Re: Dinosaurs and birds

[Michael Habib <mhabib5@jhmi.edu>]

On Wednesday, April 4, 2007, at 05:50  PM, dinoboygraphics@aol.com 
wrote:

> Now imagine that wing assisted incline running (WAIR) opened up 
> arborreal niches (because all of the biomechanical arguements against 
> dinosaurs in trees go out the window if you can walk up a trunk) for 
> the first time, leading to an explosion in arborreal but largely 
> non-volant winged theropods.  What would that have looked like?  I'd 
> imagine that you would have winged birdy critters that had well 
> developed deltoid muscles, and little need for large pectoral muscles 
> on the sternum, nor for a stabilizing role of the AHL on the 
> downstroke (which would be much weaker), nor even a need for the 
> glenoid to face as upward as it does in volant birds.  Sound familiar?

It is a cool scenario, and I've had similar thoughts myself.  Up until 
quite recently, I had favored essentially the same idea.  There is a 
catch, however.  WAIR actually requires rather significant power 
output, and a very powerful upstroke with significant amplitude.  That 
means that WAIR actually has pretty steep prerequisites.  It requires a 
supracoracoideus pulley, or some other character to generate a rapid 
upstroke.  It requires strong elevation of the wing relative to the 
body axis, and it probably does require the AHL because both the 
downstroke and upstroke actually have to be rather rapid.  WAIR tends 
to emphasize the power on the upstroke, and upstroke-enhancing 
adaptations are actually *more* derived in avians than downstroke 
enhancing characters.

To put it another way, WAIR tends to require ring vortex gait related 
characteristics.  Ring vortex gait usage is probably fairly derived, 
however.  If basal avians and near-avians were able to get into the air 
at all, it would probably be via a continuous vortex gait (which is 
"easier" in many ways).  Archaeopteryx, for example, might have been 
able to manage a continuous vortex gait, but has few characteristics to 
indicate usage of a ring vortex gait (see Rayner, 1991, 2001)

The one thing that WAIR doesn't require is a fully formed airfoil.  
That is, it can be used by birds with wing loadings so high that they 
cannot launch.  Thus, juvenile birds without the ability to launch or 
sustain flight can still use WAIR, if they have a relatively 
well-formed flight apparatus otherwise.  As you have probably already 
noted, the birds in which WAIR is recorded are generally precocial 
species in which the flight apparatus is well developed early in 
ontogeny, excepting airfoil size.  Altricial species can use WAIR as 
well, but only later in ontogeny.

It is also notable that species like Microraptor have high aspect ratio 
wings, and probably lack slotting, which is not expected in an animal 
adapted to WAIR behavior (at least not as an adult; perhaps juvenile 
Microraptor individuals had more appropriate planforms).  The planform 
in Archaeopteryx is closer to expectations (though its wing loading 
seems a bit low for what we might expect in a WAIR-adapted animal).

All that said, I do think that the general idea of derived 
maniraptorans moving between the ground and arboreal habitats has a lot 
of merit, and I think it nicely connects the previous dichotomies of 
arboreal and cursorial launching systems.  I also think that it fits 
well with the growing picture of early flight evolution in birds.  
However, actual WAIR mechanics are unlikely to be the critical link in 
question, unless basal birds actually had more flight ability than we 
give them credit for, rather than less.

> Did Sapeornis and Jeholornis fly?  Quite possibly, but shouldn't we 
> perhaps test between the above hypothesis and the prevaling assumption 
> of volancy?  As other non-volant, winged maniraptorans show, there are 
> a wide range of things you can do with wings that don't require actual 
> flight, and we need to be critically examining them rather that 
> grandfathering in Archaeopteyrx, and therefore all animals closer to 
> birds than Archaeopteryx, on little more than 150 years of tradition.

This I definitely agree with.  I'm working on a different angle for 
testing that long-running assumption for Archaeopteryx as well, and 
that may also make an appearance at SVP.  I'm happy that I'm not the 
only one!

Cheers,

--Mike H.