[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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 toes.

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.

Major problems:

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) the wrist.

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.


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
(443) 280-0181