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- Epithelial-mesenchymal interactions yet a deeper homology than anatomical placodes
Our comparative micro-anatomical and molecular developmental study allowed identifying the anatomical placode as a deep homology for the keratinised epidermal skin appendages that are found in all amniotes. It is entirely unclear what was the morphology of the placodal skin appendage from which modern hairs, scales and feathers evolved. Using hand-waving arguments, one could argue that the first placodes that evolved in stem reptiles might have generated simple nodular keratinised scales that provided extra protection against desiccation and mechanical abrasion. Note that, morphologically speaking, the scales of extant reptiles are likely not less derived, than hairs and feathers, from these ancestral skin appendages.
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What we have shown is that all these keratinised skin appendages initially develop from an epidermal placode. On the other hand, most scales in fish are bony structures derived from mesodermal tissue. This major developmental difference makes it difficult to envision homology between fish scales and reptile scales. Particularly, there is no data suggesting that dermal scales in fish require the formation of an epidermal placode for their development.
Although it is likely that the anatomical placode does not exist beyond amniotes (but amphibians should be investigated further), some level of homology is possible among all skin appendages in all gnathostomes (or even all vertebrates). After all, the developments of, among others, vertebrate teeth...Competing Interests: None declared. - RE: earliest common ancestor
Since lungfish, coelacanths, ray finned fish, and probably all forms of extinct lobe finned fish have and had scales, the earliest common ancestor of all amniotes that had placodes was probably at least as early as the first tetrapods, and probably was even a lobe finned fish if not earlier, an Osteichthyes found around 420 mya. This is an interesting article and well done science. Thanks,
William F. HagenCompeting Interests: None declared.
