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.
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.
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.
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.
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.