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An early whale had large back legs, a tail like a dog's, and a hip-wiggling swimming style, according to a new fossil study. The discovery helps pinpoint the advent of "modern" whale flukes to between 38 and 40 million years ago, scientists say.

Flukes are the two wide, flat triangular lobes on a whale's back end and are made of skin and connective tissue, with bones in the middle.

Scientists have known whales evolved from semiaquatic, four-footed creatures with long, thin tails to today's fully aquatic mammals with fluked tails, no back legs, and flippers instead of front legs.

But it was previously unknown when the tail flukes first arose in the whale family tree.

"What's interesting about this animal is that it had these back legs that it used to push itself through the water," said study author Mark D. Uhen, a paleontologist from the Alabama Museum of Natural History.

"This animal didn't have flukes, but the ones just a little bit younger [geologically] did. So we can really narrow that time frame now."

Uhen's study is detailed in the latest issue of the Journal of Vertebrate Paleontology.

Fossil Puzzle

Amateurs found different parts of the newly described fossils over time in Coffeeville Landing, Alabama.

After the various fossil parts were brought to the University of Alabama in 2005, Uhen realized that all the pieces belonged to the same individual of the species Georgiacetus vogtlensis. "This is not a new species," Uhen said. "What's significant about it is that we learned more about a species that we already knew." After analyzing the fossils for almost three years, Uhen concluded the individual had a tail, but no fluke, and that Georgiacetus wiggled its hips and moved its entire trunk up and down through the water to move forward—a swim stroke whales no longer use.

"We knew some fossil whales had a tail fluke from slightly younger [fossils]. But we hadn't had decent tail vertebrae to tell where [in time] the whales had tails and where they didn't," Uhen said. "This one little vertebra tells us that Georgiacetus vogtlensis didn't have a tail fluke."

Surprise

Jonathan Geisler, of Georgia Southern University, said the new findings, if true, would force a rethink of early whale migrations.

In a 2005 paper Geisler and colleauges hypothesized that the evolution of flukes helped early whales scatter around the world from their birthplace in South Asia.

"If Georgiacetus, which is known only from North America, did not have tail flukes,"—as the new study suggests—"then our hypothesis would be wrong, and we would have to look elsewhere to explain the dispersal of early whales into the different ocean basins," said Geisler, who was not involved with the current study.

According to Geisler, the findings are a surprise, because previously evidence about the base of the tail in Georgiacetus suggested tail flukes.

"I would have guessed that it did have flukes," he said. "Of course that is the great aspect of paleontology—new fossils can lead to new understandings."