Early web-footed bird made impression.

Scientists don't know if many birds 110 million years ago looked like a duck, walked like a duck, or quacked like a duck, but they're now sure of one thing--some had webbed feet.

Researchers discovered the fossil tracks of an otherwise unknown bird in sediments near Jinju, South Korea. The scientists, who described their find in the June 21 online issue of NATURWISSENSCHAFTEN, say the imprints push back evidence of web-footed birds by at least 25 million years.

Birds probably laid down the tracks as they waded in mud, wet sand, or shallow water to search for food, says Jong-Deock Lim of the Natural History Museum at the University of Kansas at Lawrence. The areas between the mystery birds' toes weren't fully webbed, which suggests that the animals were only occasional swimmers, says Lim, who is lead author of the report.

The webbed fossil footprints, which appear alongside fossil prints of unwebbed birds and dinosaurs, also show evidence of a rear-pointing hallux, or first toe. Such an opposable toe would enable birds to grip branches more effectively. Scientists suspect that the earliest shore birds, having recently evolved from tree-dwelling relatives, would have had such a toe, Lim says. No dinosaur tracks found to date show a rear-facing hallux, he adds.

The variety of tracks found at the South Korean site indicates that there was already considerable diversity among shorebirds 110 million years ago, Lim says.

"We know from Chinese deposits of that time that there were quite a few birds not similar to modern birds," says Storrs L. Olson, curator of birds at the National Museum of Natural History in Washington, D.C. "Now, it would be nice to find the bones to go with [these South Korean tracks]."