It had a broad scaly tail, fur, swimmer’s limbs and seal-like teeth for eating fish, and lived 164 million years ago, researchers say.

The discovery of an extraordinarily well-preserved beaver-like mammal fossil suggests mammals may have been a more diverse group during the dinosaur era than previously thought, a study suggests.

The researchers said the finding shows mammals were expanding into a variety of ecological niches even then.

The species, Castorocauda lutrasimilis, has provided a fossil that is unusual in several ways, said the scientists, Qiang Ji of Nanjing University in Nanjing, China and colleagues.

Their study appears in the Feb. 24 issue of the research journal Science.

The discovery “pushes back the mammalian conquest of the waters by more than 100 million years,” wrote Thomas Martin of The Senckenberg Research Institute in Frankfurt am Main, Germany, in a commentary in the journal.

The fossil was found in deposits of rock from the middle Jurassic era in China, scientists reported.

They wrote that the fossil provides a wealth of information compared compared to the teeth and few scraps of skull known from most mammal fossils of its time period. Castorocauda has uniquely well preserved fur and scale imprints, along with the suggestion of soft tissue webbing in the hind limbs and partial skeleton, they observed.

Castorocauda is also the largest known Jurassic early mammal, they added. It was about the size of a small female platypus, or a bit under two feet (60 cm) long.

The primitive skull features and specialized features of fur, swimming and burrowing adaptations, and fish-eating all suggest specialization and invasion of new environments, the scientists argued. The dinosaurs died out about 100 million years later, except for birds, which are believed to be their descendants.

Traditional thinking has it that mammals had to wait until the dinosaur extinction to find significant new ecological niches to move into. But recent findings have been chipping away at that belief, including finding last year of a mammal that ate dinosaur babies.

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.