Jurassic “Beaver” From China

Castorcauda lustrasimilis is a docodont mammaliaform from the Middle Jurrassic (~165 million years old). Characterized by an evolutionary convergence of its "beaver-like" tail, and some dental feautres for feeding small fish and invertebrates convergent to that of an otter. This artistic reconstruction of Castorcauda lustrasimilis shows it diving into water. [Image courtesy of Mark A. Klinger/CMNH]

It had a broad scaly tail, fur, swimmer’s limbs and seal-like teeth for eating fish — and it lived 164 million years ago.

The discovery of an extraordinarily well-preserved beaver-like mammal fossil from Middle Jurassic deposits in China suggests early mammals may have been a more diverse group than previously thought, expanding into a variety of environmental niches even as dinosaurs still ruled the earth. The new species, Castorocauda lutrasimilis is an unusual fossil in a number of ways, say Qiang Ji and colleagues in the 24 February 2006 issue of the journal Science.

First, the unique preservation of fur and scale imprints, along with the suggestion of soft tissue webbing in the hind limbs and its partial skeleton provide a wealth of information compared to the teeth and few scraps of skull known from most mammal fossils of this age. Castorocauda is also the largest known Jurassic early mammal, about the size of a small female platypus.

The combination of some primitive skull features and the specialized features of fur, swimming and burrowing adaptations and fish-eating all indicate that early mammals had begun to specialize and move into new environments long before the dinosaurs’ end 65 million years ago opened up new niches for mammals, the researchers say.

In a related “Perspective” article, Thomas Martin says the discovery “pushes back the mammalian conquest of the waters by more than 100 million years.”

24 February 2006