Scientists have long thought that during dinosaurs' time mammals were tiny, rat-like creatures that kept a low profile in order to survive. However, a discovery in northern China of a furry aquatic creature with seal-like teeth and a flat tail like a beaver has demolished that image. This creature, that lived around 164 million years ago, weighted more than half a kilogram and was around half a meter in length. It used to swim in lakes, eating fish and lived alongside dinosaurs. This is the largest mammal from this time period discovered so far.

The Chinese archaeologists led by Qiang Ji of Nanjing University found the well-preserved fossil in the Jiulongshan Formation in Inner Mongolia. Besides the skeleton including teeth, there were also impressions of soft tissue and fur, both guard hairs and short, dense under-fur that probably kept the water from the skin.

"Based on its relatively large size, swimming body structure, and anterior molars specialized for [fish] feeding, Castorocauda was a semiacquatic carnivore, similar to the modern river otter," the team writes in the paper announcing the find in today's issue of Science.
"Its lifestyle was probably very similar to the modern-day platypus," Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, said in a statement. "It probably lived along river or lake banks. It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nest."

According to Thomas Martin of the Research Institute Senckenberg in Frankfurt, Germany, who was not part of Luo's team, this discovery pushes back the date of the mammal conquest of the waters by more than 100 million years. "We stand at the threshold of a dramatic change in the picture of mammalian evolutionary history," he said. "The potential of fossil-rich deposits like the Jehol group in Liaoning Province in China or the Jiulongshan Formation in Inner Mongolia is only just beginning to be exploited."

Modern semi-aquatic mammals such as beavers and otters and aquatic mammals like whales did not appear until between 55 million years ago and 25 million years ago. This is the first evidence of much earlier semi-aquatic mammals and indicates a greater diversification than previously thought.

Although the animal looks in many ways like a beaver, it is not actually related to modern beavers. Nonetheless, scientists named it Castorocauda lutrasimilis - Castoro from the Latin for beaver, cauda for tail, lutra for river otter and similis meaning similar.

Image credit: Mark A. Klinger / CMNH