Most significantly, the animal's fur and soft tissue are also fossilized. There are clear impressions of both hair and underfur, along with hair-related skin structures.
Younger, Cretaceous-period (146 to 66 million years ago) mammals typically had naked tails, like rats. But Castorocauda's scaly tail was covered with hairs in varying concentrations along its entire length.
The tail bones are very flat and compressed, seemingly specialized for swimming, just as they are in modern beavers and otters.
There are remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. The forelimbs may have been used for rowing, and they were almost certainly used for digging and burrowing.
Castorocauda's plated ribs suggest that it was not completely aquatic. The bones could not have provided the buoyancy control required in mammals that never leave the water. The "beaver" probably walked awkwardly on land with splayed limbs.
There are no apparent mammary glands preserved on the specimen; Luo has concluded that this individual was mostly likely a male.
Castorocauda has the ankle spurs characteristic of its nearest living relative, the platypus, which uses them for territorial defense. And like the platypus, Castorocauda was probably an egg-layer, Luo says.
Hans-Dieter Sues is associate director for research and collections of the National Museum of Natural History in Washington, D.C. Sues was not involved in the study and considers the discovery significant.
"Castorocauda now shows that mammals returned to the water much earlier than anyone had previously thought," he said.
"The unusual preservation of the fossil provides clear evidence of beaver- or platypus-like adaptations to a semiaquatic life in a Mesozoic animal," he said. The Mesozoic era lasted from about 251 to 66 million years ago and includes the Triassic, Jurassic, and Cretaceous periods.
Castorocauda was not large by modern mammal standardsonly about 17 inches (43 centimeters) from its nose to the tip of its 5-inch (13-centimeter) tail. The study authors estimate that the largest Castorocauda could not have weighed more than about 1.75 pounds (800 grams).
But as small as Castorocauda seems, other Jurassic mammals were even smaller, some of them weighing less than 2 ounces (57 grams). They were limited to dining on plants, insects, and perhaps small vertebrates.
Fish for Dinner?
Castorocauda, on the other hand, probably snacked on seafood.
"The interesting thing is that we always have the stereotype that Mesozoic mammals are small and constrained by their size, specialized for not much of anything, except just living on the ground," said study co-author Luo, who is a National Geographic Society grantee.
"But now we have Castorocauda, capable of swimming. So the broader picture with this discovery is that, with its larger body size and its aquatic adaptation, this find signifies that early mammals were in the aquatic niche and had developed more diverse feeding and locomotive adaptations."
Luo emphasized that this animal, beaverlike though it may be, is not a beaver.
"People are fascinated by beavers in general," he said, "so they like to associate beavers with this very cute animal."
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