Sorry I'm a bit late to this party. You've probably already read Darren Naish's and Jaime Headden's excellent takes on Yi qi, but I'm going to try something a little different. It's a FAQ. Let me know if you like this format or not for covering news.
So I heard there’s a new dinosaur in town!
That's no sauropodomorph...
Indeed there is! It’s a basal tetanurine from Chile named…Chilesaurus. It’s weird because it’s a theropod that has reverse-engineered some sauropodomorph and ornithischian features.
Wait, what? Did it have bat wings?
Yeah, that’s the one! Tell me about it!
Awright, it’s a new scansoriopterygid described by Xu et al. and was published in Nature. It’s a really weird animal because—
Wait. It’s a what?
No idea what that means. It’s a raptor, right?
Admittedly, this is a fairly obscure group of dinosaurs. Okay, so scansoriopterygids are a poorly-known family of Chinese feathered theropods. Prior to this, only two genera were known: Scansoriopteryx—also known as Epidendrosaurus—and Epidexipteryx. They are ridiculously small animals. Scansoriopteryx is a juvenile, so it’s expected to be tiny, but Epidexipteryx is a subadult and it’s only 20 cm long.
Epidexipteryx houi, a very small scansoriopterygid.
If you make a “hang ten” sign with your fingers, the distance between the tips of your thumb and pinkie is roughly 20 cm. It’s a miracle these animals were even noticed.
They are long-limbed animals with curious hands: the third finger is about two to three times the length of the 2nd finger. They also have weird feather types that aren’t seen in other maniraptor. Their skulls are characterized by highly-situated nose-holes and procumbent dentition. Given that and their small stature, scansoriopterygids were almost certainly insectivorous. Scansoriopteryx appears to have a reversed hallux, so its describers (Czerkas & Yuan, 2002) believe it was arboreal.
Scansoriopteryx, a baby scansoriopterygid--and the first described.
That would make sense for a tiny, insectivorous animal.
I know, right?
So who’s this new guy?
Yi qi is a new scansoriopterygid that’s a little bigger, overall, than Epidexipteryx. It’s tough to tell because the skeleton is broken apart and the only well-preserved bits are the skull, neck, and arms. There’s a little bit of femur and a little bit of metatarsal, but clearly not in their natural positions. Based on measurements of the skull and neck, though, Yi qi would have been roughly the same length as Epidexipteryx, but with longer (proportionately) limbs.
Yi qi, he of ridiculously short name and elongate 3rd fingers.
It’s got a very dense feather pelage. Its hands are well-preserved, so we can tie that morphology to an adult specimen—unlike Scansoriopteryx, who is a juvenile, maybe a hatchling.
Well that’s nice. So we’ve got a third kind of scansoriopterygid. Yay?
Well, as you’ve probably seen in the news, that’s not necessarily what makes Yi qi so interesting.
Oh, right, the bat wings. It’s got bat wings?
I think it’s incorrect to call them “bat wings,” and I’ve been a little irritated at all the news stories calling them that. Yi qi is really more like a dinosaurian flying squirrel.
This muppet-like reconstruction posits a wyvern-winged model for Yi qi.
Oh, a glider?
Yeah. So there’s one family of flying squirrels—the “scaly tail” squirrels—that have fairly sizable cartilage prongs sticking out of their elbows. This prong helps anchor their “flight membrane” that grows between their wrists, this prong, and their ankles. Another family of flying squirrels has a sizable prong growing out of their wrist, which serves the same purpose. Similarly, many bats have a prong on their ankles that connects the flight membrane to the tail. And of course pterosaurs have a wrist bone called the pteroid that connects the flight membrane to the base of the neck. You could also argue that the flying squirrel’s wrist prong is similar to the pterosaur’s very long fourth finger.
Okay, so gliding animals have prongs. Are you saying that Yee kee had prongs?
First off all, let’s do a quick pronunciation guide. I know that’s how it’s spelled, but it’s pronounced “ee chee.”
Yup. So easy, a babbling infant could accidentally say it.
And yes, it looks like Yi qi also had bony prongs, but they are really big. Each prong is almost the length of the entire arm (minus the hand). And the prongs appear to grow from the wrist, as in flying squirrels. Unlike squirrels or bats, the prongs in Yi qi appear to be bone, not cartilage. I guess they could also be calcified cartilage, though. Either way, this structure is totally unknown in any other dinosaur. There’s no indication of a prong in Scansoriopteryx or Epidexipteryx.
Flying animals and prongs. (a-c) Possible Yi qi restorations; (d) Bat; (e) Bird; (f) Pterosaur; (g) Flying squirrel
So they didn’t have prongs?
Let’s not be too hasty. Keep in mind that Scansoriopteryx is a baby, so maybe it had small prongs that weren’t preserved or hadn’t ossified yet. The skeleton assigned to Epidendrosaurus is pretty poorly preserved, but Scansoriopteryx is pretty well preserved and there’s no sign of a prong. I’d say the Epidexipteryx fossil is not well-preserved enough in the area you’d expect to find prongs to tell for sure one way or the other. Future fossils will be able to say whether or not prongs were more widespread in scansoriopterygids.
