“Why we think giant pterosaurs could fly” (…NOT!)

Yesterday the Dinosaur Mailing List
linked a MarkWitton.com blogspot.com post titled, “Why we think giant pterosaurs could fly.” It’s worthwhile looking (once again) at the arguments Dr. Witton most recently put forth to test them against the evidence presented by pterosaurs here at PterosaurHereseies. After all, it’s not fair to dredge up arguments Dr. Witton may have long ago abandoned. Alas, Dr. Witton is holding fast to his old arguments and pet hypotheses, many of which paint a false picture of pterosaur biology and behavior, based on evidence to the contrary (see below).

Dr. Witton precedes his arguments
with the admission that, “Giant azhdarchids are invariably known from scant remains, sometimes a handful of fragments representing bones from across the skeleton or, in the case of Quetzalcoatlus northropi, an incomplete left wing.” We looked at Q. northropi wing elements earlier here (Fig. 1). They are indeed scant, but nevertheless, impressive.

Figure 1. Quetzalcoatlus specimens to scale. Q. sp. is also enlarged to the humerus length of Q. northropi. Gray zones are hypothetical and/or restored. Reduction of the wing, even in the smaller species, argues against flight in giant azhdarchid pterosaurs, as it does in much smaller flightless pterosaurs.

Dr. Witton reports,
…just a few bones can betray volant habits. It’s evident that even the largest pterosaurs bore wing anatomy comparable to their smaller, incontrovertibly flightworthy relatives. The huge deltopectoral crest…is a clear correlate for powered flight in giant species.” 

Unfortunately
Dr. Witton does not acknowledge the presence of any flightless pterosaurs (taxon exclusion). Flightless pterosaurs could test Dr. Witton’s ‘dp crest clear correlate’ hypothesis. Three flightless pterosaurs have been reported here based on their relatively short wings: SoS2428, PIN 2585-4, and Alcione (Fig. 2). Notably, all three have an unreduced deltopectoral crest.

Figure 2. Flightless pterosaurs, SOS24248, PIN2584-4, and Alcione, to scale. Reducing the span of the wing is the easiest and most common way to become flightless in pterosaurs.

Wing length vs body size
provides the best argument for flightlessness in the case of SoS2428 (Fig. 3), itself a pre-azhdarchid. The same argument works for the other two flightless pterosaurs when comparisons to flighted sisters are presented.

Figure 3. Lateral, ventral and dorsal views of SoS 2428 alongside No. 42, a volant sister taxon. In dorsal view it becomes very apparent which one would be flightless.

Figure 4. Arthurdactylus in dorsal view. Note the rather small deltopectoral crest in this taxon.

It’s a good time to remember
that hatchling pterosaurs had adult proportions. They were able to fly shortly after hatching. This also means that small to tiny pterosaurs had wing/body ratios comparable to those of the largest incontrovertibly flying pterosaurs, the ornithocheirids (Fig. 4) and pteranodontids. Notably, the deltopectoral crest of the ornithocheirid, Arthurdactylus, is relatively smaller than one would predict using Witton’s hypothesis, and quite variable in other members of this clade.

Dr. Witton reports,
“for large azhdarchids: their functional morphology and trackways show strong terrestrial abilities and they probably spent a lot of time grounded, only flying when harassed, or wanting to move far and fast. Indeed, in all likelihood giant pterosaurs couldn’t launch every few moments.”

Unfortunately
Dr. Witton does not consider the possibility that large azhdarchids could have employed wing thrust to hasten their getaways on the ground, like many large birds do (Fig. 5).

Figure 5. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

Witton and Habib 2010
used software designed to model bird flight to predict that giant azhdarchids could fly faster than 90 kph and were easily able to sustain long distance glides.

Witton reports: “The key to everything: quad launch”
and provided a helpful illustration (Fig. 6) to show the moment of takeoff. Remember, in pterosaurs the wing finger never makes an imprint, so the three tiny free fingers must bear some multiple of the entire weight of the pterosaur at the moment of lift-off, then the ventrally-oriented wing finger must circle around to provide at least one upward lift and one downward flap before the otherwise inevitable crash. Not even a heavily muscled kangaroo can lift itself to such a height on the first leap. Not even a body builder can perform such a push-up… but a tiny vampire bat can, and does so routinely.

Figure 6. In the ‘quad launch’ hypothesis, for which there is currently no fossil imprint evidence, the pterosaur does a sort of leaping push-up using its tiny free fingers to bear some multiple of its entire weight during the acceleration, without flapping, to takeoff speed. Then the dangerous part begins. The pterosaur has to swing its wings up and down to creat aerial thrust before crashing (see figs. 7, 8). The short humerus provides little leverage to do this. Among tetrapods, only tiny highly derived bats are able to succeed with this sort of takeoff scenario. All other pterosaurs flap first, then fly.

What happens
if pterosaurs don’t make altitude every time they attempt a launch? (Fig. 7) Calamity (Fig. 8). There is no room for error, no evolutionary path to perfection, even if possible. Can one enhanced pushup provide the necessary airspeed and altitude without wing assistance? Witton and Habib think so? Look what those giant wings have to do before contributing to thrust and lift. Much better to get those wing providing thrust and lift at the moment of takeoff, rather than waiting until, perhaps, too late.

Figure 7. Successful Pteranodon wing launch based on work by Habib (2008). Best case scenario.

Figure 8. Unsuccessul Pteranodon wing launch based on Habib (2008) in which the initial propulsion was not enough to permit wing unfolding and the first downstroke.

