The discovery of a well-preserved beaver-like mammal fossil from Middle Jurassic deposits in China suggests early mammals may have been a more diverse group than previously thought, Chinese and US scientists reported on Thursday.

The new species, Castorocauda lutrasimilis, is an unusual fossil in a number of ways, the researchers reported in the Feb. 24 issue of the journal Science. The first author of the study, Qiang Ji, is a professor at Nanjing University in China.

The mammal had a broad scaly tail, fur, swimmer's limbs and seal-like teeth for eating fish, and it lived 164 million years ago, noted the scientists.

Its well-preserved fur and scale imprints, along with the suggestion of soft tissue webbing in the hind limbs and its partial skeleton provide a wealth of information compared to the teeth and few scraps of skull known from most mammal fossils of this age.

"The animal is the earliest swimming mammal to have been found, and was the most primitive to be preserved with fur," said Zhe-xi Luo, co-author of the paper at the Carnegie Museum of Natural History in the United States.

It is also the largest known Jurassic early mammal, about the size of a small female platypus, while all other Jurassic mammals are small, said the researchers.

"Castorocauda is the largest known Jurassic mammalia form," they wrote in the Science paper. "By its preserved skull length and the well-established scaling relation of skull and body mass, we estimate that the body mass of the holotype specimen was at least 500 g."

"The preserved length from rostrum to tail is 425 mm, but the actual body length is certainly greater...We estimate the upper limit of body mass to be approximately 800 g for Castorocauda."

The combination of some primitive skull features and the specialized features of fur, swimming and burrowing adaptations and fish-eating, all indicate that early mammals had begun to specialize and move into new environments long before the dinosaurs' end 65 million years ago, the researchers said.

This new discovery is "exciting," commented Thomas Martin, a paleontologist at the Forschungsinstitut Senckenberg in Frankfurt, Germany.

It "pushes back the mammalian conquest of the waters by more than 100 million years," Martin wrote in a commentary article in the Science journal.

"These exciting discoveries may just be a glimpse of what is to come. They dramatically demonstrate how many gaps remain in our knowledge of Mesozoic mammalian diversity."

(Xinhua News Agency February 24, 2006)