When you think about the Jurassic Period, you probably think of massive, lumbering dinosaurs.
But now scientists say there were also gliders — early relatives of mammals, akin to today's flying squirrels – whizzing through the trees.
Fossils of two glider species, found in the Tiaojishan Formation in northeastern China, are particularly well-preserved, so the impressions left of skin membranes and hairs immediately show they are gliders, University of Chicago Paleontologist Zhe-Xi Luo tells The Two-Way.
Gliders have a wing-like flap of skin membrane which allows them to gracefully launch themselves from tree limb to tree limb without plummeting to the ground. At about 160 million years old, these fossils are the oldest known examples of early mammals with the ability to glide — in an evolutionary lineage that was wiped out during the extinction event that killed the dinosaurs.
The fossil record for the forerunners of mammals in the Jurassic period has been poor, Luo said, "so we have this biased impression that early mammals are not terribly diverse, not terribly interesting because they are dominated by dinosaurs."
Their team's research – published in two papers in the journal Nature – adds further weight to the idea that "early mammals even living in a dinosaur-dominated ecosystem are incredibly diverse," Luo says. Other recent mammalian discoveries from the time include creatures that lived in trees, in the water and underground.
The larger of the two gliders, named Maiopatagium furculiferum, measured about 9 inches from head to tail and has skinny, elongated limbs, Luo says. It has a membrane between its wrist and ankle, "so we imagine that that they just glide around in between the trees just like the flying squirrels," Luo says.
The smaller of the two, Vilevolodon diplomylos, is three inches long, and its fossil is also preserved with a skin membrane "as if it's a museum-prepared specimen," he adds.
These animals likely evolved the ability to glide in order to quickly move through trees to "sample their favorite plant foods," Luo says, without the need to risk running into a predator on the ground.
The animals come from a dead-end evolutionary branch, because it was wiped out at the same time as the dinosaurs. That means that they have no relationship to modern gliders, which emerged about 50 million years ago.
"This very ancient Jurassic glider developed the first gliding adaptation, and then they went extinct," Luo says. "Some hundred million years later, the modern mammals re-evolved this gliding adaptation several times over."
This is evidence of convergent evolution, he says — or the ability of animals to independently evolve the same feature if it offers them an advantage. He points to the diverse mammals that currently can glide, like North American flying squirrels, African scaly-tailed gliders, southeast Asian colugos, and Australian marsupial sugar gliders.
These diverse species support the notion that mammals have carried out so-called evolutionary experiments. As Luo put it, "if there is an opportunity ... some extinct lineages of mammals has already tried a long time ago."