Friday, February 5, 2010

On the structure of hind wings

Above: Anchiornis model from NatGeo. Something is wrong with those foot feathers...

With all the recent hubbub over Anchiornis and its coloration, have come several pretty cool life reconstructions showing the animal as it would, supposedly, look in life. I posted one in my blog yesterday on the topic, and NatGeo has a cool 3D model on their site, here (and figured above).

One thing I've noticed about all these images is the very... standard rendering of the infamous hind wings (shared with Microraptor and Pedopenna). Much debate has surrounded these appendages, mainly relating to their ability to glide. A whole Nova special was made showing different attempts to reconstruct gliding posture, from biplane-like, to sprawled-out-sideways, to rear-swept canopy. And especially in that special, the same thing bugged me about the hind wings:

Every configuration required the primary foot feathers to emerge at a different angle from different parts of the limb!

It shouldn't be so hard to figure out how these feathers attached, which would help us suss out their potential uses. First of all, the reconstruction of Anchiornis above, and by extension reconstructions of Microraptor with the legs sprawled out, make little anatomical sense to me. Maybe I'm mistaken, but I think we should look to the example of how front wings are put together to inform on this, as developmentally the two are almost certainly related.

Above: Incorrect, pronated-handed Velociraptor skeletal by Frederik Spindler, licensed. Just where are those primary feathers supposed to be attaching? To the underside of the fingers?

Back in the days of pronated theropod hands, many similary ridiculous renderings of feathered theropods were made, many by me. Of course, we all know how wings 'look' when folded - primary and secondary feathers swept back along the side of the body. The problem is that, if the hand is incorrectly shown as pronated, this arrangement makes no sense. On a pronated hand, the feathers emerge laterally from the edge of the finger and hand, and from the lateral side of the ulna. Only when the hand is not pronated does the proper wing configuration emerge. If real wing anatomy were taken into consideration, artists of bunny-wrist theropods would show the primary feathers sicking out to the side of the animal, not flush with the body, when the arms are folded. Otherwise, the feathers would be attached to the palm of the hand, which is ridiculous.
Above: Probably incorrect rendering of Microraptor. Just how is this animal supposed to sit down?

So why isn't this also true of the foot? My guess: it is. For all intents and purposes, the foot is the same configuration as the hand, only it really is pronated. The toes and metatarsals are equivalent to the fingers and metacarpals. I'll put this to the experts out there, because it's something I've never seen addressed and makes no sense to me. If the hind wing feathers projected posteriorly from the metatarsals, how are these animals supposed to have sat, or brooded their eggs? We have several fossil examples of theropods sitting, sleeping, or brooding, all with metatarsals fully in contact with the ground. If there were very large, rigid feathers pointing straight down from the foot, that would be impossible. Furthermore, developmentally, it seems we would expect the legs to mirror the arms, with primary leg feathers articulating from the top of pedal digit 2 and metacarpal 2. Therefore, sticking straight out to the side when standing.

Above: An animal capable of sitting, with sticky-out hind wings. AMNH.

Now, maybe I missed something so please correct me if any of my reasoning is wrong. But in short, I believe the biplane or semi-biplane model is the only one anatomically feasible, barring some novel adaptation like feathers with a hinge at their base to allow them to deploy outward like an X-Wing from Star Wars. The pedal primaries have to have pointed out to the side, while the tibial primaries of the lower leg would still point backward, as in the model above. So all those recons purporting to show Anchiornis as it really was miss a pretty important point. Not to mention the naked, beak-like snout, doesn't anybody look at fossils anymore? ;)

Above: This pigeon with sticky-out, biplane configured foot feathers is apparently able to sit down.

Another note on this--maybe my whole reasoning about comparing the front to hind wings is off. Can't we then compare to modern hind winged animals? Like the one pictured above? Those foot feathers do not point backward and, as you can see, the animal is therefore able to sit. Imagine those feathers configured like the Anchiornis model at the top of this post. It just doesn't work.


