It's a bird! It's a plane! It's a … Jurassic squirrel mimic! Today, in the pages of Nature, palaeontologists have announced a pair of ancient beasts that now stand as the earliest known fuzzballs to take to the air.

Left: Fossil of gliding mammaliaform Maiopatagium furculiferum; Right: Jurassic gliding mammaliaform Vilevolodon diplomylos. Photos: Zhe-Xi Luo/UChicago
Locator’s map of the Daxishan fossil site in Liaoning Province and Nanshimen fossil site in Hebei Province, China.

The petrified duo were found in the 160-million-year-old rock of China, and both are as exquisitely preserved as the feathered dinosaurs that made this part of the world famous. Named Maiopatagium furculiferum and Vilevolodon diplomylos, respectively, these critters are represented by nearly complete skeletons surrounded by remnants of their skin and fur. Both, palaeontologists found, had membranes stretching from their ankles and wrists that made them the Jurassic equivalents of flying squirrels.

Still, Maiopatagium and Vilevolodon aren't quite like anything alive today. They weren't true flying squirrels, like the ones that soar through our modern forests. And, in fact, they weren't even mammals just yet. Both of these newly named creatures were mammaliforms, or members of the broader group of almost – but not quite – mammals that proliferated during the early chapters of the Age of Dinosaurs. More specifically, these new fossils belonged to a totally extinct mammaliform group called eleutherodonts, many of which are thought to have spent their time scampering around in the trees.

There's more to the new fossils than delicately preserved soft tissues, though. In addition to the membranes that made up the wings of Maiopatagium and Vilevolodon, both fossils show skeletal features related to gliding and hanging around also seen in fliers and gliders that are with us today. Parts of their ankles, for example, have a flange where their wing membrane attached, just like in bats. The proportions of their fingers and toes are strange, too, more like those of bats than other mammals. This, the palaeontologists propose, means that Maiopatagium and Vilecolodon could roost by hanging upside down from both their hands and feet.

Maiopatagium in Jurassic forest in crepuscular (dawn and dusk) light: A mother with a baby in suspending roosting posture, climbing on tree trunk, and in gliding (Reconstruction by April I. Neander/UChicago).

Despite some similarities to bats, though, the ancient eleutherodonts moved through the air in a different way. They didn't flap their arms in powered flight like bats do, but instead launched themselves into the air to glide: soaring from tree to tree in forests of conifers and over ground carpeted with ferns and cycads. Their teeth, meanwhile, show these gliders were munching on the bounty of plants that grew in this forest primeval.

According to University of Chicago palaeontologist Zhe-Xi Luo, this makes Maiopatagium and Vilecolodon the earliest airborne members of the greater mammalian family – over 100 million years before true mammals independently evolved their own method of gliding.

The two Jurassic gliders do more than push back the date when little furry beasts started to glide over the heads of dinosaurs. They also add to a bigger picture that changes traditional wisdom about the role of mammals and their ancestors during the Mesozoic.

Mammals and their ancestors are traditionally thought of as eking out a living under the feet of dinosaurs, usually in the form of little insectivores. In recent years, however, palaeontologists have found that mammals and their precursors underwent an evolutionary explosion that spun off archaic versions of more familiar, modern animals.

"With every new mammal fossil from the Age of Dinosaurs," Luo says, "we continue to be surprised by how diverse mammalian forerunners were in both feeding and locomotor adaptations." There were the equivalents of shrews, squirrels, beavers, aardvarks and flying squirrels, providing what Luo sees as the groundwork for the eventual triumph of mammals once an asteroid ended the days of dinosaurian dominance.



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