Quetzalcoatlus Forelimb Leap Questioned by Chatterjee et al. (2012)

Mike Habib started this argument a few years ago by proposing that all pterosaurs were built strong enough in the forelimbs and weak enough in the hind limbs to adopt a forelimb launch technique similar to that of the vampire bat, Desmodus.

We discussed this earlier wherein I produced launch trajectories for successful attempts and unsuccessful ones. The margin for error was pretty darn small. A leaping or running take-off like birds employ was considered better on all counts, enabling three flaps in the space that one would take place if the forelimbs were employed for leaping.

Now Chatterjee et al (2012) have argued against a forelimb takeoff in Quetzalcoatlus. This isn’t Chatterjee and Templin’s first foray into pterosaur biomechanics. Their 2004 book was 64 pages of nothing much new and not much valid.

The Dinosaur Mailing List took up the Q. leaping arguments starting here. The two sides differ on estimated mass  for Q. Chatterjee et al. (2012) suggest 79 kg. Others (Witton, Habib) propose 250kg, which seems much more reasonable.

Here Mike Habib proposes that a 3 meter leap is more than sufficient for flapping clearance in a giant pterosaur. To my eyes three meters (10 feet) is one helluva leap for a pterosaur forelimb where most of the musculature is proximal to the elbows (Fig. 1). How high can a human catapult when doing clapping pushups? Not very high and not for very long.

There are several specimens of Zhejiangopterus.

Figure 1. Click to enlarge. There are several specimens in the growth series of Zhejiangopterus. Note the very small size of the humerus, which is supposed to pogo this pterosaur 3 meters in height during a forelimb take-off?? It’s the smallest bone in the wing. Seems impossible. Better to take off like the proposal in figure 2.

Certainly flapping from low altitudes (within a wing’s length) does not entail lowering the humerus very far in birds and pterosaurs during takeoff  (Fig. 2). The big problem with the forelimb leap hypothesis lies in the fact that if pterosaurs are launching quadrupedally they initiate their leap with the humerus oriented ventrally and it remains ventral during the entire leaping phase and for a certain amount of time during the follow-through extension after the leap — and for a certain amount of time during the wing folding recovery phase, getting set for the first major extended flap. During these phases, prior to the first flap, the wing finger has to open. While the forelimb is ventral and vertical the wing finger is also ventral and vertical, potentially banging into the ground until the humerus is raised after the leap extension and after the recovery follow-through. That takes time. Perhaps more time than the puny leap from the puny humerus would allow.

Habib’s model proposes a preloading of the catapult by initially launching the hindlimbs and compressing the folded forelimbs. Habib also figures that the wing finger extensor tendon was compressed beneath the body during launch. We know from hundreds of ptero tracks there’s no indication at all of the wing finger making any impressions in the matrix. The illustration he and his artist produced cheated this bit of morphology as I pointed out earlier. Their invention served their own cause but was completely bogus based on bones and manus impressions.

Quetzalcoatlus running like a lizard prior to takeoff.

Figure 2. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

There’s not much preloading possible on this the shortest of the wing bones, the humerus (Fig. 1). I don’t see how such a pogostick could rise more than a few centimeters, which is how high pogo sticks actually rise.

The main argument of the Habib model is the scaling of limb bones. From the Witton note:

“…bird hindlimb bones become proportionally much larger with body mass, because the increased mechanical stress of launching at larger body masses means the hindlimb bones must be much stronger. By contrast, bird wings are rather slender, as the forces incurred in flight, even at large size, are much less than those incurred in takeoff. If pterosaurs launched with their hindlimbs, like birds, their limbs would show the same scaling relationship. They don’t. Instead, like quadrupedally launching bats, their forelimb bones get proportionally bigger with body size, while their legs remain slender. Mechanical testing of pterosaur leg bone strength suggest they would buckle if they were used in launch alone, but their forelimbs are incredibly powerful.”

First of all, there are no larger bats that launch quadrupedally, like tiny vampire bats do. Second, I can’t imagine any creature built so fragile that it’s hind leg bones would buckle if used like hind limbs are typically used. Third, the diameter of the humerus in some pterosaurs, like Zhejiangopterus (Fig. 1), is comparable with the much longer femur. Which one was most capable of producing the greatest leap? In this case, it is clearly the femur — longer and therefore better leveraged.

There is no doubt that in most pterosaurs the humerus is indeed thicker than the femur (but there are exceptions!). Sometimes the humerus is ridiculously thick (see Arthurdactylus). Sometimes, in similarly-sized and built pterosaurs, the humerus is not so thick (at its narrowest) and can be shorter than the femur, as in Pteranodon.

