You are viewing the content for Friday 24 February 2006

‘Jurassic beaver’ changing scientists’ view of early mammals

By John von Radowitz
A ‘JURASSIC beaver’ that was paddling around before the hey-day of the dinosaurs is changing scientists’ view of early mammals.

Well-preserved fossil remains of the creature, which lived 164 million years ago, were discovered in China.

Like modern beavers, the animal had fur, a broad scaly tail and webbed feet. It also had forelimbs adapted both for burrowing and swimming, and seal-like teeth designed for eating fish.

The advanced features are leading scientists to rethink the early evolution of mammals.

They indicate that mammals had already begun to specialise and move into new environments long before the dinosaurs’ reign ended 65 million years ago.

Previously it was thought that mammals only really began to develop and diversify once dinosaurs had left the planet.

Until they had a niche to walk into, most were believed to be primitive shrew-like creatures scuttling between the feet of the giant reptiles.

But the newly discovered creature, named Castorocauda lutrasimilis, was a highly developed animal with sophisticated adaptations for aquatic life.

It is also the largest known Jurassic early mammal, estimated to be more than 45 centimetres long and to weigh just over 500 grams.

Castorocauda’s most beaver-like feature is its tail, which is broad and flat and partially covered in scales.

Describing the find in the journal Science, researchers led by Dr Zhe-Xi Luo, from the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, the US, and Nanjing University, China, wrote: "The broad and scaly tail of Castorocauda was similar to that of the modern beaver Castor canadensis, a semi-aquatic placental mammal well adapted for swimming."

One reason why the Castorocauda fossil is so important is that it is unusually well-preserved.

Most of what is known about mammals from this era has been scratched together from teeth and a few skull fragments.

But the new fossil includes the unique preservation of fur and scale imprints, as well as a partial skeleton and the suggestion of soft tissue webbing in the hind limbs.

The creature’s vertebrae and tail are very similar to those of modern beavers and otters.

"All other Jurassic mammals are small," wrote the scientists. "Constrained by their small size, most were generalised terrestrial insectivores or omnivores.

"This fossil shows that basal mammals occupied more diverse niches than just those of small insectivorous or omnivorous mammals with generalised terrestrial locomotory features."