In the past, at least once, we have discussed the rodent family Castoridae which includes the two extant members of beaver (North American and Eurasian Beaver both). This week we will be discussing a beaver-like animal from the Jurassic that, despite appearing to look very much like an extant beaver, was highly specialized for a semi-aquatic lifestyle similar to that of a beaver. There is also a great deal of similarity between river otters and our animal this week, which earned this cynodont mammal its specific epithet. The only animal that this animal convergently shares traits with, but not a name reference, is the platypus. The animal in question, Castorocauda lutrasimilis, actually translates to "Beaver Tail, Otter-like" and therefore directly references both similar extant animals. This animal, as stated, is actually a cynodont, a group of therapsids that appeared during the Permian and includes modern mammals as well. The closest family members of Castorocauda, other docodonts, are also extinct, with the family completely disappearing from the fossil record in the late Mesozoic. Castorocauda itself is found in 164 MA Jurassic rocks from Inner Mongolian fossil lakebed sediments. A wonderfully preserved fossil of Castorocauda was recovered in 2004 that possessed hair, including an undercoat of fur. Another difficult to preserve portion of mammal anatomy that was wonderfully preserved in Castorocauda is the delicate and tiny middle ear including the ossicles.