  1. I think the case in Anchiornis is far less problematical than the case of Microraptor.
    The tibial and metatarsal feathers of Anchiornis seem quite flimsy structures and I would think those things would definately bend under the weight of the animal. Also since the feathers of Anchiornis are symetrical there is no airfoil to consider. Microraptor is the real problem case.

    But it seems that many people suffer from the same problem while interperting these structures. And that is they seem to think that these feathers were always extended. If these things worked as airfoil there must have been a way to protect them, there must have been someway to fold them. Taking that in to consideration it is especially interesting that Microraptor has really long mt 5 with expanded distal head that almost seems like the distal head of typical pedal phalange. I think there must be some connection with Microraptors unusual mt5 and the leg feathers. If the feathers could be folded there is no sitting problem as proposed by you.

    I also find your pidgeon example rather unconvincing. That breed was produced by selective breeding by man not by nature. We can produce all sorts of things with all sorts of structures that you wouldn't normally find in nature becaue it is we who controll the sellective pressures as we have freed the population from natural pressures and subjected the population under our own whimsical selection.

  2. I dunno Ville, I don't think we can simply create things that aren't already allowed by the underlying developmental pathways. Is it possible to artificially select for feathers pointing back from the mt as in the drawings? I would guess no.

    And I think you're underestimating the rigidity of the leg feathers in Anchiornis. After all these seem to form a plane, not just a haphazard 'bell bottom' pants leg as in say, hawks or Archaeopteryx.

    Additionally, I wonder if it would be possible to look for quill knobs on the legs in these forms, or something similar. But, if pointed backward or mobile, where would the quill knobs even be? The ventral side of mt2 or what? All primary feathers we know of need to anchor to bone or ligaments, so a mobile structure as you suggest really would require extraordinary evidence.

  3. Interesting discussion, I've always wondered how to make a Microraptor sit with those big hind wings.

    However, I also agree with the above poster: what if the hind wings could be folded up?

  4. Okay, your reply popped up after I finished typing...

  5. As I discussed with Ville in my blog, I think that the most plausible site for anchoring the metatarsal feathers in microraptors was the lateraloplantar margin of MT IV. It shows a flange that is more robust than in other deinonychosaurs. Interestingly, the MT V of microraptors is more elongate than in most theropods, being bowed and distally robust, recalling superficially the bowed metacarpal II of many maniraptorans, where remiges are anchored. This configuration is a serial homologue to the lateral insertion in the forelimb feathers, and supports the "biplane configuration".

  6. I forgot to mention this, as a conclusion:
    I think that "the Mt IV flange + bowed Mt V (the latter contacting distally the former) complex" should be the site for feather anchoring.
    Was this complex able to fold the feathers? Maybe, feather folding would be linked to the action of the pedal flexor muscles.

  7. Matt,

    Well I guess the real question is does the metatarsal feathers of that pigeon breed require some developmental specialities? or is that just the same thing as turning scaled areas into feathered areas? My guess is the latter.
    As for posteriorly projecting feathers....does this count?

    Actually IIRC the feather pants of raptors are actually pretty neatly organized. and besides the neat plane seen in Anchiornis could simply be illusion of the preservation as feathers are squished in the sedimetary bedding plane.

    I made my estimate of the rigidity of these structures based on the rachis which to my eye looks particularly slim.

  8. I made a similar observation on (great site btw, I recommend it)

    IE, wouldn't the leg feathers of certain ornamental breeds of bird (such as the pigeon you pictured and certain varieties of chickens and canaries) be a good starting point to see how they might have articulated in 'feathered dinosaurs'?

    Also, I wonder if the genes for leg feathers in modern birds (again, referring to the pigeons and the chickens, which were selectively bred to be this way, as opposed to ptarmigans and certain types of owls, which presumably evolved in their own direction) might be.... almost the same idea as the switched off genes they've recently discovered for teeth in birds? IE a 'throwback' to the leg feathers of certain theropods?

    Sorry if that was all a bit garbled.... the concepts all make sense in my head but being as I'm just an amateur enthusiast, sometimes I have trouble expressing them in proper 'scientific' language.