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Re: pteros have lift-off
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
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).
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181