With regard to Quetzalcoatlus, it is important to note the great increase in thickness in the large specimen humerus versus the small specimen humerus. We don’t know much of the rest of this giant pterosaur, but if similar to the smaller specimen, there was a large skull and long neck to deal with. We don’t know anything about the hind limbs of the giant. It is possible that the much more robust humerus was needed for flight. It is also possible that such a robust humerus was needed for quadrupedal activities.

For comparison sake, take Zhejiangopterus (Fig 1), a pterosaur known from small specimens and large ones. The small humerus was dwarfed by the much longer and nearly equally thick femur. The size of the humerus in proportion to the rest of the body seems too small to generate enough extension propulsion that it could catapult this pterosaur into the air in the manner that Habib and others suggest.

As Mike Habib noted, even large vultures can take off from a standing start by a simple leap into an updraft.

Dimorphodon.

Figure 3. Dimorphodon. Not much difference in the diameter of the humerus and femur.

Finally, in terms of humerus vs. femur diameter, Dimorphodon (Fig. 3) demonstrates very little difference. The relatively greater length of the humerus (relative to Zhejiangopterus) was more likely to produce a greater forelimb leap in this taxon, but to plant the hand on the matrix depresses the spine and skull creating awkward angles for subsequent actions.

See more of the problems of a forelimb launch illustrated here. And Chatterjee et al (2012) isn’t right either. Pterosaurs did not have to run downhill!! (how ridiculous…)

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Chatterjee S, Alexander DE and Templin RJ 2012. Flight-initiating quadrupedal jumps in the giant pterodactyloid Quetzalcoatlu: Fact or Fantasy? 2012 GSA Annual Meeting in Charlotte.
Chatterjee S and Templin RJ 2004. Posture, locomotion, and paleoecology of pterosaurs. – Boulder, Geological Society of America (Special Paper 376). 64 pp. ISBN 0-8137-2376-0.

5 thoughts on “Quetzalcoatlus Forelimb Leap Questioned by Chatterjee et al. (2012)

  1. Finally someone questioning the flight of the Pterosaur! I first entered into this area by pure chance in the early 1980’s (approached by an eminent palaeontologist to see whether Pterosaurs could fly). Based upon the information available at that time, I could only make the Pterosaur “fly” if the atmospheric density was about twice what it is today. I have again been approached some 30 years later to see if I can still come up with the same conclusion. Scanning through recent work, seeking essentially parameters pertinent to aerodynamics, I am finding increasingly that nobody seems to have a firm set of body measurements ( e.g. mass, wing span, wing area etc. of the Pterosaur) and start with high mass estimates only having to reduce them to levels, unsubstantiated by fossil findings or other data, so that this “bird” can fly in today’s atmosphere! Perhaps I was right all along!

  2. Greetings Dave. Well, we agree on one thing: Chatterjee et al. don’t have a very good model. A 75 kg mass is unrealistically low, and running downhill is indeed ridiculous.

    I am going to have to take some issue once again with your assertions, though. I have been pondering whether to write a reply to your other page on launch for some time, and I admit that it just wasn’t a high priority. Since you have repeated the same mistakes here, however, I think it’s worthy pitching in.

    1) The biomechanics of leaping do not work they way you suppose. Having short, powerful proximal limb segments attached to longer, more tendinous distal segments is, in fact, the best morphology for leaping, not the worst. This is why many of the best leapers (antelope, vampire bats, tarsiers, etc) have such a build.

    2) If you are going to presume that a given leaping height is exceptional, then you need to do the calculations. To put things in perspective, though, consider that a 3 meter leap is more than workable for a 200 kg antelope (see Biewener’s biomech book; chapter on jumping) and a 200 kg Quetz would be even better suited for leaping than an antelope, since it wasn’t carrying a huge gut as payload.

    3) Julia and I based our hand reconstruction in Anhanguera on the reconstruction available at the AMNH at the time. Yes, as it turns out, the free fingers were much longer than was thought at that time for ornithocheirids. The thing is, for the catapult to work, the animal need only be *able* to push down on MCIV, it does not have to be habitual during walking. Oddly enough, your very own finger reconstruction improves the catapult by providing a better tendon pinch.

    4) Your animations look superb in terms of rendering (you are a great artist, regardless of our disagreements). However, they do not seem to be very accurate. The flapping rate you use is far too fast (or the leaping is far to slow; you don’t have a calibration given). If you want to work out the flapping rate, you need to do the calculations. You can use a two-equation system to do this fairly simply (Pennycuick 2008 combined with Strouhal Number constraints; see Taylor 2003). The expected flapping frequency for Quetzalcoatlus northropi in steady flight, is just over one hertz.

