It is not uncommon today to find swans in rivers and lakes, dividing their time between lifting aquatic plants and punishing the unwise with powerful blows of their bony-elbow wings.
Eleven million years ago, however, the swans in what is now Japan did something unexpected: they went to the oceans. In an article published this week in The Bulletin of Gunma Museum of Natural History, Japanese paleontologists formally described this family or genus of swans, Annakacygna, which had long, filter-feeding heads, small wings and very odd hips — all of which lead the way. researchers call it the “ultimate bird.”
The first set of Annakacygna remains — a nearly articulated skeleton in a stone slab from a riverbed in Japan’s Gunma Prefecture — was unearthed in 2000 by a Japanese fossil hunter. After the fossil hunter donated the remains to the Gunma Museum of Natural History, museum director, Hasegawa Yoshikazu, enlisted Hiroshige Matsuoka, a paleontologist, to examine them.
Initially thought Dr. Matsuoka that he was looking at a strange duck, perhaps an animal that dove into the oceans just off the coast of the then newly emerged Japanese archipelago. But when the bones were taken off the plate, he concluded that the short-winged skeleton belonged to a flightless swan.
The species, which he and his co-author Dr. Yoshikazu Annakacygna hajimei was about four feet long, the size of the modern black swan. Another set of remains from a related species, which they named A. yoshiiensis, suggested a bird as long as the largest living swan species, the 5½-foot trumpeter swan.
Both birds were “thicker and heavier than these modern swans,” said Dr. Matsuoka. When he compared their remains to the dissected body of an existing swan, he found that the birds differed from each other in other ways as well. Their tails were very mobile. Their hips were unusually wide and strong, and their bones were thicker than normal for a water bird, allowing them to ride low in the water.
Strangest thing were the wings. Flying birds usually lose some of the usefulness of their wings, said Dr. Matsuoka, a process called degeneracy. But in Annakacygna, the shoulder joints and muscle attachments that pull the arms back were unexpectedly well-developed, with uniquely shaped wrists that kept the digits—and thus the wings—permanently flexed.
At first, these wings amazed the team. But while watching a video of a mute swan holding a chick on her back, Dr. Matsuoka received a brain wave. Many modern swans usually carry their young piggybacks, he said, with their wings back and up to protect the chicks. That attitude in Ananakacygna’s modern relatives suggested a new possibility: that the flightless swans might have captured this behavior in their anatomy, converting their curved wings and broad hips into specially adapted cradles to safely transport chicks across the salty depths.
The swans were also well-adapted to a coastal lifestyle in another way: long, filter-feeding beaks resembling those of shovelers, allowing them to forage for plankton in the cool, rich seas off Japan’s coast. Modern swans, on the other hand, have straight beaks that nibble on vegetation.
Flightlessness is not uncommon in waterfowl; modern steamer ducks, a few species of teal, and several extinct species of geese closed the air to the water. Some of these waterfowl reached remarkable sizes: Malta’s Pleistocene giant swan, which some researchers have suggested was land-bound, was 30 percent larger than a living mute swan.
But while it’s smaller, said Dr. Matsuoko, Annakacygna is in a class of its own. “I think all wild animals live for two purposes,” he said, which is to maintain the self (by eating) and the species (by procreating). is something special.
“It’s the best way to survive as an animal,” he said. “That’s why we call it the ‘ultimate bird.'”