This image provided by the journal Science shows a Castorocauda lutrasimilis, a furry aquatic creature with seal-like teeth and a flat tail like a beaver that lived some 164 million years ago, when dinosaurs ruled.

WASHINGTON — For years the mammals living in the era of dinosaurs have been thought of as tiny shrew-like creatures scurrying through the underbrush. Now the discovery of a furry aquatic creature with seal-like teeth and a flat tail like a beaver has demolished that image.

Some 164 million years ago the newly discovered mammal was swimming in lakes in what is now northern China, eating fish and living with dinosaurs.

"Its lifestyle was probably very similar to the modern day platypus," Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, said in a statement. "It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest."

Luo was part of a team led by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing that discovered the remains in the Inner Mongolia region of China. They report their findings in today's issue of the journal Science.

It's the first evidence that some ancient mammals were semi-aquatic, indicating a greater diversification than previously thought, according to the researchers.

The animal is not related to modern beavers or otters but has features similar to them. Thus the researchers named it Castorocauda lutrasimilis. Castoro from the Latin for beaver, cauda for tail, lutra for river otter and similis meaning similar.

The animal had fur, a broad scaly tail with vertebra similar to those in a beaver or otter, swimmer's limbs and seal-like teeth for eating fish, they said.