Figure 9. Successful heretical bird-style Pteranodon wing launch in which the hind limbs produce far less initial thrust because the first downstroke of the already upraised wing provides the necessary thrust for takeoff in the manner of birds. This assumes a standing start and not a running start in the manner of lizards. Note three wing beats take place in the same space and time that only one wing beat takes place in the Habib/Molnar model.

re: the pelvis
Witton reports, “The avian skeleton has two large girdles for limb muscles: an enlarged shoulder and chest region for flight muscles, and an enhanced pelvic region to anchor those powerful hindlimb launch muscles. Pterosaurs, in contrast, have only one large limb girdle – their shoulders, making this the de facto likely candidate for powering their launch cycles.”

Figure 10 Standing Pteranodon (the Triebold specimen). Note the robust and extended pelvis supported by at least nine sacrals.

It may be traditional to discount the pelvic region
of pterosaurs, but in all cases, the pelvis is also enhanced (Fig. 10) with fused sacrals, prepubes and an anteriorly expanded ilium anchoring powerful, and under appreciated muscles.

Ignoring evidence that does not serve a pet hypothesis.
Witton ignores the hard evidence of bipedal pterosaur trackways, when he quotes Habib 2008, who “also notes that launch in living tetrapod fliers correlates to terrestrial gait: the number of limbs used to locomote on the ground is the same as the number used to take-off. Birds walk and launch with two legs, while bats walk and launch using all four. An extensive record of pterosaur trackways shows that pterosaurs were quadrupedal animals like bats, and it stands to reason that they also launched from four limbs: they would contrast with our living fliers if they had to shift gaits to take off.” 

Witton calls the quad-launch
“the most efficient launch mechanism conceivable for a tetrapod,” ideal for such a strong humerus and such a weak femur. Julia Molnar produced a video of a quad launch.  You might remember that the Molnar pterosaur free fingers were incorrectly reduced (Fig. 11) and relocated to the dorsal (in flight) surface of the wing in order to get that big wing finger on the ground and ready to snap like a grasshopper’s hind limb. Yes, they cheated the anatomy to make their pet hypothesis work… and Dr. Witton warmly embraced, rather than pointing out its faults.

Figure 11a. Left: The so-called catapult mechanism in pterosaurs. The fingers are in the wrong place and cheated small in order to let the wing finger make contact with the substrate – which never happens according to hundreds of pterosaur tracks. Right. The actual design of pterosaur (in this case Anhanguera/Santandactylus) fingers. Click to enlarge.

Figure 11b. Errors in the Habib/Molnar reconstruction of the pterosaur manus

The infamous animation by Molnar
(click to play YouTube video) apparently assumes a nearly weightless mass, a super powerful pushup, and a suspension of the moment of inertia required to drag that big pool stick of a wing finger around to the flying position after it has just been oriented ventrally to say nothing about the effects of drag while opening that less than aerodynamic wing membrane. Isn’t it better to completely extend that wing and set it in the upward position before launch?

Summary of points ignored by Dr. Witton

1. The largest flying pterosaurs have the largest/longest wings
   
2. Flightless pterosaurs do exist and they are identified by their short wings
3. Flightless pterosaurs retain a large deltopectoral crest and continued flapping to provide thrust for fast getaways and threat displays
4. The quad launch hypothesis was built on the false premise of wing finger contact with the substrate
5. The quad launch is dangerous for its participant every time they perform it. Much better to generate wing thrust at the moment of takeoff, not some time later. Such takeoffs can be aborted or diverted without the danger of a crash landing.
6. The quad launch hypothesis works well for small  bats, ankle high to a Dimorphodon (Fig. 12), which fly in a different fashion from other volant tetrapods, but this ability does not scale up well for giraffe-sized or other pterosaurs.
7. Dr. Witton cherry-picks the data that fits his hypotheses and ignores data that invalidates the last few years of his work.
8. Given the paucity of data at present for giant azhdarchids, it would have been appropriate to restore Q. northropi as flightless AND volant, and tell us where the dividing line would be if the missing bones were one way or the other, making comparisons to smaller azhdarchids and to other fully volant large pterosaurs, like ornithocheirids and pteranodontids.
9. It would have been professional and appropriate for Dr. Witton to alert us to the (perhaps inadvertent) cheating Molnar and Habib did to their pterosaur manus (Fig. 11) before some rank amateur brought it to our attention, and not to adopt this bogus and untenable idea with such gusto (Fig. 6), perhaps out of friendship.

Figure 12. Dimorphodon and Desmodus (the vampire bat) compared in size. It’s more difficult for larger, heavier creatures to leap, as the mass increases by the cube of the height. Size matters.  Note the toes fall directly beneath the center of balance, the shoulder glenoid, on this pterosaur, And it would have been awkward to get down on all fours, especially with giant finger claws.

At what stage(s) did azhdarchids lose the ability to fly?
If we just look at wing length (reduction of distal elements) then this clade appears to have become flightless at least twice (Fig. 13). In both instances that happens when the wing finger tip is no higher (when folded) than the dorsal rim of the dorsal vertebrae. And that happens the second time when azhdarchids double in size to standing over a meter tall. If valid, then the doubling and doubling in size of azhdarchids was possible because they gave up aerial pursuits in favor of a fully terrestrial and/or wading niche, as in the many giant flightless birds we are more familiar with.

Figure 13. Click to enlarge. Here’s the 6 foot 1 inch former President of the USA alongside several azhdarchids and their predecessors. Most were knee high. The earliest examples were cuff high. The tallest was twice as tall as our former President. The doubling and doubling again in size was made possible by giving up the constraints of flying. 

Figure 1. Click to enlarge. The largest flying and non-flying birds and pterosaurs to scale.

References
Habib MB 2008. Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana, 159-166.
Witton MP and Habib MB 2010. On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PloS one, 5(11), e13982.

markwitton-com.blogspot.com/2018/05/
Seven problems with the quad launch hypothesis