Jurassic Beaver

Prehistoric mammals were originally thought to have flourished when the dinosaurs lost their dominance 65 million years ago. However, many scientists were forced to rethink this theory with the discovery of Castorocauda lutrasimilis. The unearthing of this ‘Jurassic Beaver’ pushes the date of the emergence of mammals back by 110 million years.

Castorocauda lutrasimilis comes from the Latin words castor (beaver), cauda (tail), lutra (otter), and similis (like/similar to). More than anything, the Castorocauda lutrasimilis resembles modern-day beavers. However, it is likely that they belonged to a whole different line of mammals, the Docodonta that died out even before the first rodents came about. The Docodonta is an extinct group of proto-mammals, also known as Mammaliaformes, and it has no known existing relative.

The most well preserved fossil of the Castorocauda was found in 2004 by a research team led by Qiang Ji of Nanjing University and the Chinese Academy of Geological Sciences found in Beijing. Their discovery was made in the Middle Jurassic Jiulongshan Formation, in the Inner Mongolia Region of China. Interestingly enough, fossils of lissamphibians, coelurosaurian dinosaurs, pterosaurs and other numerous invertebrates have also been found in this same location.

The fossil that Qiang Ji and his team found dates back to 164 million years ago. It was in such great condition that even the animal’s hair was preserved. They also found evidence of a broad tail with scales and a few bits of hair towards the tip. There was also an impression of webbed feet found in the fossils that was most likely used for swimming. This important feature suggests that the mammals may have adapted faster to different environments—in this case, the marine environment—faster than we thought. Before, it was thought that the mammals of the time were confined to land. It was assumed that the that mammals during the Jurassic were tiny. They were ground and tree-dwelling. These animals were also thought to have been nocturnal in order to escape the much larger predators of the time.

The Castrocauda that was found possessed molars that suggest that these animals were piscivores, or that they fed on fish. In size it is quite similar to a small platypus. These animals probably weighed from about 500-800 grams and grew in length for about 17 inches. Compared to the rodents living at the time, the Castrocauda was easily the largest mammal of the Jurassic period.