Re: pteros have lift-off

[Michael Habib <mhabib5@jhmi.edu>]

David Peters wrote:
> I'm wondering where does the ability to leap several torso lengths 
> above the ground come from? I have little problem with Quetz. Huge 
> pectoral girdle. Huge humeri. Massive antebrachium. Presumeably a 
> large radius of movement and store energy at the metacarpus.

Yes, azhdarchids are disproportionately good launchers - which is one 
reason that can get so very large.  One strength of the quad launch 
model is that it rather accurately predicts which clades of pterosaurs 
should be able to attain especially large size.  For example, the quad 
launch model predicts that the max size in Pteranodontids and 
Ornithocheirids should be greater than for some of the more basal 
groups, but lower than the max size of azhdarchids.  So far, this seems 
to hold up.  Of course, it also predicts that some groups could have 
produced rather larger members than they seem to have done (tapejarids, 
for example), but that's not really shocking (not every group will be 
selected for gigantism).

> Using the same criteria and model, I have a problem seeing 
> Istiodactylus leaping. Pectoral girdle smaller than half the size and 
> not fused. Humerii huge, okay. Massive antebrachium, okay. Wing 
> finger, nearly three times longer relative to torso. Big problem. It 
> has to leap three times higher to follow the same wing opening 
> trajectory, whatever the lateral angle, on forelimbs no larger than Q. 
> and hindlimbs half the length relative to torso. So comparing apples 
> to apples, Istio needs a 3x larger effort on weaker equipment than 
> Quetz.

This has already been addressed, but the basic error here is that you 
are making the wing finger open far to early.  Quetz requires that 
robust equipment because it is so large.  Istiodactylus does not have 
nearly so much mass to accelerate.  More importantly, the launch is 
essentially a "leap first, flap second" situation - which is true of 
most avian launches, too (it just looks simultaneous to us, because 
they're quick).  Technically, the wing finger opens on the way into the 
first upstroke (after it has been raised a fair bit), but the basic 
idea is that the leap precedes the upstroke.

So, the summary comparison for quad versus biped launching, with 
regards to timing and clearance is that quad launching adds an upstroke 
period after the leap (while bipeds can upstroke a bit earlier), but 
purchases much greater launching power for this price.  The question 
then becomes rather straight-forward: does the extra power give enough 
height and speed to offset the slight cost of the extra upstroke time?  
For all pterosaurs I have yet tested, the answer is yes.  I believe 
this is true for Jim's work, as well.  This becomes especially true 
when one considers the lack of power in the hindlimbs of many 
pterosaurs (big hips or not, they couldn't be generating more force 
than the femur could handle).


> I'm curious as to what you and Jim say about wing clearance when 
> bipedal. Seems to me to be no problem if a pterosaur deploys its wings 
> while standing on two legs, then either leaping with hind limbs, like 
> a bird, or running to take-off speed, like other birds.

It's simply a matter of the fact that bipeds don't leap as high.  The 
ability to hold the wings out while standing looks like it gives better 
clearance, but it doesn't actually do so.  The first flap occurs near 
the top of the leap, in both biped launchers and quad launchers, so the 
clearance is set by the height of the leap.  More leaping power means 
more clearance.  Again, it's very important to remember that even birds 
leap BEFORE they deploy the first downstroke.  The only thing they get 
to do earlier than a quad launcher is the initial upstroke, and the 
expected upstroke time can be calculated alongside the estimated 
leaping height, so the hypothesis of clearance advantage is testable.  
This isn't to say that biped launching never has advantages - burst 
launching birds can take off at extremely steep angles, which the vast 
majority of pterosaurs probably couldn't manage (possible exceptions 
being very tiny things, like anurognathids).

> There's also the case of Rhaeticodactylus in which the forelimbs were 
> no more robust than the hind limbs and the humerus was unusually 
> elongated. Was it different, as a leaper, or the same?

Haven't worked on it, so can't really say.  Based on the external 
measurements, which is all I currently have, it is quite plausible that 
it had a unique launch strategy among known pterosaurs.

> Finally, if there was a change from leaping with the hind limbs to 
> leaping with the forelimbs, this should be visible in the anatomy and 
> phylogeny of the Pterosauria. Since outgroups had to be hindlimb 
> leapers, which anatomical marker tells us that the forelimbs have 
> taken over? And which is the first taxon to have this character?

As soon as the forelimb strength scaling rate exceeds the femoral 
scaling rate, a given lineage has likely made the transition to quad 
launching.  Of course, that requires a comparative sample.  If you want 
to do "single-point" markers, then one estimate is where the 
mass-specific forelimb strength starts to exceed two standard 
deviations from the measured biped trend.  Other markers include 
bracing in the forelimb for compressive loads (seen in relative joint 
areas, synovial sac attachments, dp crest orientation, and trabecular 
strut orientation within the dp crest).

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