Re: pterosaur launching tactics

[Michael Habib <mhabib5@jhmi.edu>]

> MPUM 6009, as you can see, retains the long legs of its ancestors, 
> something all other pterosaurs lack. A pterosaur with such proportions 
> would have about as much luck with a quadrupedal launch as a lemur.

Actually, it looks to me like it could crouch with the hindlimbs, tip 
forward onto the wings, and launch from that position.  Just a quick 
impulse, look, however.  I haven't worked on that animal - maybe it was 
a bipedal launcher.  If so, I imagine it was among the only biped 
launching pterosaurs (though other small species could biped launch, on 
occasion, if they really needed to - not as effective as quad launch 
though, by a long shot).

> On the other hand, Quetzalcoatlus, IMHO, seems well equipped for 
> terrestrial locomotion, able to move about quadrupedally and to lift 
> its wings readily. It also seems to have the best proportions in the 
> hind limb and pelvis of the latter three to launch bipedally. I

Except that it is simply to large to launch in that matter - much too 
large.  Also, Quetzalcoatlus (and Azhdarchidae in general, really) has 
more adaptations for powerful quad launch than practically any other 
pterosaur.  It's actually the extreme example of quad launch ability - 
the mechanical signal is very strong.  Even simple calculations 
indicate that quad launch would be very powerful, but bipedal launch 
would be essentially impossible.

I caution against getting too caught up in the qualitative look of limb 
lengths and resting position - one really needs to look at the 
quantitative mechanics to make assessments of launch ability.  The 
hindlimbs in Quetz, for example, may be nice and long for a pterosaur, 
but they are also unable to sustain the forces required for a bipedal 
launch.

> n any case, among the pterodacs, the ability to extend the wings 
> laterally while bipedal might have been sufficient to enable that 
> first flap to get airborne (without hind limb assist), and thus they 
> may have been, in a strict sense, launched by their forelimbs.

No flying vertebrate alive today launches in this manner, not even 
hummingbirds (which still get some hindlimb impulse).  In fact, that 
launch kinematic would fail to get almost any mid to large sized 
pterosaur into the air at all, unless they had one heck of a gust.  
Even small pterosaurs could not launch bipedally without a hindlimb 
impulse.  Remember, flying animals do not just flap themselves into the 
air - they have to generate some airspeed to reach their launch window, 
and this is done by pushing off the ground.  90% of the initial impulse 
for launch comes from the hindlimb in terrestrial birds - even in small 
species (Earls, 2001).  The impulse can be either bipedal or 
quadrupedal, but quad launching is much more powerful and is much more 
effective for elevating the animal high enough for flapping.  A quad 
launch is required to get to large azhdarchid sizes - birds maxed at 
around 80 kg, and that was near the limit given structural scaling and 
mechanical capacity.  Quetz, by contrast, wasn't even at the size 
limits for a quad launching pterosaur at 200-250 kg or so (maximum, 
fully fueled mass).

Also note that because a quad launch produces a more powerful initial 
push from the ground, and thus produces faster launch speed, it also 
has the effect that the required amplitude for the first flapping 
strokes is reduced.  At the other end of the spectrum, a pterosaur 
trying to launch with no push at all would need very deep flapping 
strokes, and be at ground level, and thus would impact the ground with 
its wings.  The initial push from the ground is very important, and a 
harder push is much better: quad launches produces the best initial 
pushes.

> I might suggest that the relative brevity of the humerus in Q. 
> provides less leverage for forelimb launch, but perhaps you guys don't 
> rely strictly on this lever for your launch. Still, while I don't 
> discount it, I'd like to see your version before guessing wrongly.

The short, stout humerus is well adapted for resisting the massive 
forces of the forelimb muscles during launch.  The excursion distance 
for the leap unload, by contrast, comes from the forearm and MCIV (the 
latter is, in azhdarchids, quite elongated and this improves both 
terrestrial mobility and launch ability).  Birds do the same thing, but 
in the hindlimb (because that's the primary launch module in birds): 
the proximal portion of the launching limb (the femur) is short and 
stout, and the more distal elements (which provide the excursion 
distance) are long.

> With regard to the vampire bat, Desmodus, despite its remarkable 
> secondary abilities, phylogenetically, the muscularity and 
> configuration of the hind limbs is so out of touch with any sort of 
> standard tetrapod upright locomotion that there really is no hope for 
> the hind limbs to assist in takeoff.

Not true; according to the force plate data, the hindlimbs do 
contribute to the launch in a significant fashion - the forelimbs 
simply contribute even more.  In fact, Desmodus femora are fairly 
robust by bat standards (though the humerus is also "overbuilt" 
compared to the norm).

> In this case, they act more like the tail wheel of an airplane, 
> keeping the butt off the ground.

The uropatagium probably does provide some lift after the hindlimb 
push-off, but only after the hindlimbs have imparted force to the 
substrate for launch - again, see force plate data.

> More importantly, please note that the fingers of Desmodus are beneath 
> the humeral glenoid, the bat's center of gravity. The hindlimbs and 
> its toes are far from it, which is not the case in pterosaurs, as you 
> can see by the drawings. That's why in Desmodus the launch has to 
> originate with the forelimbs.

Actually, Desmodus has to launch in the manner that it does because 
blood feeding produces very high wing loadings, and it therefore needs 
a powerful launch.  I rather doubt the cg is at the glenoid when it's 
full, but either way, a launching animal can reposition the cg a fair 
bit during launch, pterosaurs included.

In any case, you seem to be under the impression that bipedal launch is 
"better" unless the animal is forced to quad launch by historical 
constraints on anatomy.  Actually, it's closer to the other way around: 
quad launching is more powerful than bipedal launching.  The primary 
advantage to bipedal launching is the allowance for greater 
forelimb:hindlimb disparity - a value that is much larger in birds than 
in pterosaurs or bats (see work by Dyke and Nudds).


> Addendum: I do note that in Q., like Pterodactylus, in the 
> bipedal/quadrupedal configuration, the fingers are also below the 
> center of gravity, the humeral glenoid. That is not the case with 
> Nyctosaurus and Arthurdactylus. So what works for Q. and P. may not 
> work for N. and A.

The animals don't have to launch from the typical "standing" stance.  
Nyctosaurus and Arthurdactylus, and other especially long-winged forms, 
can easily shift the hands relative to the cg, and vice versa.  The 
launch cycle does not rely on statically stable poses (just as in bats 
and birds).  I think you may be a bit too caught up in cg position 
during standard, resting stance - limbs and cg loci can be moved a 
great deal.  Having the hand below the cg in Q. and P. does simplify 
things, but other configurations are not precluded from launch.  For 
one thing, the animal has forward motion from the hindlimb impulse 
before the forelimbs unload and give the primary power.  Thus, a quad 
launching pterosaur has a sort of vaulting phase at the launch 
initiation- the long forelimbs of things like Arthurdactylus would 
exaggerate that phase, but make the unloading slower.  Not quite as 
powerful a launch as in azhdarchids (hence the smaller max size), but 
still quite effective, and presumably a compromise for having limb 
proportions adapted to extremely efficient marine soaring with low 
payloads (hindlimbs become payload in flight, so reduction is helpful).

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu