When mammals first evolved some 200 million years ago, the dinosaurs reigned supreme. For eons, conventional wisdom has it, mammals survived by staying small, nocturnal, and unspecialized. Only after dinos went extinct 65 million years ago did mammals come into their own. In the past few years, however, a string of new discoveries has shown that mammals actually diversified early on and developed a variety of tricks for making a living. The finds include an insectivore specialized for digging and a fish-eating, beaver-like creature.
The newest addition to the crowd is Volitcotheria antiquus, a 14-centimeter-long mammal from Inner Mongolia. Its most impressive feature is a large membrane of skin that stretched between the front and rear limbs. "It was really a surprise when we saw this," says paleontologist Jin Meng of the American Museum of Natural History in New York City, who describes the fossil this week in Nature with several colleagues.
The skin is clearly an adaptation for gliding, Meng says, and would have allowed the tree-climber to glide from branch to branch. With its squarish wing-surface, Volitcotheria probably wasn't chasing prey while in the air. But gliding would have been useful for escaping predators, like the flying pterosaurs that lived at the same time. At somewhere between 130 million and 164 million years old, the fossil is by far the oldest known flying mammal. The next oldest is a 51-million-year-old bat from Wyoming.
"The total range of ecological diversification is so much greater than we thought possible 5 years ago," says Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. "This paints a new picture."