Fishermen caught the four-finned dolphin off the coast of Wakayama prefecture in Western Japan on October 28 and alerted the nearby Taiji whaling museum to their strange find, according to Katsuki Hayashi, the museum director.
Fossil remains show that dolphins and whales were four-footed land animals about 50 million years ago and share the same common ancestor as hippos and deer. Scientists believe they later became aquatic mammals and their hind limbs disappeared.
Though odd-shaped protrusions have been found near the tails of dolphins and whales in the past, researchers thought it was the first time one had been found with well-developed, symmetrical fins, Hayashi said.
"I believe the fins may be remains from the time when dolphins' ancient ancestors lived on land ... this is an unprecedented discovery," Seiji Osumi, an adviser at Tokyo's Institute of Cetacean Research, said on Sunday.
The second set of fins - much smaller than the dolphin's front fins - are about the size of human hands and protrude from near the tail on the underside of the dolphin.