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Re: Pterosaur take-off movie on the NG site
David Peters wrote:
Minor problems (Tupuxuara painting):
1. When quadrupedal, the palms would be in the neutral position,
neither suppinated nor pronated, but positioned as in clapping or
tree grappling. When pressed against the ground then, the fingers
splay laterally to posterolaterally. The painting of Tupuxuara has
trouble with putting the fingers "pads and claw tips down."
The fingers are indeed a bit off. It was an improvement from earlier
versions. I did not do the painting, and neither did my student,
Julia (who produced the animation).
2. Chest is way too shallow on painting, should be 3x neck depth.
Upper arms way too gracile. Beef them up!
The torso is actually overall a bit too large, albeit possibly too
shallow as you suggest. Again, the artwork was done by an individual
learning pterosaur anatomy from scratch. I'd cut them some slack.
3. Should be a shallow keel on the dentary for Tupuxuara.
True, but not a big deal.
4. No hair?? Too little hair? Needs more fuzz.
We have no idea how much "fuzz" Tupuxuara had. The current rendering
is perfectly plausible. Similarly, if you use more pycnofibres, then
that's great, too.
5. Wings should nearly disappear whenever folded. Wings don't extend
onto the tibia, but are stretched between the elbow and wingtip with
a fuselage fillet extending back toward the femur. No fossil shows
it any other way.
Actually, no fossil shows the inboard wing clearly at all; the ankle/
tibia attachment is a plausible reconstruction that several authors
currently support. I generally use a more narrow wing that than
preferred by the paleoartist that did the NG work. I have no
particular problem with it, though. I disagree that the wing goes
from the outboard tip the elbow - there are several fossils showing
that the trailing edge passes about 50% of the humeral length behind
the elbow. Where it goes from there is a matter of debate, because
the more elastic mesopatagium is generally a mess (or missing) in most
specimens, even those that have a nice outboard wing preservation.
Minor problems (Anhanguera movie):
1. Ordinarily on the ground, the knees are flexed at 90 degrees,
extending only during leaps and in flight. Move feet beneath the
torso. This happens when you bend the knees. Footprints show
pterosaurs are not wide-splayed like this. (We don't have Anhanguera
tracks, but we do have beachcombing pterosaur tracks). When the
knees are bent the feet move further forward, beneath the center of
balance near the shoulder joint.
The hindlimbs are not particularly splayed, and actually line up with
some of the beachcombing tracks, so I consider the position
reasonable. Other positions can also be hypothesized, and also work.
We don't know what the "ordinary" hindlimb posture is for any large
pterodactyloid. The more vertical posture you prefer is quite
plausible; it is not the only plausible posture, however, nor does it
actually affect the overall launch sequence: if the starting posture
is more vertical, then the animal can simply shift weight forward into
a more horizontal posture first. This adds a small amount of time to
the overall sequence. It is not of major consequence.
2. Needs prepubic bones. I know, they're unknown in Anhanuera, but
all other pterosaurs have them.
They were probably there, yes - but we worked directly from the AMNH
22555 specimen, which doesn't have them.
3. Anhanguera feet are much smaller than you show. The smallest feet
of all pterosaurs. During the leap, the feet should elevate from
heel to toe in sequence, delivering a final push with the tips of
The feet were reconstructed from the cast of 22555 and some estimation
to fill in gaps. Data subsequent to that production does seem to show
that the feet are smaller, though I'm not sure I agree they were as
short as you imply. Again, an interesting tidbit but not fundamental.
4. The forelimbs are quite wide. A more efficient vector would have
the limbs directly beneath the shoulders.
Yes, but they cannot get there. We constrained the forelimb postures
according to published estimates of possible joint excursions (which
we confirmed with the original specimen as best we could). Anhanguera
appears to have been a relatively wide-gauge animal up front. Other
pterosaurs were probably not so wide gauge.
5. Anhanguera did not have a typical tetrapod gait, nor a typical
pterosaur gait. With such short legs, the force vectors from fingers
to shoulder could never point forward. This relegates Anhanguera to
a weak bipedal hindlimb propulsion (knees bent) with support, not
propulsion, coming from the wings. Like using crutches, except the
crutch tips have to stay in front of the shoulder the whole time on
Anhanguera. My guess is it stayed airborne as much as possible.
We don't know what the gait was in Anhanguera. There are multiple
possible gaits. One of them is the near-bipedal gait you describe. A
more thoroughly quad gait is also plausible, and is what we used.
Contrary to your assertion, it is possible for the forelimbs to
provide walking power in Anhanguera, albeit not a great deal of power
because of their position. I agree it was likely airborne most of the
time, and I would not expect it to be capable of a canter or gallop,
as I suspect that most azhdarchids were.
1. The narrator reports the "knuckles of the wing fingers rest upon
the ground", but this is not so according to tracks. Only would be
so in the case of Nyctosaurus (and pterosaurs like it), which has
vestiges for fingers and a much longer metacarpal.
Actually, the fingers are small and weak enough in Anhanguera that it
almost certainly put body weight on the mcIV-phIV joint. In fact,
this was probably true of most large pterodactyloids, and is apparent
from some of the better tracks when viewed in detail. While there is
no obvious "divot" for the knuckle (actually palmar side, but in any
case), there is a broad enough contact area that the mc/phIV joint
must be in contact with the ground. If it were not, then pterosaur
tracks would have a "hollowed" appearance in the middle of the manus
print, as the base of the digits 1-3 cannot touch the ground. The
reason that the tracks look filled, presumably, is that the mcIV/phIV
joint is touching the ground. I base this on both personal
observation of original tracks, and on published data. If you have
examined a trackway that shows otherwise, that can be attributed to a
large pterodactyloid (esp. an ornithocheirid), please let me know.