    5) Larger bats do launch quadrupedally. Cynopterus, for example, can quad launch almost vertically even with 25% of its body mass added as saline, giving it a wing loading not terribly different from a large pterosaur.

    6) Your comment above: “I can’t imagine any creature built so fragile that it’s hind leg bones would buckle if used like hind limbs are typically used. Third, the diameter of the humerus in some pterosaurs, like Zhejiangopterus (Fig. 1), is comparable with the much longer femur. Which one was most capable of producing the greatest leap? In this case, it is clearly the femur — longer and therefore better leveraged”

    I can’t imagine that they would buckle their hindlimbs either, which is why I suggest they didn’t use the hindlimbs as the primary launch module. I’m sorry, but unless you have reworked the cross sectional properties calculations and have a different result, I am going to have to go with the data and say that the femur just couldn’t handle what you’re suggesting as a takeoff strategy in giant pterosaurs.

    You also do not seem to follow what is going on with the strength of the bone elements. If two elements are similar in diameter, but different in length, then the longer one will be easier to break (assuming the moment arm is proportional to the difference in length). That leverage you mention is the leverage working against the femur to snap it. So, in fact, you have the right data but the wrong conclusion: the humerus is the stronger element. It can therefore sustain the greater reaction forces. To get a good leap out of it, that stout element needs to carry most of the muscle but be connected to longer elements more distally – which is exactly what we see.

    7) I still don’t understand why you insist on a running launch. The only living animals with a long takeoff run are aquatic birds. Azhdarchids lived inland; I see no reason to have them run. If you cannot find a plausible way for them to launch by bipedal leaping, then bipedal running won’t fix the problem.

    8) It is also possible to calculate the timing for the initial upstroke. This is, in fact, the *only* downside to quadrupedal launching compared with bipedal launching. You are absolutely correct to ask if there is enough time. To get the timing, we can again go to flapping frequency publications, and this time use the area and span of the *folded* wing (since it would still be mostly folded during the initial upstroke) to get the timing. The result is that not only does Quetz have enough time for that initial upstroke, it has almost double the time it likely needed.

    With the timing problem solved, quad launch does everything else better than bipedal launch. More clearance, more speed, and better standing-start acceleration (important for beating the Wagner Effect). I have no idea how you calculated your clearance “tests”; it appears you simply drew out animations that seemed right to you intuitively. The wings cannot “catch” the animal as you have illustrated for Pteranodon on your other takeoff commentary page.

  3. Mike,
    The best leapers: vampire bats, antelope, tarsiers, appear to have leaping limb segments of more equal length, not short powerful proximal segments. Azhdarchids have a short powerful proximal segment, like a giraffe, which is not known for leaping.

    Your hypothesis MUST be put into a phylogenetic perspective that shows the evolutionary moment or grade when the forelimb became the dominant leaping engine. The basal forms were definite hind limb leapers, with forelimbs too short to reach the ground as you propose.

    Your dataset needs to broaden to include those pterosaurs that had slimmer forelimbs relative to their hindlimbs. Then figure out the strength of those specimens based on their bone thickness. If they fall within the bird range, that has to be noted, to be fair.

    I don’t insist on a running launch. Leaping is just fine. It’s simply part of the repertoire based on the ilium size which produces huge thighs great for running. Many pterosaurs could run, but ornithocheirids seem to be the poorest of that bunch based on the reduced hind limbs. They may have been insulated from ordinary pterosaur needs by a pterosaur-friendly environment, like a windy highpoint isolated from predators. If ornithocheirids are your poster children for forelimb leaps, as your video indicates, I’m still going to have to see the key frames of your animation showing the wings trailing vertically for several frames following the initial take-off (as in all leaping animals there’s that thing called follow-through), then I’ll need to see how the huge and massive wings can rise to provide that first downstroke, while unfolding and produce that initial downstroke, all frame-rate timed. You’ll need to show where the takeoff succeeds and where it would fail by banging the wings against the substrate when the pterosaur grows too old to produce sufficient leaping power, or the point where the hypothesis would break down. Hopefully that would provide sufficient space and time for a safety factor during each and every takeoff in a variety of taxa, but one would be sufficient for a start.

    My estimates of flapping rates are based on what I’m seeing on videos of pelican takeoffs from water, from vultures and from other smaller birds. It’s a blur it happens so fast, as you know.

