Scientists discover early mammalian fossil

More than 164 million years ago, when dinosaurs dominated the Earth, a small beaver-like creature with a tail most Canadians would recognize was holding its own in world of Jurassic giants.

The discovery in China of a fossilized skeleton of a creature with a flat, paddle-shaped tail is significant because, until now, scientists believed the mammals that lived alongside the dinosaurs were tiny, shrew-like generalists.

This animal, with webbed hind feet, strong arms for digging and otter-like teeth, shows that mammals became highly specialized long before the dinosaurs disappeared 65 million years ago.

"We now know that early mammals are really far more diverse. They got into the water. They were good diggers. Previously, we thought this would be possible only with far more advanced mammals," says Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh.

He and his colleagues named it Castorocauda lutrasimilis, but say the animal isn't the direct ancestor of the North American beaver or European beavers. Dr. Luo says its line appears to have died out long before the ancestors of modern beavers emerged, about 25 million years ago, reports Globe and Mail.

According to Xinhua, 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."


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