As the dinosaurs crashed around on the forest floor, the early relatives of mammals were flitting from tree to tree above their heads. Two new beautifully preserved fossils show that the extinct relatives to modern mammals had evolved gliding flight at least 100 million years before our own direct lineage would do so itself.

Dating around 160 million years old, the fossil gliders are part of what is known as the “crown group” of mammals, or the animals that gave rise to “true” mammals, which include marsupials and placentals. They are known to have been amazingly diverse, having not only taken to the air, but also evolving a subterranean lifestyle, and some even becoming aquatic.  

“With every new mammal fossil from the Age of Dinosaurs, we continue to be surprised by how diverse mammalian forerunners were in both feeding and locomotor adaptations,” explained the lead author of the two papers describing both fossils, Zhe-Xi Luo. “The groundwork for mammals' successful diversification today appears to have been laid long ago.”

The researchers have analyzed both fossils from the Jurassic, representing the species Maiopatagium furculiferum and Vilevolodon diplomylos, in two papers published in Nature this week. The incredibly preserved remains show in exquisite detail the soft tissue, the fur that once covered them, and the membrane of skin that stretched between the arms and legs.

But the adaptations to this novel way of life are also reflected in their bones. Both display multiple skeletal features relating to the shoulder joints and forelimbs that would have aided in their ability to fly through the canopy, suggesting that they shared a similar ecology with modern gliders, such as colugos, sugar gliders, and flying squirrels. But the researchers are keen to point out that there are some significant differences.

While most of today’s modern gliding mammals are mainly herbivorous, feeding on the seeds, fruits, and other soft parts of flowering plants, this ecological niche would not have been available to these ancient gliders. When Maiopatagium and Vivlevolodon were gliding through the trees, flowering plants were yet to exist, and so they instead adapted to feed on the cycads and ferns that dominated at the time.

They also appear to have hands and wrists that infer they would have hung from branches, not unlike modern bats do today, suggesting that they used to roost upside down in the trees in which they lived.

These incredible finds lend weight to the notion that rather than being limited and stifled during the Jurassic, the early mammals were instead able to successfully compete with the dinosaurs of the time, and evolve to fill a whole host of different niches.