Japanese researchers captured last month a bottlenose dolphin with an extra pair of fins, possible remains of hind legs. This find confirms the fact that ocean-dwelling mammals once lived on land.

Such traits - which are genetic accidents revealing ancient features - are called atavisms. Human atavisms are: tail, face hair or ear moving muscles.

The four-finned dolphin was caught alive off the coast of Wakayama prefecture in western Japan on October 28, and called the nearby Taiji Whaling Museum. Fossil tracking proved that cetaceans (dolphins and whales) originated in four-footed land even hoofed mammals which lived about 50 million years ago and have an common origin with hippos.

Today, other even hoofed mammals are pigs, camels, deer, cattle, sheep, goats, antolpes and giraffes. Evolving to a more advanced aquatic lifestyle, cetaceans lost their hind limbs. Cetacean fetuses show signs of hind protrusions, but these disappear before birth. "Though odd-shaped protrusions have been found near the tails of dolphins and whales captured in the past, researchers say this was the first time one had been found with well-developed, symmetrical fins," said the Museum's director Katsuki Hayashi.

"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.

The hind fins, much smaller than the dolphin's front fins, are about the size of human hands and protrude from near the tail base on the dolphin's underside. The 5 years old dolphin measures 8.92 feet. Till now, the researchers have not determined whether the dolphin used its back fins to swim. "A freak mutation may have caused the ancient trait to reassert itself," Osumi said. "The dolphin will be kept at the Taiji museum to undergo X-ray and DNA tests," said Hayashi.

Photo credit: Taiji Whale Museum