How do we know the prongs were connected to flight membranes?
That’s a good question—the authors detail skin impressions that are clearly not feathers between the prongs and the elongated 3rd fingers. It’s true that reconstructions showing a membrane stretching from the back of the prong to the body (or thigh) are conjectural, but from what we know of gliding animals today (and extinct ones) it’s a pretty safe bet. Czerkas & Yuan also noticed tissue impressions coming off the ulna and third finger of Scansoriopteryx—those might be better interpreted (now) as a flight membrane.
Where would the rest of the membrane attach? Body? Hip? Leg?
Hard to say. It’s tempting to restore the membrane as attaching to the ankle as in bats, flying squirrels, and pterosaurs, but doing so would imply that Yi qi could “sprawl” its femora. I don’t think Scansoriopteryx or Epidexipteryx are well-preserved enough to say whether that was possible or not. If sprawling was infeasible, I’d imagine the flight membrane would attach the body wall, approximating the shape of a bird’s wing.
The bizarre skeleton of Yi qi, which may be "inspired" by Jaime Headden's Epidexipteryx drawing.
I suppose this solves the mystery of that long third finger in the family.
It probably does. What’s really bizarre is that Andrea Cau and Lukas Panzarin came up with this hypothesis years ago. Science lets us make predictions, and sometimes those predictions play out!
That's insane! So you said that Scansoriopteryx was a baby...could it be baby Yi qi?
I wondered that, myself, so I went back to all the scansoriopterygid papers (there are only four) to see where and when they're found.
Scansoriopteryx: Daohugou Beds, western China.
Epidendrosaurus: Daohugou Beds, western China.
Epidexipteryx: Daohugou Beds, Inner Mongolia (northern China).
Yi: Daohugou Beds, western China.
So all three genera (note that Scansoriopteryx and Epidendrosaurus are most likely synonymous) are from the same formation, although Epidexipteryx is from Mongolia. It's certainly possible that the three are synonymous, in which case Scansoriopteryx would have priority...I think. But if the lack of a styliform prong is genuine in Scansoriopteryx and Epidexipteryx, then Yi qi is definitely distinct.
But wait, in your handy pictures above, Scansoriopteryx has a long tail, but Epidexipteryx doesn't. That seems pretty distinct to me!
True, that may be a genuine difference. There's a break in the caudal series of Epidexipteryx, so we don't have the whole tail. However, its ribbon-like tail feathers sprout right from the tip of what would be missing, so my sense is that it's a very short-tailed animal. And I doubt that Scansoriopteryx lost its tail as it grew up. That's not a thing that happens.
And there's just no tail preserved in Yi qi.
Anything else I should know about?
Whatever these animals were doing, they were insanely weird. They may give us some insight into the early evolution of pterosaurs, what with that elongated third finger (pterosaurs have an elongated 4th finger). One thing that's really got me puzzled is that scansoriopterygids appear to be right at the base of Paraves, so their closest relatives were birds and deinonychosaurs, but they don't have pinnate feathers.
Broad feathers that cover the body and wings of birds. Flight feathers are modified pinnate feathers. Basically, when you're looking at a duck, you're seeing an animal covered in pinnate feathers.
So what if scansoriopterygids don't have them?
Because their closest relatives on either side (oviraptorosaurs and paravians) were decked out with pinnate feathers. So if the phylogeny is correct, scansoriopterygids descended from theropods who were covered with pinnate feathers, but for whatever reason lost them completely. They developed their own bizarre feathers (not seen on any other feathered animal) and then took an entirely different path to the skies--or maybe between the trees.
That IS strange. And how did the membrane even develop?
That's a little easier to imagine. Birds and, presumably, bird-like theropods, already had a propatagium and some kind of cheiropatagium. Have you ever seen a plucked bird's arm?
This is what a chicken wing looks like before being fried.
See how it's covered in skin already? There's a tendon running between the shoulder and the wrist, just like in pterosaurs, that anchors a skin membrane called the propatagium. The ulna is covered in thick skin that anchors the flight feathers. In many birds, the bottom of the upper arm is loosely connected to the body by skin as well. So you can imagine these membranes becoming larger and more parachute-like.
How would the styliform spur evolve?
Probably the same way it did in flying squirrels--as a cartilage wrist element that provided additional support and square footage for the main membrane. Now, in flying squirrels, the spur isn't as ridiculously large as in Yi qi, but evolution produces some strange things once and awhile. And keep in mind these are sparrow-sized dinosaurs. Flying squirrels could eat them for lunch.
Awesome. Well, I hope more, and better specimens of Epidexipteryx are found.
Me too. I think that could answer a lot of questions about scansoriopterygids. Because there are still a LOT of questions about scansoriopterygids.
See you next time!