Castorocauda lutrasimilis : Largest known Jurassic mammal discovered

Discovery of the fossil of a large beaver-like mammal - Castorocauda lutrasimilis - shows that some large mammals lived alongside dinosaurs.

13 February 2006
MUMBAI, INDIA

Evolution theories received a jolt with the discovery of a beaver fossil that dates back to 164 million years. Scientists believe that at that period, all mammals were confined to land. However, the fossil indicates that this particular mammal was adapted to aquatic life, a discovery that suggests that mammals played a greater role in the ecosystem than has thus far been understood.

The fossil is being called Castorocauda lutrasimilis (castoro means beaver in Latin, casuda means tail, lutra means river and similis means similar) and was discovered by a research team led led by Qiang Ji of the Chinese Academy of Geological Sciences in Beijing. The discovery was made in in the Inner Mongolia region of China.

It was found in the Middle Jurassic Jiulongshan Formation, had fur, a broad scaly tail, and webbed feet for swimming. In size it is comparable to a small platypus, and possessed seal like molars for fish consumption. It also showed webbed hind feet, and strong arms for digging.

Such advanced features are leading to speculations about how advanced mammals actually were at the time of dinosaurs. Earlier myths (especially since Jurassic Park) about mammals suggested that mammals were primitive, shrew-like creatures at the time of dinosaurs and only began to flourish 65 million years ago, once dinosaurs began dying out.

According to the scientists, the animal is not a direct ancestor of the North American beaver or European beavers, which are estimated to have originated 25 million years ago. This beaver's line died a long time ago.

Thomas Martin of the Forschungsinstitut Senckenberg in Frankfurt, Germany, commented in Science that the Castorocauda lutrasimilis 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." He also said that this meant that mammals had conquered the water 100 million years prior to the previously estimated period. The increased diversity is suggested by the indication that mammals were semi-aquatic at such an early stage.

Castorocauda lutrasimilis is the largest mammalian fossil to be discovered from the Jurassic period, weghing 500 to 800 grams, which is at least 10 times larger than its contemporaries, the shrew like mammals. It is also the earliest fur-possessing mammal to have been discovered and scientists believe that this indicates that mammalian skin is an ancient formation.