Jurassic Fossil Breaks the Mold

By Guy Gugliotta
Washington Post Staff Writer
Friday, February 24, 2006

In modern times it would be called a chimera, a furry, fish-eating swimmer and burrower a bit bigger than a tree squirrel, with an otterlike body, teeth like a seal, webbed hind feet like a platypus and a flat tail just like a beaver.

But it lived -- and died -- about 164 million years ago in a swampy Jurassic lake bed northeast of Beijing in what is now the Inner Mongolia Autonomous Region of China. Its multinational team of discoverers called the fossil Castorocauda lutrasimilis -- "beaver-tailed sort-of otter."

Its discovery, reported in today's issue of the journal Science, showed for the first time that mammals diversified much earlier than was thought, even in an age dominated by dinosaurs.

"This guy swam like a beaver or a modern-day platypus; he was mostly a carnivore, but certainly capable of eating plants," said team member Zhe-Xi Luo, a paleontologist at Pittsburgh's Carnegie Museum of Natural History. "What kind of modern animal is he most like? Your guess is as good as mine."

Castorocauda , though hardly formidable, is the largest mammal of its time ever found, and by a wide margin is the oldest known aquatic mammal, predating the river otter by more than 100 million years.

Most early mammals were tiny shrewlike insect-eaters with all-purpose physical characteristics and habits that probably made it easier to survive in hostile environments, said Michael Novacek, curator of paleontology for New York's American Museum of Natural History. By contrast, he added in an interview, the new animal is a specialist, "well outside that box."

Luo said the team led by Qiang Ji, of China's Nanjing University, found Castorocauda in Inner Mongolia's Jiulongshan Formation, so well preserved in lake deposits that the fossil imprint showed evidence of fur, beaverlike scales on the wide, flat tail and soft tissue webbing between the hind toes.

"The fur is a key mammalian indicator," Luo said in a telephone interview. "This guy was warm-blooded all the way." The teeth, he added, included specialized molars enabling Castorocauda to eat both plants and animals, a characteristic of Docodontans, an ancient mammalian lineage that appears to have died out 125 million years ago.

Castorocauda was about 17 inches long and probably weighed 1 to 1 3/4 pounds, Luo said. It had front paws capable of burrowing, and its tail and webbed hind feet enabled it to swim in "doggy-paddle" fashion, Luo added.

Paleontologist Jason A. Lillegraven, a Docodontans specialist retired from the University of Wyoming, noted that the world's first mammals appear to have arisen about 200 million years ago, with their history paralleling that of the dinosaurs right up to the moment of the dinosaurs' extinction 65 million years ago.

Only after that did Earth begin to see horses, elephants, rhinoceroses and other large mammals, Lillegraven said in a telephone interview, and in this context Castorocauda , "while very old, is not near the beginning," and although it is "large for its time, it is not huge."

Instead, he said, the new animal's significance lies in its ability to "carve out its own niche" at a time when dinosaurs were dominant. The Jiulongshan Formation also has produced the fossils of flying dinosaurs, known as pterosaurs, and coelurosaurs, an early carnivorous dinosaur.

"I can say, though, that this new fossil to me is wholly expected," Lillegraven said. "For years, we've had this biased viewpoint toward mammals, thinking they were much more ecologically restrictive -- that they were too small to specialize. I've thought for a long time that that was totally wrong, and this proves it. It's great stuff."

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