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DEVONIAN PERIOD

416-359 million years ago

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finfin

To transform from a fin to an arm, the ancestral fins could have made the existing humerus, radius, and ulna bones more robust, added additional carpal bones, and developed an additional set of bony elements which would compose the digits. Lengthening the area of expression of genes such as HoxA13 and HoxD13 in the embryonic fin may have played an important role in this transition.