Mammals have always found inventive ways to move across the landscape — walking, hopping, swimming, flying — and a select few, such as the marsupial sugar gliders of Australia, have mastered the art of gliding.
But with fifty-two species scattered across the Northern Hemisphere, flying squirrels are the most successful gliders ever to take to the trees.
They are not true fliers, at least not in the way bats or birds command the air. Instead, these diminutive rodents hurl themselves into space with astonishing confidence, stretching their limbs wide to transform their bodies into living parachutes. It is a leap that looks both reckless and charming: an adorable woodland pilot bounding into the night inside a furry paper airplane, with just enough tooth and claw to remind you they are still wild.
Their improbable flight depends on an extraordinary bit of anatomy — a thin membrane of skin, the patagium, that stretches from wrist to ankle. When they leap, the membrane balloons outward, turning their entire body into a gliding surface.
Hidden within their tiny wrists are elongated, cartilaginous struts, unique among squirrels, that help spread and stabilize the winglike skin. These distinctive wrist bones mark them as gliders and set them apart from their earthbound cousins.
The oldest remains—mostly cheek teeth—hint that gliding squirrels were already slicing through forest air thirty-six million years ago.
To complicate matters further, the subtle dental traits used to distinguish gliding squirrels from non-gliding ones may not be exclusive after all. Teeth, it seems, do not always tell the whole truth.
In 2002, a routine excavation at a dumpsite near Barcelona, Spain, brought the mystery into sharper focus. As workers peeled back layers of clay and debris, a peculiar skeleton began to emerge.
First came a remarkably long tail. Then two robust thigh bones, so unexpectedly large that the team briefly wondered whether they belonged to a small primate. But as each bone was freed and reassembled, the truth took shape. This was no primate. It was a rodent.
The breakthrough came during preparation, when screen-washing the surrounding sediment revealed a set of minute, exquisitely specialized wrist bones — the unmistakable calling card of a glider. From that mud rose the tiny, ancient hands of Miopetaurista neogrivensis, an extinct flying squirrel whose nearly complete skeleton would become the oldest known representative of its kind.
Studied in detail by Casanovas-Vilar and colleagues, the 11.6-million-year-old fossil revealed an animal belonging to the lineage of large flying squirrels, the same branch that today includes the giant gliders of Asia. Molecular and paleontological data, when combined with this new find, painted a richer story: flying squirrels may have arisen between thirty-one and twenty-five million years ago — and perhaps even earlier.
The skeleton of Miopetaurista was so similar to those of modern Petaurista that the living giants of Asia might fairly be called “living fossils,” their basic form barely altered across nearly twelve million years of evolutionary time.
It is rare for molecular and fossil evidence to agree so neatly, yet in this case, both strands appear to weave the same narrative. The Barcelona specimen anchors the timeline, offering a crucial calibration point that reconciles genetic divergence estimates with the scattered hints found in teeth alone. It also underscores how conservative evolution can be: once perfected, the gliding design of flying squirrels changed little through the ages.
Still, much remains hidden in the shadows of deep time. Older fossils, or transitional forms showing the first experimental steps toward gliding, could help illuminate how these rodents took to the air. What combination of strength, membrane, and courage first allowed a squirrel to turn a fall into a flight? And how did these early pioneers spread so widely across the forests of the Northern Hemisphere?
Flying squirrels remain unique among mammals that glide, remarkable for both their diversity and their broad geographical reach. Yet their lineage is a riddle still missing key chapters. For now, the fossil from Barcelona stands as a rare and precious window into their past — the moment when a small rodent stretched its skin, trusted the air, and opened an entirely new evolutionary pathway between the branches.
