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.
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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.