Beaver: Castorocauda lutrasimilis life reconstruction. The artwork of the reconstructed animal is 50% of actual fossil size. Illustration: Mark A. Klingler/CMNHMost early fossil mammals were tiny, rodent-like, and ate primarily insects. Paleontologists assumed that mammals didn’t diversify much or grow to large sizes until their dinosaurian competition was eliminated. Recent fossil finds from China have included larger early mammals that have changed this view of mammal evolution. And now, with the new discovery of the beaver-like Castorocauda lutrasimilis, we know that a hefty mammal (about a half-meter long) evolved to live a semi-aquatic lifestyle way back in the Jurassic Period, a time on earth best known for enormous dinosaurs!

Assembly by committee?
Castorocauda is not directly related to any modern mammals. But it resembles several! Castorocauda looks like a combination of otter, beaver, and platypus. With webbed hind feet and a scaly paddle-shaped tail, Castorocauda was clearly a semi-aquatic animal. This early mammal ate more than just insects. Its seal-like teeth suggest it ate fish. Other aquatic mammals aren’t found in the fossil record until many millions of years later, making Castorocauda the first of its kind.

Record for the oldest fur pelt
Early fossil mammals are known primarily from their teeth. Other skeletal remains are rare. Castorocauda has a near-complete skeleton and its fur was remarkably fossilized, too! The fossil fur is similar to that of modern mammals that have adapted to swimming in cold water. At 164 million years old, this fur is the oldest ever discovered.

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