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In 2007, in a quarry just outside a village in northeastern China,
a farmer stumbled upon what seemed like some kind of small dinosaur fossil.
A museum bought the fossil from him, and in 2015, scientists announced that this chance discovery
was a dinosaur unlike any found before. It was a pigeon-sized, feathered creature
from the Late Jurassic Period, around 159 million years ago. And it seemed like a
very close relative of the earliest birds, which also got their start around this time.
But it had one bizarre feature that had never been seen before in any ancient
bird relative, or in any other kind of dinosaur: bat-like membraned wings!
They named it Yi qi, which means ‘Strange Wing’ in Mandarin.
Overnight, it showed us that there was a lot more going on near the origin of
birds than we had previously thought. And its differences from birds actually
makes the birds we know seem like the weird ones in comparison.
'Cause if we take a step back, the discovery of Yi qi forces us to ask an important question:
from an evolutionary perspective, who really has the stranger wings?
When we think of wings, many of us
might picture feathers, probably because birds are so familiar to us.
We're just used to feathered wings being the default for airborne dinosaurs.
And for decades, the fossil record seemed to back this up, with paleontologists finding
feathered dinosaurs in China and elsewhere. And though we knew we didn't have the full
picture, the evolution of feathered wings in theropod dinosaurs, eventually leading to flight,
was thought to have happened in a relatively linear process of gradual refinement…
…From poor flyers at first like Anchiornis, to Archaeopteryx which could flap its wings
enough to fly in short bursts, to the birds that fill the skies today.
And there was no evidence to suggest otherwise, no reason to think that other bird relatives might
have taken an evolutionary detour and developed totally different kinds of wings altogether.
But in the early 2000s, a couple of interesting members of one
particular dinosaur family were described. The first, in 2002, was the sparrow-sized
juvenile specimen of Scansoriopteryx, which is also called Epidendrosaurus,
depending on which researchers you ask. It was followed by the magpie-sized
Epidexipteryx in 2008. And, based on some of the
unique skeletal traits of these two species, researchers hypothesized that this family,
the scansoriopterygids, were small dinosaurs with an arboreal, or tree-climbing, lifestyle.
These traits included their long arms, an extended digit on each hand, and feet
that seemed adapted for perching. So for over a decade, this family,
known only from a handful of other small fossils from the Mid to Late Jurassic Period of China,
was just considered a strange group of dinosaurs… …One that was closely related to the lineage
that gave rise to birds and that had adapted to life in the trees.
Now, this was already plenty fascinating - no other non-avian dinosaurs are
thought to have been so arboreal. But the discovery of Yi qi revealed
that there was even more to this group than we originally thought.
The incredible level of preservation of this specimen meant that some soft tissue elements were
still visible – including clear evidence of a membraned wing
attached to a rod-like extended wrist bone that gave it structural support.
It suggested that Yi qi, and probably some of the other members of its family, were not just
climbing, but also taking to the air! And other scansoriopterygids that were
found later provided more evidence of this. Like Ambopteryx, which was also found by a farmer
in northeastern China and described in 2019. It’s a close relative of Yi qi that also has
evidence of a membraned wing attached to the same extended wrist bone.
So researchers proposed that these dinosaurs represent a previously unknown
independent experiment in dinosaur flight. Maybe even one of many experiments near the
origin of birds - the group that usually comes to mind when we think of ‘winged, flying dinosaurs’.
Though, it’s hard to tell from fossils alone whether they were capable of
powered flight – like birds and bats – or gliding flight like flying squirrels,
though gliding seems much more likely. One study from 2020 that modeled their
flying abilities based on their wing structure even found that they probably
couldn’t do much more than very clumsy, short-distance gliding between branches.
They argued that Yi qi, Ambopteryx, and the rest of their family were not just an experiment in
dinosaur flight, but a failed experiment. And that might be why all the species found
to date are from roughly the same time and place in the Late Jurassic: the experiment hit a dead
end due to competition with other groups, like the ancestors and closest relatives of birds.
It was in the Late Jurassic, after all, that feathered wings also first appeared and gradually
became more refined for flight over time. Eventually, this gave feather-winged
dinosaurs flying skills that were much more impressive than anything the membrane-winged
Yi qi and its relatives had managed. Now, while the scansoriopterygids had both
feathers and wings, they didn’t have feathered wings, and they certainly didn't have the best
of both worlds when it came to flight. Because their feathers, preserved
in some specimens like Yi qi, were simple, stiff, and paintbrush-like.
They were probably used for thermal insulation and for display, as they
were in many other non-avian dinosaurs. In contrast, the flight feathers of birds
that cover their wings have a totally different and more complex structure.
And because feathered wings are so successful today, we take them for granted, and think of
membrane-winged dinosaurs as strange and unusual. But if we take a broader view, maybe the way
we think about Yi qi and its relatives is totally backwards.
Membraned wings aren't strange at all, really. They’ve convergently evolved many times across
the animal kingdom, from pterosaurs - the sister group of dinosaurs - to
mammals like bats and flying squirrels. They show up so often for a simple reason,
too - their simplicity. Membraned wings don't require
nearly as much evolutionary innovation. The right mutations come together for
an area of skin to grow more than usual, and for the arms to change to support it,
and boom, you've got yourself a membraned wing. And these membranes often enable gliding flight,
and in some cases even powered flight, to quickly evolve in previously flightless groups.
So, is it really surprising that some dinos, like so many others,
went in this direction too, experimenting with gliding flight via a membraned wing?
The fact that we saw them as strange is more like a testament to the success
of feathered wings instead. But birds are not normal, folks.
Flight feathers are just about the weirdest and most complex things to
ever sprout from vertebrate skin. And, as far as we know, in the entire
history of life on Earth, using feathered wings as a means of flight evolved only in
birds and their closest dinosaur relatives. And if birds were to go extinct, the skies of
planet Earth would be feather-free and might remain that way forever.
But if every animal with a membraned wing went extinct today, on the other hand,
you can comfortably bet that very similar structures would show up again before too long.
They just don't require nearly as much evolutionary innovation.
So, from a certain point of view, Yi qi – our so-called ‘Strange Wing’ – and
the other scansoriopterygids weren't so strange after all.
They were just one of the many diverse groups that independently experimented with this same, simple
approach to becoming at least kinda airborne. The unique feathered wings of birds, instead,
are actually the strangest wings of all.
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Thanks to the following Eontologists for their support this month!
Raphael Haase, Jake Hart, Juan M, Annie & Eric Higgins,
John Davison Ng, Melanie Lam Carnevale, and Colton.
By becoming an Eonite at patreon.com/eons you can
get fun perks like submitting a joke for me to read, like this one from Kassy W.
What do you call crooners on parasails? Dino-soars!
I don't get it...
What?
Crooners?
So they're saying they're old?
Well, that's rude.
I like crooners!
And as always thanks for joining me in the Adam Lowe studio. Subscribe
at youtube.com/eons for more ancient adventures.
Epidexipteryx...
Epidexipteryx.
...magpie-sized Epidexipteryx [said with uncertainty]
[laughs]
Believe in yourself!
Don't get scared and stop halfway through the sentence, you're gonna say the WHOLE sentence...
blind confidence...
It was followed by the magpie-sized Epide-bic-ter...
Aaahhh!
It was followed by the magpie-sized Epidexipteryx in 2008.
Huh?
[off-screen] I think that was it.
[sighs in relief] Oh, the outtakes you'll have for this one!