An artist's impression of Castorocauda lustrasimilis
The discovery of a beaver-like fossil that lived when the dinosaurs ruled the Earth could challenge some currently accepted ideas on mammal evolution.
Castorocauda lutrasimilis, which was unearthed in China, is a species previously unknown to science.
It dates back to 164 million years ago, a time when mammals were thought to be primitive creatures confined to land.
But this animal was adapted to life in water, meaning scientists may now have to rethink their theories.
The fossil was found in the Middle Jurassic Jiulongshan Formation, a deposit rich in the remains of dinosaurs, early insects and other organisms.
The creature had fur, a broad scaly tail, and webbed feet for swimming. It was about 42cm in length and had seal-like teeth for eating fish.
Castorocauda lutrasimilis resembled a modern-day beaver, but belonged to a group that became extinct long before rodents appeared.
Such advanced features have surprised many scientists, suggesting mammals that lived during the hey-day of the dinosaurs had already conquered a variety of environments.
The mammals of the time were once thought to be largely primitive shrew-like creatures, scuttling at the feet of dinosaurs, and only flourishing when the dinosaurs died out some 65 million years ago.
Commenting on the find, revealed in the journal Science, Thomas Martin of the Forschungsinstitut Senckenberg in Frankfurt, Germany, said it showed mammals had conquered the water 100 million years earlier than previously thought.
"This exciting fossil is a further jigsaw-puzzle piece in a series of recent discoveries, demonstrating that the diversity and early evolutionary history of mammals were much more complex than perceived less than a decade ago," he wrote.
Image courtesy of Mark A Klinger/CMNH.