Art by Mark Klingler
When: Middle Jurassic (~164 million years ago)
What: Castorocauda is an aquatic mesozoic mammal. It is known from a very well preserved slab specimen, which shows a suite of features indicating an adaptation to aquatic life. Its limbs were powerful, and easily capable of strong swimming or digging. Its tail was flattened, shown both by the soft tissues preserved on the slab, but also by its flattened caudal vertebrae, which are very simular to that of living swimming mammals, such as the beaver, otter, or platypus. This tail is what gives this Jurassic animal its name: Castorocauda translates to ‘beaver tail’. Its teeth were also specalized for aquatic life; they have primary cusps which curve backwards, which is seen in fish eating animals today, such as the seals. Castorocauda was about 17 inches (~43 cm) long, making it one of the larget Jurassic mammals.
Castorocauda would have looked somewhat like a platypus lacking a bill, but its diet was more like that of a seal. In the mammal family tree Castorocauda is far far removed from any living taxa. It is a member of a group called the Docodonta, just a few branches removed from one of the first Mammaliforms: Morganucodon. As Castorocauda clearly had fur in life, this puts fur extremely far down on the mammalian lineage. It has been hypothesized previously that even more basal taxa had fur, but there has been no conclusive evidence. As Castorocauda was completely coated in fur, except for much of its tail, it is extremely likely fur originated much deeper in the mammalian lineage, possibly in the non-mammalian synapsids.
The discovery of Castorocauda also shows that mesozoic mammals were much more ecologically diverse than has been previously proposed. Our ancient kin were not only small rat like insectivorous creatures that ran in the shadows of the dinosaurs; they swam, preyed upon dinosaurs themselves, and even flew (more on that one later)!