Proc. R. Soc. B doi:10.1098/rspb.2010.1148 (2010)

Some of the best hearing in the animal kingdom belongs to mammals, including humans and bats, thanks to the snail-shaped coiling of the cochlea, a key part of the inner ear. A 150-million-year-old fossil of a mammal, Dryolestes leiriensis, has revealed how this key innovation evolved.

The fossil has a bony inner ear structure (pictured) containing auditory nerves, similar to that of its contemporary relatives. But this structure is curved rather than coiled. Zhe-Xi Luo at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and his colleagues suggest that the cochlea was innervated before it evolved into today's curved shape.

Credit: LUO (CARNEGIE MUSEUM ) & I. RUF (UNIV. BONN)/R. SOC.