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Pangolin scales are certainly not retained from fish. Pangolins are within the clade Pholidota (Tree of Life page for Eutheria) and their nearest relatives are anteaters, sloths, and armadillos. According to the University of California Museum of Paleontology, similarities between pangolin and armadillos are the results of convergent evolution. So the ...


From a little research I have been able to find some of the hypotheses and speculations but no papers expressly confirming or denying the matter. One theory displayed in "The evolution of mammalian keratinized structures" (abstract only) is that mammalian hair developed along the following pathway: Hair follicles develop between reptilian like scales. A ...


As to the early evolution of mammalian hair, Rowe et al. (2011) hypothesized that the primitive function for hair was not thermoregulatory, but rather for tactile sensation (contra the hypotheses of Spearman and Maderson). Rowe et al. say: Body hair develops as migrating neural crest cells induce patterns of tiny placodes that mature into hair ...


I believe it is mainly due to hydrodynamics. Scales reduce drag while allowing for the body to be able to still move. Drag only occurs in the back part of the fish, so there is where you need scales. Please refer to http://darwin.wcupa.edu/~biology/fish/pubs/pdf/ISSDR(drag%20reduction).pdf for other interesting biological solutions for drag reduction.

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