Jurassic "Beaver" Found; Rewrites History of Mammals

Nicholas Bakalar
for National Geographic News
February 23, 2006

It looks a lot like a beaver—hairy body, flat tail, limbs and webbed feet adapted for swimming—but it lived 164 million years ago.

A well-preserved fossil mammal discovered in northeastern China (map) has pushed the history of aquatic mammals back a hundred million years, a new study says.

It is the oldest swimming mammal ever found and the oldest known animal preserved with fur, the researchers say in their report, which will be published in tomorrow's issue of the journal Science.

"The origin of fur predates the origin of modern mammals," said study co-author Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

"This discovery has pushed fur-bearing nearly 40 million years further into the past," Luo said.

Until now, the oldest mammal fossils found with fur, such as the mammal Eomaia scansoria, were about 125 million years old.

The newfound specimen is called Castorocauda lutrasimilis—the name, loosely translated, means "beaver-tailed and looks like a river otter."

Scientists call Castorocauda a mammaliaform or a basal mammal, although the term "mammal" is often used for these Jurassic forms. (The Jurassic period ran from about 200 to 146 million years ago.)

So Castorocauda isn't a mammal in the modern sense. But it is the immediate predecessor of modern mammals, such as the platypus and the echidna.

Almost Complete Skeleton

Most mammal specimens this old consist of no more than a few broken, fossilized bones. But this specimen is an almost complete skeleton.

Even tiny middle-ear bones are intact. The well-preserved teeth—incisors, canines, premolars, and molars—look to have been ideal for feeding on fish and aquatic invertebrates, somewhat like the teeth of modern seals.

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