Re: FOUR, THREE, TWO, ONE . . . PTEROSAURS HAVE LIFT OFF!

[Mike Habib <habib@jhmi.edu>]

Thanks Dave!

Couple of general comments regarding your concerns, and then more  
specific replies indicated below:

With regards to clearance issues, bipedal launch actually makes  
clearance more problematic, not less.  Jim has already responded to  
the specific kinematic issues, and did so just as well as I can, so  
I'll let that stand.  More generally, though, a biped launch would  
require that the same clearance be generated with less muscle power.   
This appears to have had substantial influence on planform evolution  
in birds, with steeper launchers requiring reduced spans in order to  
get the clearance for early onset of flapping strokes.  It feels a bit  
like the biped launch gives more clearance, because the animal is more  
erect and all.  But, it's the top of the leap that matters for  
flapping clearance (be it a biped launch or quad launch), and more  
power means more height.

It's also worth noting that running launch (among birds) is not really  
a way of "building up" flapping power, either - it's mostly an  
adaptation to water launching, and occurs in species with posteriorly  
located hips, shortened hindlimbs, strong femora, high wing loadings,  
and gracile forelimbs.  The only one of these that matches pterosaurs  
(some) is the high wing loading.


David Peters wrote:

> While Mike makes a good case, I was still a little disappointed not  
> to see a step-by-step sequence illustrating this launch sequence in  
> a variety of pterosaurs. In the spirit of 'spirited discussion' I'd  
> like to challenge Mike a little, again.

Well, the paper went in some time ago, and I've been working on  
illustrations since that time.  I also don't want to scoop my Masters  
student, who is doing a launch animation for her thesis.  Still, your  
point stands: an illustration would be very helpful for lots of folks.

> At manus lift off, the metacarpus would, of necessity, still be  
> vertical, having just pushed off the earth like a airborne pole  
> vault. A vertical metacarpus means the wing finger was still in the  
> vertical plane, but no doubt beginning its rotation snap to the  
> flight position. The question is: how long does this take? After  
> passing the horizon, the wing finger would be on a collision course  
> with the earth--unless the proximal wing had rotated laterally  
> sufficiently to enable passage of the entire wing above the  
> substrate. The clearance shrinks with every passing nanosecond.

The positions aren't quite correct, but your basic question is  
perfectly valid: is there enough time and clearance for a stroke  
cycle?  While we cannot calculate the amplitude and flapping frequency  
exactly, rather robust estimates can be produced using known scaling  
trends in flapping flight (that hold across multiple clades).  Having  
run these calculations, I find that clearance distance and time were  
more than sufficient, even when I use very conservative numbers for  
pterosaur flapping frequency.


> The quad launch of Istiodatylus, with a much smaller  
> scapulocoracoid, tucked wing fingers several times longer and  
> relatively weak legs several times shorter presents quite another  
> scenario. Istiodactylus would have had to leap several times higher,  
> relative to its torso, than Quetz would have on ostensibly weaker  
> landing/launching gear.

It's launching gear isn't as weak as it seems: most of the power is in  
the forelimbs.  Now, that said, azhdarchs do generally have more  
powerful launching systems (not so much because they have stronger  
hindlimbs as much as they have more powerful forelimbs).  This  
explains the larger observed max size in azhdarchs, and it makes the  
launch sequence faster.  Istiodactylus ends up with a slower, longer  
vault phase, but clearance still seems to be fine.

If, on the other hand, you make Istiodactylus launch with the  
hindlimbs, then it does run into a power and clearance problem: now  
those weak legs you mentioned are a serious handicap.  Again, if  
clearance and height are the issue, then more power helps.

As a rough comparison to give everyone an idea of how much this helps:  
a passerine bird just barely clears on its first flight stroke when  
launching.  A vampire bat of roughly the same size, full of a blood  
meal, quad launches up to three feet vertically.

> There is some evidence (which I have not seen) for a bipedal launch  
> in the literature. Unfortunately I don't find it in Mike's reference  
> list.
>
>
> A New Pterosaur Tracksite from the Jurassic Summerville Formation,  
> Near Ferron, Utah
>
> Authors: Debra Mickelson1; Martin Lockley2; John Bishop3; James  
> Kirkland4
>
> Ichnos, Volume 11, Numbers 1-2, Numbers 1-2/January-June 2004 , pp.  
> 125-142(18)

I probably should have referenced it, but it didn't happen because I  
haven't actually looked at the tracks myself, and (more importantly)  
one of the authors indicated to me at SVP a while back that these are  
probably landing tracks.  It's also worth noting here that when  
looking for trackway evidence of launch sequences, we should keep our  
eyes open for evidence of leaping (probably more so than running).   
Even if pterosaurs somehow managed to launch bipedally, it still  
doesn't suggest a running launch - most birds launch by leaping, for  
instance.

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