Since most dinosaur names consist of long, polysyllabic gargles—Parasaurolophus, Therizinosaurus, Pachycephalosaurus—it is refreshing that the latest addition to the family has the shortest one yet.
It is simply Yi.
In full, it is Yi qi, which comes from the Mandarin for
“strange wing” and can be roughly butchered as “ee chee”. The name hints
at this pigeon-sized animal’s most remarkable feature. Each of its
hands had a long bony rod extending from the wrist. This rod seemed to
support a membrane, much like that of a flying squirrel or bat.
Yi was a dinosaur with bat-like wings! What an astonishing find!
Until now, the assumption was that prehistoric reptiles took to the
skies in one of two different ways. The dinosaurs did so with feathers.
Many species were covered in downy insulating fuzz, and some small
predatory species elaborated these into true, flight-capable feathers—long, flat vanes that protruded from their arms (and sometimes their legs). These winged creatures gave rise to the first birds.
Meanwhile, the pterosaurs evolved a very different type of wing, by
greatly lengthening their fourth fingers to support a membrane of skin
and muscle. (Pterosaurs are often lumped with dinosaurs but belonged to a
totally separate group.)
These wings were mutually exclusive: dinosaur or pterosaur, feathery or leathery. But Yi went
for both options! It had membrane wings with a feathery covering on the
leading edge. It shows that at least some dinosaurs had independently
evolved the same kind of wings as pterosaurs—an extraordinary example of
convergent evolution.
“This is refreshingly weird,” says Daniel Ksepka from the Bruce Museum, who was not involved in the study. “Paleontologists will be thinking about Yi qi for a long time, and we can surely expect some interesting research into the structure and function of the wing.”
There’s only one known fossil of Yi. A farmer in China’s
Hebei Province found it around eight years ago., and the Shandong Tianyu
Museum of Nature bought it shortly after. Xing Xu
and Xioating Zheng from Linyi University, who discovered the creature,
first laid eyes on it in 2009 and started working on it in 2013. “It
looked special to me,” Xu recalls.
As the team exposed and analysed the specimen, they worked out that it was a scansoriopterygid—a
group of small, feathered dinosaurs with very long third fingers. These
species were reputedly good climbers (their name means “climbing wing”)
but there was no evidence that their feathers were good enough for
flight.
The same applied to Yi—its feathers, covering its skull,
neck and limbs, were stiff filaments that ended in paintbrush-like tips.
They were very different to the flight-capable plumes of birds.
Then, the team noticed the weird rod. It stuck out from each of the
dinosaur’s wrists and was longer than its forearm. It’s not a finger,
but its chemical composition revealed that it is indeed a bone, or
perhaps a piece of hardened cartilage. The team had no idea what it was.
“When I saw the bone, I was really confused,” says Xu. “There is
nothing comparable in any other dinosaur.”
But, as Corwin Sullivan from the Chinese Academy of Sciences realised, there is something
comparable in flying squirrels. These rodents glide from tree to tree
by expanding a membrane that stretches from their wrists to their
ankles. They deploy this membrane by splaying their limbs and extending a
long piece of cartilage (called the styliform process) that protrudes
from their wrists. Bats have a similar piece of cartilage (the calcar)
on their feet, and pterosaurs had a similar bone (the pteroid) on their
arms.
All of these structures do the same thing: they support a membrane that keeps their owner in the air. Yi’s
wrist rod was almost certainly fulfilling the same role. “As far as i
know, this is the only plausible interpretation,” says Xu. He even found
several patches of what look like sheet-like membranes, surrounding the
rods and fingers of both hands. Again, they’re unlike anything seen in
other dinosaurs.
“The ‘bone’ seems to be what they say it is, and they have made
appropriate studies to show it isn’t something else,” says Michael
Benton from the University of Bristol. “So, yes, it seems real, and, my
goodness, what a further broadening of flight capabilities in
paravians!” (That’s the group of dinosaurs that includes the
scansoriopterygids, celebrities like Velociraptor, and all birds.)
But Yi “is not necessarily as weird as it might first seem, in an evolutionary sense,” says Michael Habib
from the University of Southern California. “Living birds actually have
membranes around their forelimbs, including a well-developed membrane
in front of the elbow called a propatagium. Feathers cover [these] parts of the wing, obscuring the soft tissues.” Yi simply extended these membranous parts with the help of their weird extra bone.
“Yi might have moved through the air with a combination of
flapping and gliding flight, though it probably relied more on gliding,”
says Xu, who is planning to search for more specimens. “There are many
questions remaining to answer about this bizarre dinosaur.”
For now, this discovery reminds us that the evolution of flight among
birds and other dinosaurs was not a simple story. In the late Jurassic
period, when Yi lived, there were all manner of dinosaurs with
varying shapes, sizes, and numbers of wings. It was a world of
not-quite-birds and just-about-birds—and now bat-winged dinosaurs, too!
“What a grand age of experimentation!“ says Ksepka.
“This may also be evidence that flight evolved multiple times within dinosaurs—perhaps three or more times,” adds Habib.