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Re: pterosaur launching tactics
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
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
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).
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