    With regards to bone length, strength and snapping points, I don’t see anything in pterosaurs more fragile or gracile than the forelimbs of vampire bats, which appear to be less heavily muscled.

    I appreciate your loyalty to your numbers, but these issues and taxa must be included. Plus, it would be great to find a fossilized takeoff point as you describe.

  4. “I appreciate your loyalty to your numbers, but these issues and taxa must be included”

    That would be more concerning if you raised any real issues.

    The animation Julia produced is already time-calibrated, including initial upstroke. I’m not sure why you insist that the wing has to open before the upstroke: I have reiterated numerous times that the wing of quad launchers stays mostly folded until well into the upstroke. More importantly, you’re doing things backwards: you want the animation to be your answer, but animations need to be tied to quantitative results. The *numbers* are the answer. When we study kinematics in living animals, you use the high speed video to get at the numbers, not the other way around. The timing numbers come out emphatically in favor of sufficient upstroke time and clearance, even when I am conservative. If you want to challenge this, that’s great – but you need to do the calculations for it. I have already supplied sources for some of the fundamental equations.

    While you are at it, be sure to provide solutions to the numerous problems of biped launch (again, quantitatively as possible):

    – The angle of attack problem for bipedally launching pterosaurs
    – The potential flutter problem for bipedally launching pterosaurs
    – The lack of sufficient skeletal reinforcement for biped launching in large pterosaurs
    – The lack of sufficient clearance during a bipedal launch for most mid to large pterosaurs
    – The reversed allometry of the forelimb and hindlimb compared to bipedal launchers

    What I take home from your response is that because you recover bipedal outgroups for pterosaurs, you insist that they were bipeds and did everything bipedally. It makes no sense to simply forbid gait transitions to happen in this manner, and is particularly odd given that the trackways from pterosaurs are, well, all quadrupedal.

    As for a phylogenetic pattern analysis: that is a great idea, but no, I do not have to do one to demonstrate that a quadrupedal launch was likely for many (especially derived) taxa. It certainly would be neat to know where the quad launch shows up, but we don’t have to know the origin of a trait to examine it biomechanically. I am estimating performance, and while origin of the trait in question is an important evolutionary question, it is not needed to estimate raw performance. Incidentally, the reason you can’t get basal taxa to touch the ground with the forelimbs is, I suspect, that you force such an upright stance on your pterosaurs. All the basal forms I have so far analyzed seem to indicate a quad launch modality. That said, I’ve focused much more on derived taxa, and it may be that quadrupedality shows up in pterosaurs further up than the base of the clade. I don’t think that’s true, but it could be.

    What frustrates me the most about your response is that while I have, in fact, numerically checked the timing and clearance expectations for both bipedal and quadrupedal models, you have not done the same – but you keep insisting I need to do more of them. If you think bipedal takeoff would have the clearance and power required, then show me a compelling quantitative analysis. The second piece of frustration is the fact that nearly every constraint you have raised would be *worse* for a biped launch. In particular, wing clearance is less under a bipedal launch (it has to be: the animal is generating force from only two limbs instead of four, so it won’t go as high, to be colloquial about it).

    Regarding flapping rates, I appreciate the candid response. I see now why you are getting unrealistic wing flapping rates for your pterosaur models. You cannot model them as pelicans or vultures for that; you’ll need to calculate expected flapping frequency from a base principle equation (or two, ideally). I will give you this: at least you haven’t grossly *underestimated* the flapping frequency, which I see a lot.

    Oh, and vampire bats have very strong humeri relative to mass (yes, they are long, but also have a huge diameter and comparatively thick walls). You can’t just eyeball it. I gathered full long bone data on 8 vampire bats at the USNM. Taking allometry into account, they’re not as robust as in large pterosaurs, but that’s not surprising.

    Look more closely at the skeletons of leapers (or, better yet, do as I have and measure them in collections): the proximal segments are typically shorter and stouter than the distal ones. Is it as extreme as in pterosaurs? Usually not, though some of the birds that leap very well come close with regards to the hindlimb. The extreme disparity in pterosaurs could, in fact, make their jumping ability even *better* in many ways, though other constraints would probably make this a wash. Either way, in my calculations I only allow giant pterosaurs a fraction of the total preload factor of dedicated living leapers, so I’m not suggesting they were great – only decent (in reality, they might have been quite good).

    Finally, I never said that the hindlimbs in pterosaurs are useless. They were still important for walking, running, and launching, and so it should be no surprise that they reasonably broad thighs and appreciably muscled crura. But that doesn’t mean they were bipedal.

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