungfishes, and in various extinct sarcopterygians. The name derives from cosmine, a dentine-like, highly vascularized tissue that forms a distinctive outer portion of the scale. Cosmoid scales are relatively thick and heavy compared with many modern teleost scales.

The histology of cosmoid scales shows a layered structure. The outer surface is often coated by a thin enamel-like or dentine-like layer containing cosmine. Beneath this lies a cosmine-dentine complex that is rich in vascular canals, contributing to growth and repair. A basal layer of bone anchors the scale to the dermis, providing rigidity and stability.

Distribution and evolution: Cosmoid scales occur in fossil sarcopterygians and are a characteristic feature of lungfishes. Modern lungfishes retain cosmoid scales, whereas many other osteichthyan lineages evolved different scale types, such as ganoid or elasmoid scales. The presence of cosmoid scales is thus informative for understanding the early evolution of the dermal skeleton in jawed fishes and the relationships among major vertebrate lineages.

Cosmoid scales are distinct from other scale types, such as ganoid scales with ganoine surfaces and elasmoid scales found in most modern teleosts, reflecting diverse evolutionary paths in dermal armor. Etymologically, the term centers on the cosmine tissue that gives the scale its defining character.