Distant relatives of today's flying squirrels probably existed at least 135 million years ago; much earlier than anyone had suspected. That is the conclusion of a team of researchers led by Jin Meng, who have found the fossilized remains of a new species of mammal (see page 889). The fossil indicates that early in their evolution, mammals, as a group, had very different ways of getting around — some on land, some in water and some gliding from tree to tree.

Like many fossil finds, the discovery of these remains was largely a result of good luck. “You never get exactly what you are looking for, so you have to look at everything,” says Meng, who is a curator of palaeontology at the American Museum of Natural History in New York.

That's exactly what Meng was doing in March when he visited the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. Meng completed his graduate studies at the institute in 1980s and, after moving to the United States, has continued to visit and collaborate with his colleagues there. He was inspecting specimens of mammalian fossils his colleagues had collected a few months earlier in the eastern part of Inner Mongolia.

The team noticed a squashed skeleton preserved in a split slab sitting on a desk. “It had not caught anyone's attention until then,” recalls Meng. His colleagues had initially thought it might be a triconodont, a common Mesozoic mammal. But the team noticed that the fossil's teeth were unusual. Triconodont teeth have pointed cusps in a straight line, but the specimen's teeth had sharper cusps that curved backwards.

This led Meng and his colleagues to think they might be looking a new species. Their suspicions were confirmed when they examined the fossil under a microscope. They could see the outline of a membrane of skin covered in hair, a membrane that was adapted for flight. “It was a big surprise,” says Meng.

The scientists spent the next six months working day and night to characterize their find. “It is a very competitive area, so we had to work as fast as we could. But we also had to be very thorough,” says Meng.

The biggest challenge for the researchers was to work out where the new mammal fitted in the evolutionary tree. They added the animal's measurements and properties to a database containing 435 characteristics for each of 58 different mammalian species, including both Mesozoic and current mammals, looking for similarities. They discovered that their specimen has little in common with any other mammal. This suggests that it belongs to its own mammalian order, says Meng, an order that became extinct a long time ago. In the process the group also concluded that the animal probably ate insects and was active at night.

The finding will raise questions about the early evolution of mammals, says Meng. In particular, why did gliding behaviour evolve so early in mammalian life? It might also explain the origin of some mysterious teeth fossils that scientists have identified over the years. These can now be re-examined to see if they belonged to an ancient glider. Meng, meanwhile, plans to continue his visits to China two or three times a year, and hopes to stumble across another find. “We will keep looking in the same area,” he says. “I don't know what will show up next.”