An international team of researchers have discovered the remains of a new species of prehistoric mammal, which challenges the conventional theories of mammalian evolution.

The team, from the Carnegie Museum of Natural History, Nanjing University and the Chinese Academy of Geological Sciences, found the remains of a beaver-like creature referred to as Castorocauda lutrasimilis in excavations taking place in the Middle Jurassic Jiulongshan Formation of the Inner Mongolia region of China.

The remains date back 164 million years, making the animal - which is widely being referred to as the "Jurassic beaver" – a contemporary of the dinosaurs.

Like the modern beaver, to which it is not closely related, Castorocauda lutrasimilis had a scaly tail, webbed feed for swimming and fur – some of which was preserved with the fossil remains. It also had seal-like teeth adapted for eating fish.

The discovery is causing consternation amongst scientists, who had previously believed that the only mammals living at this time were land-dwelling.

But the Jurassic beaver was clearly adapted to living in the water, and was bigger than any known mammals of that era – the researchers state that it was around the size of a platypus - posing a challenge to current thinking about how our oldest ancestors developed.

"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," Thomas Martin of the Forschungsinstitut Senckenberg in Frankfurt declared in the academic journal, Science.