Re: Archaeopteryx not the first bird, is the earliest known (powered) flying dinosaur

[Michael Habib <mhabib5@jhmi.edu>]

> Both sets of wings are very large, the hindwing is at least as large 
> as the
> fore, and the chord of both is constant and broad along the entire 
> span.

By 'large', I assume that you mean the area seems roughly equal.  The 
measured foil span is presumably quite different (as are the tip 
shapes).

> The forewing alone has an area/total mass ratio that falls
> in the middle of the avian range and is similar to that of 
> Archaeopteryx...

Do you mean for all birds, or for birds of similar mass to that which 
you have estimated for Microraptor?  Wing loading scales negatively 
with body mass in birds, so 'middle' of the range can mean multiple 
things.

> ...the hindwing halves the wing loading of course.

This actually doesn't follow, necessarily.  The hind wings would only 
halve the effective wing loading if the lift they generated supported 
an equal portion of the weight (relative to the forewings).  In modern 
birds, since the forewings generate essentially all of the measurable 
total lift, taking wing area/weight at face value works fine.  However, 
without knowing how the hind wings were deployed, and without taking 
into account center of mass, it would be inappropriate to simply use 
the total airfoil area from both forelimb and hindlimb and divide by 
total weight.  This is especially true when doing a biomechanical 
analysis of flight ability.  Wing loading is, after all, a way of 
estimating how body weight is supported in the air, and with a pair of 
possible lifting surfaces that may be significantly offset from one 
another, this becomes more complicated.

> The leg wing is so well developed
> that suggestions it was not frequently fully deployed as an airfoil 
> are not
> credible (it would be a plausible display device only if there were no 
> highly
> asymmetrical feathers), There is no viable means to deploy any part of 
> the leg
> feathers with the legs even partly folded (all such suggested 
> arrangements are
> awkward and contrived at best, if not anatomically improbable).

But then the question becomes how they can be deployed such that the 
hindlimbs generate lift but are in an anatomically probable position.  
Most reconstructions so far suffer from being either anatomically 
unlikely or biomechanically problematic.  I am not certain what you 
mean by 'fully deployed'.

>  The humerus is very robust and strong, with large muscle attachment 
> areas. So
> the arm wings alone are too large, strong and well muscled to be just 
> for
> gliding. Nor does gliding explain the evolution of advanced flight 
> features of
> the sort found in flying dromaeosaurs not present in Archaeopteryx. 
> Gliders are
> passive aerialists and do not need to updgrade their flight apparatus 
> beyond
> the limits seen in modern examples...

Well, while I agree that it is likely that Archaeopteryx and 
Microraptor were powered fliers, the comment that modern gliders set 
the bar for gliding adaptations is false.  For example, Icarosaurus had 
appreciably higher aspect ratio airfoils than modern Draco.  
Additionally, high aspect ratio wings with some fine control are 
generally quite good for gliding, so it really does come down to the 
muscle power argument.  Which leads me to ask: what metric you are 
using for the arms being too strong and/or well muscled for gliding?  
To an extent, a glider needs strong arms just to resist the forces 
associated with lift and drag.  Naturally, a powered flier will 
generally have appreciably stronger bones because muscle forces during 
flapping will be much higher than lift forces.  So what animals are you 
comparing robustness to, and how are you measuring robustness?

Of course, the issue of bone robustness is also more complicated, 
because how muscle force actually relates to skeletal element 
robustness is not entirely understood.  For example, shearwaters 
(marine soarers/divers) and grebes (obligate flappers which can barely 
glide) have roughly the same humeral bone strength in both bending and 
torsion (slightly higher for shearwaters).

The differences can be detected by taking into account mores specific 
qualities, such as cortical bone thickness, and whether the bones are 
equally robust in all directions (ie. Ix vs Iy).  But in general, the 
fact that one flaps a great deal more than the other in steady flight 
does not result in appreciably greater forelimb robustness.  On the 
other hand, grebes have much weaker humeri than say, ravens, and this 
is probably because ravens simply fly more often and with much greater 
agility (for reasons I won't get into now, agility and maneuverability 
are probably quite important in determine limb bone robustness for 
birds).

In any case, I look forward to seeing the reconstruction; the overall 
questions regarding flight ability in Microraptor are difficult but not 
untenable.  With luck they will be worked out in time.

Cheers,

--Mike Habib