Ambopteryx had a long wristbone that likely supported leathery membranes, allowing it to glide between trees.
Two years ago, a farmer from Wubaiding Village, in northeastern China, came across a beautiful fossil. The only animal fossils that had ever been found at this site were a pair of salamanders, but when Min Wang from the Chinese Academy of Sciences saw the new specimen, he was sure it was a dinosaur.
Studying the beautifully preserved and nigh-complete skeleton, Wang took note of the creature’s sparrow-size body, the quill-like feathers on its neck, and its stubby tail. But when he looked more closely at the left arm, he saw a thin bone coming down from its wrist—a rod as long as the entire forearm, but not jointed like a finger. “I shouted, and my heartbeat elevated,” he says.
He knew he had found another bat-winged dinosaur.
Hold your arm out to the side, palm facing forward. Imagine a bony rod extending downward from your wrist. Now imagine that rod supports a membrane that stretches from your fingertips to your side. That’s how Wang saw his new dinosaur—a feathered animal with a pair of bat-like wings. He named it Ambopteryx longibranchus, from the Latin for “both wings, long upper arm.”
Ambopteryx is actually the second bat-winged dinosaur to be found. The first was also spotted by a farmer in northeastern China, and eventually described in 2015 by Xing Xu and Xioating Zheng. Named Yi qi, after the Mandarin for “strange wing,” it also had a long wrist-rod that likely supported a leathery membrane.
It was a stunning discovery, which broke a neat divide between two styles of prehistoric flight. The dinosaurs took to the skies by transforming insulating fuzz into elegant, flattened feathers, and eventually giving rise to birds. The pterosaurs—often bundled together with dinosaurs, but actually a very different kind of reptile—did so by lengthening their fingers to support membranes, creating a style of wing that bats later reinvented. But Yi straddled both worlds. It was a dinosaur that independently evolved the leathery wings of pterosaurs, and covered their leading edges with fuzzy proto-feathers.
Since its unveiling, “paleontologists have hoped that something similar might show itself to either confirm or refute the interpretation of Yi qi,” says Michael Habib of the University of Southern California, who studies prehistoric fliers. The discovery of Ambopteryx does the former.
Its fossilized arms are folded, so it’s hard to reconstruct their outstretched shape. But the fact that the wrist-rod stows away into a sensible position also backs up the idea that it supported a leathery wing. “After all, an animal wing has to fold up properly when not in use,” Habib says.
“It’s a fascinating find, which confirms that dinosaurs took flight multiple times and very different ways,” says Julia Clarke of the University of Texas at Austin. “It’s not an animal or a set of [traits] I would have predicted.”
Yi and Ambopteryx both belong to the scansoriopterygids—an obscure group of small, two-legged creatures that were closely related to Velociraptor and birds, and that were among the smallest dinosaurs. Only four species have been identified, and all of them have an extremely long third finger. When the group was first discovered, researchers suggested that they used this finger to extract grubs from wood, just like the aye-aye—a bizarre, shaggy-furred lemur—does today. But Wang and his colleagues think that this finger wouldn’t have been mobile enough for that. Instead, they argue, its main role was to support a leathery wing.
That wing almost certainly wasn’t good for flapping flight. Fliers with leathery membranes, like bats and pterosaurs, use muscles in their arms to constantly adjust the tension in their wings, to stop them from being distorted by incoming air currents. The scansoriopterygid wrist-rods had no such muscles, so these dinosaurs probably couldn’t have exerted the fine control necessary for true flight. Instead, Ambopteryx and Yi were likely gliders, much like modern-day flying squirrels, sugar gliders, and colugos. (Flying squirrels also hold their gliding membranes aloft with a long piece of cartilage that comes from their wrists.)
“The Ambopteryx-style wing was probably an evolutionary experiment that didn’t leave any descendants,” Habib says. For whatever reason, pterosaurs fared well with leathery wings (and bats still do), but dinosaurs did not. For them, the crucial innovation was the feathered wing, a feature that today’s dinosaurs—the birds—still rely on.