Thursday, March 06, 2008

Norell v. Martin: the NOVA Microraptor Documentary

Better late than never, eh? PBS' science series, NOVA, aired a documentary a few weeks ago about Microraptor gui and how it flew. This show comes five years after the little bugger's initial discovery and more than a year after the biplane posture paper was published. Because the latter was never really discussed during the documentary, I have to wonder when the show was produced. While generally entertaining, the Microraptor doc ultimately erred in a number of important ways which I'll get to later. First, though, let's talk about the show itself.

The focus of the documentary was figuring out how Microraptor gui (=zhaoianus?) used its four wings to fly. No living bird has primaries on its feet, so Microraptor represents an entirely novel flight strategy. As Xu Xing notes in the show, the fact that Microraptor's hindwings are made up of asymmetrical primaries, they must have had an aerodynamic function. The show starts as an exploration of how this unique apparatus would have worked.

But then we force Larry Martin into the picture. Martin is a University of Kansas paleontologist who is one of the few doubters of a dinosaur origin for birds. For reasons which the show never goes into, Martin believes that Microraptor is a descendant of some pre-dinosaurian avian ancestor. Even more than that, he believes its limbs sprawl, like they do in Xu Xing's original illustration (an illustration which Xu himself apologizes for during the show). And so a face-off ensues: Xu, Norell, and the crew of the AMNH build a model of Microraptor's skeleton while Martin and some other guy build their own model. Norell et al. create their skeleton by measuring and averaging the lengths and structure of every single bone in the animal's body, based on more than 30 specimens (I was unaware that so many were known!). Martin uses a single specimen that was crushed by a Cretaceous steamroller.

Not surprisingly, Norell's team creates what appears to be a normal theropod skeleton, while Martin's model looks like a paper airplane. It all comes down to the structure of the femur head and the corresponding socket in the pelvis. Independant experts are called. Norell's model actually works, and Martin's model is so squashed that the femur won't even fit in the socket. Ladies and gentlemen, we have a winner!

That entire discussion was a waste of time. In presenting Martin's crackpot idea, NOVA (and PBS) was, in a way, educating people on a controversy that doesn't exist. On the other hand, they did do a good job of showing why Martin's idea doesn't work, and how the scientific process works to reconstruct (accurately) a long-extinct dinosaur.

After determining that their model fits together properly, Norell's team puts meat on the bones and feathers on the meat. Their final model looks great, except that feathers were used rather sparsely. The Microraptor model on display at the AMNH looks a lot better than the model used for NOVA. I wonder if the NOVA model was a prototype for what was eventually used at the AMNH? Anyway...

The model, feathers and all, is placed in a wind tunnel to try and figure out the best gliding posture. Few positions were attempted:

1) Legs splayed as far as they'd go, with the hindwing directed toward the tail (performed poorly).
2) Legs swung forward, feet facing forward, and hindwing directed downward (performed poorly).
3) Legs held in various Z-shapes, and hindwing directed downward (performed poorly).
4) Legs held in Z-shape, with biplane wing configuration (performed well, but not great).
5) Legs straight and directed back, so that hindwings overlap above the tail (performed best).

The theory here being that Microraptor would use its legs to spring off a tree trunk, and it would keep its hindlimbs in that retracted position. Sadly, for all practical purposes, the hindwing is barely involved in gliding or flight in this posture. At any rate, when Microraptor approached its destination, it could have brought its legs forward into the second position to form a sort of air brake.

But as Darren Naish points out all the time, animals are not always built ideally. Flaws exist in every organism. If humans were ideally built for an upright posture, I wouldn't constantly have back problems, and Scott wouldn't have a broken pedal sesamoid. Also, the Chatterjee paper linked above gives plenty of good reasons why a biplane posture would work, and how the hindwings would actively participate in flight.

The show just sort of ends after that, which was disappointing. The real sin committed by the NOVA show, though, was that it constantly hammered forth the idea that Microraptor is some kind of transitional form in the evolution of flight, which is probably not true. Yes, Archaeopteryx has short feathers along its hindleg, and there are a few enantiornithines with hindleg feathers (some are quite long), but that does not mean that the original paravian had hindwings. It's more parsimonious, for now, to conclude that the original paravian did have hindleg feathers, but not the extent that Microraptor did. Microraptor may have developed its own specializations.

The show did do a great job of two things, though: First, explaining how traits move through a family tree. The "branching family tree" graphics was very well-done and accurate, and showed how a single trait, like a tridactyl pes, originated at the base of the Theropoda and was inhereted by the descendants of that ancestor. Also, it was extremely interesting to watch how Norell's team created their Microraptor skeleton. It must have taken forever, but the results were beautiful. This is science in action, people. I suppose the show did a good job of explaining why Martin's model didn't work, although that's pretty obvious from just looking at his steamrolled skeleton.

Overall, the show was interesting but probably moreso for somebody unfamiliar with Microraptor. While I haven't played around with it, Scott tells me that the NOVA website's interactive Microraptor media are quite intruiging. Did any of you in readerland watch the show? What did you think?

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