Japanese fisherfolk have found an unusual bottlenose dolphin with an extra set of fins, which researchers say could be an evolutionary throwback to the time when the marine mammal's ancient ancestors lived on land. The dolphin was captured alive off the southwest coast of Japan in the last week of October and is now kept at the nearby Taiji Whaling Museum for study. Researchers say the extra fins could be an atavistic trait: an ancient genetic trait that re-emerges for some reason. Fossil remains show that dolphins and whales were four-footed terrestrial mammals about 50 million years ago and have descended from the same ancestor as hippopotamus. Later they transitioned to an aquatic lifestyle and their hind limbs disappeared. Foetuses of whales and dolphins show signs of hind protrusions even now that dissapear before birth.