The actinopterygians are the group of fishes ancestral to all the ray-finned fishes. Everything from the seahorse up to tuna belongs to the ray-finned fishes, some 20,000 or more species.

The only actinopterygian found in the Middle Devonian sediments of Orkney is the species Cheirolepis trailli. This species is one of the earliest articulated examples found from this group and is described in several publications. A bit later in time (Givetian, Upper middle Devonian) is Stegotrachelis finlayi, a species found on Shetland.

Cheirolepis was a slender fish with one dorsal fin and jaws with rows of tiny but sharp teeth. The probably had to share the lake with the other top predators.

The head is covered with medium sized bony plates, with the body covered with bony scales with sharp ridges on the surface. The scales are generally smaller than one millimetre. The scales of most palaeozoic actinopterygians are covered with ganoine, a very hard enamel-like substance sometimes found in layers. The definition of ganoine differs in publications but most agree on that the structure strongly resembles enamel or enameloid. Ganoine can also be found in some Mesozoic actinopterygians and even in several recent species.

The term cosmine was introduced By Williamson in 1849 to describe the top layer of the scales of Lepidotus and palaeoniscoid actinopterygians. Later Goodrich 1907 restricted cosmine for sarcopterygians with ganoine was used for actinopterygians.

According to Friedman and Brazeau (2010) “Multilayered enamel occurs only in ganoine, even partial overlap of enamel is only found in ganoine.”

According to Schultze (2016):

Ganoine:

Thus, ganoine can be defined as multilayered or partially overlapping enamel with a tuberculated surface in Mesozoic and Recent ganoid scales.

Cosmine:

Cosmine is a combination of tissues and one structure a single thin layer of “true” enamel covers dentine interrupted by pores of the pore–canal system.

The tiny scales of Cheirolepis have a bony base with often several layers of enamel on top of each other, the ganoid type. The whole scale lies beneath the skin with only the enamel visible on the outside. The base of the scale is made up of isopedin and the scale grows by adding a layer concentrically. Also, the head plates and the finrays are having a histology that looks much  like the one seen in the scales. In the isopedin cell lacunae are visible. In later forms the ganoin is often not present.

Photomigrographs of scales in a thin section, note the layered enamel!

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