• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

Recent Aquatic Hairy Mammal Find Dates Back to Age of Dinosaurs

Posted on: Tuesday, 28 February 2006, 21:00 CST

By Wu Chong

It is the oldest swimming mammal ever found. It looks like a beaver, with a hairy body, a scaly tail, limbs, webbed feet and seal- like teeth for eating fish. But it lived 164 million years ago, alongside dinosaurs.

The discovery of a well-preserved fossil from Middle Jurassic deposits in North China has pushed the history of aquatic mammals back a hundred million years, Chinese and US scientists announced last week.

The new species, named Castorocauda Iutrasimilis meaning "beaver- tailed and river otter resembling," suggests that early mammals may have been a more diverse group than previously thought, according to the report in the February 24 issue of the journal Science.

Ji Qiang, the first author of the study and professor with Nanjing University, said the fossil is unique in many ways, being the earliest proof of mammalian conquest of the waters, compared with most Mesozoic mammals which were previously known to have inhabited land and fed on a monotonous diet of insects.

The Mesozoic Era lasted from about 251 to 66 million years ago and includes the Triassic, Jurassic (about 200 to 146 million years ago) and Cretaceous periods.

Of all the discoveries about the fossil mammal, the hairy body and signs of swimming are the most significant, said the study's co- author Luo Zhexi, also curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh, during a phone interview on Monday with China Daily.

The fossil mammal is the oldest known animal preserved with fur, he said. Carbonized in the fossil, the short and dense under-furs were to keep water from the skin; the longer guard hairs are preserved as impressions on the fossil slab.

Fossilized furs of this animal provide fresh evidence on phylogenetic evolution of mammalian fur, a specialized pelt developed well before the rise of modern mammals.

"It is anticipated that all mammals have hair, which is a fundamental feature to all mammals to keep a constant body temperature," said Luo, who was born in southern China and pursued his doctorate degree in the United States a couple of years ago.

"And the discovery has pushed fur-bearing nearly 40 million years further into the past."

Until now, the oldest mammal fossils found with fur are about 125 million years old.

However, despite some similarities with modern mammals, the new species discovered in China outsizes them all. It has no modern descendant and had no direct relationship with modern placental mammals, Luo noted.

"We call it a mammaliaform," he said.

And the striking discovery about the animal's semi-aquatic lifestyle has revealed that early mammals may have been a more diversified group than previously thought, said the scientist.

The well-preserved teeth somewhat resemble those of modern seals, considered as ideal for feeding on fish and aquatic invertebrates.

"Traditionally, we had the stereotype that Mesozoic mammals were small and shrew-like, and limited by their size, living on the ground and eating insects," Luo said.

"But now we have Castorocauda, capable of swimming."

He said the new species probably led a lifestyle very similar to the modern day platypus. "It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest," Luo said.

The tail bones of the mammal are flat and broad, covered by some scales, suggesting a perfect shape that helped propel it through water like the modern beaver.

There are remnants of soft tissue between the toes of the back feet, signifying they were webbed. The mammal may have used its forelimbs for rowing, but certainly used them for digging and burrowing.

Although it is for certain that the mammal could swim, its plated ribs imply that it was not completely aquatic. And the bones could not have provided the buoyancy control required in mammals that never leave the water.

Luo said modern semi-aquatic placental mammals, such as beavers and otters, and fully-aquatic placental mammals, such as whales, did not appear until 55 to 25 million years ago.

By comparison, Castorocauda is at least 164 million years old, which indicates that primitive docodont mammalia-forms evolved semi- aquatic swimming independently in the Mesozoic, almost 100 million years earlier than placental mammals, such as beavers and otters.

With a size almost of a small female platypus, Castorocauda is also the largest known Jurassic early mammal, about 43 centimetres from its nose to the tip of its 13-centimetre tail.

"By its preserved skull length and the well-established scaling relation of skull and boy mass, we estimate that the body mass of the holotype specimen was at least 500 grams," the study authors wrote in the Science.

They also estimate that the upper limit of body mass for this new species of mammal could not have weighed more than about 800 grams.

But, it is still much larger than other Jurassic mammals, some of which were only 57 grams in weight.

Different from most mammal specimens of this age, the fossil contains an almost complete skeleton, including intact tiny middle- ear bones.

There are no apparent mammary glands preserved on the specimen, so Luo concludes that this specimen was most likely a male.

Compared to the platypus, the mammal ancestor has something like ankle spurs for territorial defence. And it was probably an egg- layer too, said Luo.

The scientist said the fossil was disinterred in Daohugou, Ningcheng County in the Inner Mongolia Autonomous Region in North China, where a pterosaurs fossil was previously discovered.

"This latest discovery shows the location's high potential for more fossils from Middle Jurassic. We can expect more amazing discoveries there in the future," he said.

Thomas Martin of the Forschungsinstitut Senckenberg in Frankfurt, Germany, said the exciting fossil is a "further jigsaw-puzzle piece" in a series of recent discoveries.

It demonstrates that the diversity and early evolutionary history of mammals were much more complex than perceived less than a decade ago.

Source: China Daily; North American ed.

More News in this Category