2. On the closeup view of the slipping tendon, metacarpals I-III are
inaccurately shortened. In reality they should extend as far as the
big wing metacarpal (#4). If this was done for illustrative
purposes, then it cheated the whole concept. Fingers 1-3 should be
larger (1/3 of the metacarpal length in Anhanguera) with much deeper
unguals (claw bones), further lengthened by keratinous extensions.
Ordinarily, and pterosaur tracks show this, these three fingers take
ALL of the weight not taken by the hind limbs. There is no deep
track preservation of digit IV. There is no trace at all, typically,
of digit IV. If no weight is placed on this joint, then there is no
elastic recoil and the hypothesis has no power source for takeoff.
See above. The fourth metacarpal/phalanx joint was likely taking a
lot of the weight. Not only is this apparent in trackways, but we did
not shorten the fingers; the proportions were based upon
reconstruction of AMNH 22555. Because there is some estimation
involved, it could be that the fingers were a bit longer in life than
we estimated, and could grip the ground to some extent, rather than
lying above the surface. Even presuming, for a moment, that this is
the case, I can say with confidence that digits 1-3 did not take much
of the weight not accommodated by the hind limbs, as these small
digits are not structurally capable of doing so. Even if the fingers
were much longer than we reconstructed, and were much stronger than
calculated somehow, and in a position to take more weight than
reconstructed - the animal could still sink onto the mcIV/phIV joint
at the start of launch, thereby initiating the catapault.
Interestingly, the flexor tubercle on phIV-1 is shaped just right to
be walked on in every pterosaur I have ever seen that has this
structure preserved. This would be a heck of a coincidence given that
the exact flattening tracks the evolution of gait - so in more
vertical walkers the flattened aspect is still positioned to take body
weight. Seems strange if it never touched the substrate.
In any case, it turns out that the elastic storage mechanism is not
required for the launch to work - it just makes it more powerful, and
therefore faster. Without the flexor tendon recoil, the launch simply
takes longer. I have done all of the launch calculations without
assuming the lock and release, and the numbers still come out fine,
just not quite as impressive. This would mean that escape from
predation might be more difficult, but at least Anhanguera would still
get off the ground. No bipedal launch scenario gets Anhanguera into
the air, unless one assumes some very specific cliff topography is
constantly available. Even then, it's a close thing.
3. Whenever an animal leaps into the air (think of frogs, kangaroos
and basketball players), after the leap the legs extend straight
back trailing the rest of the body in the line of the trajectory
until they draw up somewhat and extend forward near the apex to
prepare for a springy landing, lined up with the anticipated landing
spot. In this pterosaur animation "the elastic recoil forcefully
extends the forelimbs," but the wing finger is not forcefully
extended. The animator holds it back, to avoid contact with the
ground, which would have been inevitable if indeed there was an
elastic recoil at this joint. But slamming the wing finger into the
ground won't work...
First of all, not all animals leap with the position you suggest. In
fact, vampire bats, the living quad launchers, don't do so. But more
to the point:
1) the wing finger *is* forcefully extended - because of its shape,
only the proximal end (the flexor tubercle itself, essentially)
actually hits the ground. The rest of the finger does not "lay flat",
as it were.
2) the long flexors for the fingers also cross the wrist. Much of the
stored power goes into mechanically extending (anatomically flexing)
3) Actually, letting the proximal wing finger phalanx hit the ground
does work, though I'm not convinced this actually happened for the
reasons noted above. But, as it turns out, PhIV-1 is reinforced in
the AP direction enough to take the estimated bending load,
interestingly enough (in part because even with the wing finger
striking the surface, the cg is placed, at that point in the launch,
such that the bending load on PhIV-1 is pretty modest).
4. So the elbow is forcefully hyper-extended and no distal (ground
contact) elements are forcefully extended. When the elbow is
hyperextended the propatagium is minimized as it is stretched to its
limit. Birds have a ligament to prevent exactly this sort of
hyperextension of the elbow. Bats do too.
The elbow is extended, but not hyper-extended. Either way, yes, the
propatagium is somewhat minimized during the push-off. This doesn't
matter in the slightest. I would not be surprised if there were a
leading edge tendon in the propatagium for pterosaurs. Note that the
presence of such a tendon does not prevent relatively heavy extension
of the elbow and a quad launch, as evidenced by the fact that bats
have such a tendon, and quad launch well.
The 1-4 sequence of an azhdarchid takeoff called "Winging it":
1. This sequence shows that all of the propulsion comes from the
elbow joint because the wing finger is not fully extended
immediately after take-off (according to the illustration), as it
would have to be if indeed a recoil of this joint is operating. Here
there's not enough coil in step #1, not enough recoil in step #2
(note the pterosaur is no higher after extension) and for some
reason it keeps on rising in step #3.
I think these are things you'd need to take up with the artist.
Personally, I think it got the point across, despite not being 100%
biomechanically accurate. The anatomy of thee azhdarchid is not
exactly precise, either. Such is life. Remember, the artist is
trying his/her (his, in this case) best to depict what the
quantitative results entail, rather than the conclusions being based
upon an art rendering.
Mike, sorry I didn't nail these problems earlier. I never realized
you had shorted the fingers and metacarpals I-III to make the recoil
pressure on the big metacarpal possible.
They aren't problems, so no need to worry.
Assistant Professor of Biology
Woodland Road, Pittsburgh PA 15232
Buhl Hall, Room 226A