February 23, 2006

Early beaver-like mammal swam with the dinosaurs

WASHINGTON (Reuters) - A creature that looked like a
beaver, right down to its flat paddle-like tail and webbed
feet, lived 164 million years ago alongside dinosaurs, U.S. and
Chinese researchers reported on Thursday.

It might not have gnawed trees as modern beavers do -- its
teeth suggest it ate fish -- but the little animal had fur and
even the inner ear structure of a mammal, they report in
Friday's issue of the journal Science.

Thomas Martin of the Forschungsinstitut Senckenberg in
Frankfurt, Germany, said the finding showed mammals had
conquered the water 100 million years earlier than anyone

"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," Martin wrote in
a commentary.

The fossil was found in the Middle Jurassic Jiulongshan
Formation in China, a deposit rich in the fossils of dinosaurs,
early insects and other creatures. It dates to 164 million
years ago.

Qiang Ji and colleagues at Nanjing University in China and
the Carnegie Museum of Natural History in Pittsburgh said the
animal, which they named Castorocauda lutrasimilis, would have
weighed just 500 grams (18 ounces).

"It is the most primitive taxon in the mammalian lineage
known to have fur and has a broad, flattened, partly scaly tail
analogous to that of modern beavers," they wrote.

They found remnants of fur, scales on the tail and, in
between the back toes, webbing.