It looked like an otter, had teeth like a seal, a tail like a beaver and could swim. And though it died 164 million years ago, it's making a big splash in 21st century studies of mammalian evolution.

Mark A. Klingler Castorocauda lutrasimilis reconstruction Click photo for larger image. Graphic: Castorocauda lutrasimilis, on land and in water

A complete fossil skeleton of a creature dubbed Castorocauda lutrasimilis (Latin for beaver tail, similarity to the otter) reveals that some mammals adapted to watery environments 100 million years earlier than scientists had thought, said Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum. Dr. Luo was a member of the research team that described the find today in the journal Science.

Castorocauda's discovery -- in what was once a shallow, freshwater lake in northern China -- also paints a fascinating new picture of mammalian life in the Jurassic period of the Mesozoic era.

Scientists have long believed that the mammals of the time, living alongside dinosaurs, were small, shrew-like creatures that munched on insects and dwelled on land.

But at about 1¹/₂ pounds, Castorocauda is "about the same size as the modern platypus, which is also a swimming and digging mammal," Dr. Luo said. Its lake also was home to small fish and aquatic invertebrates, which likely were a food source for the animal.

Castorocauda's line died out, yet some of its anatomical features appear to have been reinvented in modern-day mammals such as the beaver and the platypus.

"Many organisms derive not from the same common ancestor, but evolve the same structures with the same function," Dr. Luo said. In Castorocauda, he said, "we have one of the most dramatic examples of convergence."

The fossil is spectacular in every way and makes a strong statement about how organisms adapt to their environments, said Matthew Carrano, dinosaur curator at the Smithsonian's National Museum of Natural History. He was not involved in the research.

"To see what the whole animal looks like is extremely rare," he said. "It's something that might happen once or twice every 100 years that we get something like this."

Castorocauda also gives scientists a new marker in the timeline of mammalian fur development.

"This must have been one of the very early things that distinguished mammals from other animals in their evolutionary history," Dr. Carrano said. And it likely means that mammals that came after Castorocauda probably had fur.

The discovery of fossils like Castorocauda add a wealth of material in efforts to fill out the mammalian evolutionary tree.

"If we found two or three of these animals in the early lineages, that will really help us clarify what's going on for early mammal evolution," Dr. Carrano said. "No matter what you learn, you're never done. That's what makes it fun."

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Correction/Clarification: (Published Feb. 25, 2006) A graphic accompanying a Feb. 24, 2006 story about a new fossil find gave incorrect credit information. The source was the Carnegie Museum of Natural History, and the name of the illustrator is Mark A. Klingler of Carnegie Museum of Natural History.