As a Jurassic predator, it was hardly in the big league. But compared with the shrew-like stature of the earliest mammals, it was a fearsome giant.Meet the Jurassic 'beaver' from China. Living 164 million years ago, it's the oldest known furry member of the mammal family, and the first known to master swimming.The specimen, described in Science this week1, was found by palaeontologists trawling the collections of the Jinzhou Museum of Palaeontology in China. Zhexi-Luo, of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and his team christened the creature Castorocauda lutrasimilis, meaning beaver-tailed and otter-like. Like today's river otter, C. lutrasimilis probably lived in a burrow and hunted fish. And, a bit like a modern beaver, it sported a broad, flat, scaly tail.

At almost half a metre long, the animal is the biggest-known mammal-like creature of its time, and shows a hitherto unsuspected diversity in the shapes and sizes of the earliest mammals.C. lutrasimilis would have been dwarfed by the giant reptiles that lived alongside it - but, says Luo, its burrowing lifestyle probably kept it safe.The discovery shows that early mammals and their relatives were experimenting with different lifestyles from the start, rather than waiting for the decline of the dinosaurs before diversifying.

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