Excellent Atavism

Japanese fishermen recently netted a dolphin that posesses atavistic rear fins. This means that it has an extra set of hand sized fins in front of its fluke. These appear to likely be a result of a mutation that has turned back on the genes that produced the back legs of the dolphin's terrestrial ancestors. Since the front and back legs are serially homologous structures, if the gene is turned on, the other genes that regulate shape and structure of the limb will produce a fin shape even if ancient whales lost their rear legs before they got to look like functional fins.