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Wikipedia:

Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically.

So aren't reptilia scales also dermally derived?

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No, fish scales are dermal (= formed in derma) bones like skull roof bones. Scales in reptiles are formed by epidermis and are made primarily of protein (from keratinocytes), being similar in derivation to hair, feather and nails.

On the other hand, in reptiles one must differentiate between scales and osteoderms (= scutes). Scutes are widespread among reptiles and they are similar to fish scales in that they are produced in derma and are ossified. Similar structures are found in some amphibians and in armadillo. Nevertheless, different types of osteoderms are currently considered as converges: "it is essential to recognize that osteoderms represent non-homologous structures that have been independently evolved a number of times" and have no direct homology with fish scales.

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  • $\begingroup$ Do u mean fish scales disappeared and then reptilie scales re-emerged? $\endgroup$ – Anixx Oct 28 '14 at 18:40
  • $\begingroup$ @Anixx I don't know anything about reptile and fish evolutionary history, but it's also possible that both developed after divergence. According to wikipedia, scales have evolved multiple times. Also, some reptiles have scutes which are derived from the dermis, like fish scales. $\endgroup$ – canadianer Oct 28 '14 at 20:43
  • $\begingroup$ Scales must have been present in the ancestor of Tetrapoda. Actinistia (like Latimeria), for example, had/have cosmoid scales. Other elements of dermal ossification are skull roof bones and teeth, and only they are found in Tetrapoda. Hair, reptile scales (and feather on their basis) have absolutely no relation to dermal bony elements. $\endgroup$ – har-wradim Oct 28 '14 at 21:18
  • $\begingroup$ @canadianer can scales develop into scutes ever rather than other way around? $\endgroup$ – Anixx Oct 28 '14 at 23:08
  • $\begingroup$ @har-wradim I thought skull originates as several vertebrae. $\endgroup$ – Anixx Oct 28 '14 at 23:13

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