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Upstroke of *Archaeopteryx* and *Confuciusornis* was Re: Science feather strength debate



 If the wing is generating lift, and the bird has just completed a
 downward stroke, would not the wings rise back up? In a fixd wing
 aircraft, if your wing spar is too weak, and snaps, the wings "fold
 up" Sure, powering the up stroke could increase the flapping rate,
 and provide forward thrust too, but I can't believe it if flapped its
 wings down, that its wing would just stay that way, with massive
 anhedral. I don't see the lack of a powered upstroke as a convincing
 argument against some form of flapping behavior (I can see it as an
 argument about the upper limits to what sort of power it could get by
 flapping)

You've misunderstood the argument by confusing it with another that used to be made about *Archaeopteryx*.

*Archaeopteryx* lacks an ossified sternum. (Only in the Berlin specimen the cartilaginous sternum decayed in such a way as to be replaced by calcite crystals at least in part.) People deduced from this that the flight muscles must have been weak enough for a cartilaginous sternum to withstand their forces, and that, they thought, meant flapping flight was difficult or impossible. Various calculations have taken wind out of the sails of this argument, and the sternum takes some time to fully ossify in extant birds.

Archie further lacks an ossified sternal keel, and so do most specimens of *Confuciusornis*. Tim Williams has way exaggerated what *C.* has: Chiappe et al. (1999) counted _two_ specimens that had a faint ridge on the caudal half of the sternum, out of several hundred; in all others, the sternum is completely flat. This led people to believe that these animals were too weak for a decent upstroke. In extant birds, the pectoralis muscles, which accomplish the downstroke, do _not_ attach to the keel except at its very base, they attach to the huge plate that forms the rest of the sternum; the keel is where the supracoracoideus muscles attach, and they do the upstroke. No keel, no supracoracoideus, no upstroke powerful enough for climbing flight. _This, too, is incorrect._ First, while it's true that pigeons cannot gain height if you cut the tendons of their supracoracoidei, pigeons have a rather specialized flight mode that isn't shared by most other extant birds. A less impressive upstroke can still be accomplished by the deltoideus muscles, the ones that we use for that motion*, the ones that bats use for their upstroke** and that pterosaurs must have used too; on the humerus, they insert on the deltopectoral crest, and indeed these crests are huge in *A.* and *C.* (and even in *Ichthyornis*). And finally, in extant birds, the keel is present as cartilage before it ossifies (and indeed before the left and the right sternum fuse or even touch each other).

The new argument is something else entirely. It's that it was impossible to lift the wings of *A.* and *C.* above shoulder level without breaking bone or ripping the shoulder joints apart. Reconstructions of muscles play no role in this argument.

This new argument is, AFAIK, the only one for assuming that *A.* and especially *C.* were not powered fliers. I find it difficult to make sense of *C.* as a glider. But if that's what its shoulder joints say... :-|

* That means pulling the more or less outstretched arms backwards. Don't forget we're tilted by 90° with respect to a bird. ** Probably together with half a dozen others. Muscle-wise, the wing beats of bats are very complicated indeed.

> BTW, although _Confuciusornis sanctus_ (the type species) has a
> keel,

(mostly wrong, see above),

> other confuciusornithids (such as _C. dui_, _Changchengornis_ and
> _Eoconfuciusornis_) do not.

 So what can you say about the differences of those others, as far as
 environment, or morphology. Not knowing anything about the others, I
 could engage in pure speculation that maybe the others lived in areas
 more conducive to soaring, and confuc. lived where the soaring wasn't
 so great, and often needed to supplement its flight with some of its
 own power (hence the keel)

All of them were found together with *C. sanctus*, IIRC. So, they all lived in the same environment: _forest_. No dunes, no desert, no evidence of any open ground; lakes, but AFAIK no evidence of big lakes.

Very few differences in morphology. It amounts mostly to beak shape and a detail or two in the feet.

btw, was it ever established if archy was actually missing feathers
inward of the elbow when it was alive, or if it was a preservational artifact?

Tertials are missing in all specimens, even those that preserve contour feathers. A preservational artifact would be a bit odd.

In *Confuciusornis*, tertials are nicely preserved, IIRC.