Dermal scales

The oldest fossil evidence of scales on the exterior of vertebrates begins with the oldest fossil fishes. A dermal armor that was present in a group of jawless fish known as ostracoderms consisted of layers of mineralized material. The layers of these scales from the deepest to the most superficial were

1) lamellar bone (lacking lacunae in some of the most ancient forms)

2) vascular bone (spongy bone or cancellous bone)

3) dentine

4) enamel

Various other ostracoderms in the later fossil record reflect types of scales that differ from earlier fossil species. Scales in these show some that keep the dentine and enamel, and others that keep only the plates of lamellar tissue.

Variability is also seen in modern fish species.

The chondrichthian species have the tooth-like scales, with enamel covering dentin, and a pulp cavity occupies the core at the base of the scale.

The polypterids have scales with a layer of lamellar bone, a layer of dentine, and a layer of enamel.

Gars have a layer of lamellar bone and a layer of enamel.

Other extant fish have scales with an acellular lamellar bone layer and an added supportive collagen layer.

Dermal scales exist as bones beneath the surface in tetrapods. Sometimes as dermal bone of the skeleton, and sometimes as bony plates beneath the epidermis called osteoderms.