Dr. Mike Habib (2008) promoted the hypothesis that pterosaurs probably launched using their forelimbs by first noting the greater diameter of the humerus vs. the femur in pterosaurs as compared to birds (Fig. 1).The “forelimb launch” notion (not yet proven with any tracks) has become quite popular. Mark Witton’s upcoming book on pterosaurs will feature many such take-offs, judging by the preview viewable here. Earlier we examined forelimb launch problems here, here, here and here. Today we’ll note problems with the data in Habib (2008).
Habib (2008) portrayed the humerus : femur ratios of 14 birds and just 3 pterosaurs (Fig. 1). The birds Habib (2008) chose to use ranged from the flightless rhea and penguin to the ultra-soaring albatross. Ratios for birds ranged 2.25 : 1 (femur : humerus) for the rhea to 0.9 : 1 (humerus : femur) for the tropic bird. All three of the pterosaurs that Habib (2008) cherry-picked for this data chart (Dorygnathus, Zhejiangopterus and Anhanguera) exceeded the tropic bird in their humerus : femur ratio (Fig.1).
There is no doubt that the vast majority of pterosaurs had a humerus larger in diameter than the femur, even when scaled to a common length. In some cases, as in many ornithocheirids, the ratio is quite enormous with very thick humeri and spindly femora. See Arthurdactylus and Boreopterus for some prime examples. In some cases (Jeholopterus) the humerus was much longer than the femur. In others (Zhejiangopterus), just the reverse.
Cherry-picking works both ways
When I cherry-picked six other pterosaurs I found similar diameters in the humeri and femora. These would have had a humerus : femur ratio around 0.0 (to use Habib’s data points). Just take a look at the data for Raeticodactylus, Preondactylus, Scaphognathus, Germanodactylus (the JME Moe 12 specimen) and the small germanodactylid, No. 13 (in the Wellnhofer catalog, Fig. 1). All of these pterosaurs would have nested in the center of Habib’s chart, along with owls and hawks. Notably, all of these pterosaurs had close relatives with thicker humeri and thinner femora. So they are oddballs, not typical.
Are there phylogenetic patterns here?
Not really. However, the Pterodactylus clade all had slender humeri, some more slender than their femora, but they were all derived from tiny pterosaurs with a thicker humerus.
Cherry-picking data got Habib lots of press and notoriety.
And no one noticed or griped for the last four years. Here we can show with “twice as many pterosaurs” that Habib’s data was skewed to say the least. I like Mike. But his methods involve skewing data and cheating morphologies (which we talked about earlier) when he artificially shortened metacarpals and fingers 1-3 of his ornithocheirid example so the extensor tendon of the flight finger would be pinched between the weight of the pterosaur and the ground, which is impossible. Here using Habib’s own methods permits us to also falsify this bad hypothesis by using more inclusive data that was not originally presented.
Pre-loading the forelimb is like jumping with a pogo stick
Habib (2008) envisioned a vampire bat-like launch for pterosaurs, but that method is fraught with problems (except for vampire bats). It is especially unlikely when the humerus is the shortest element of the forelimb, as in the azhdarchid pterosaur, Zhejiangopterus.
A typical pogo-stick will only raise the rider only a foot off the ground. It takes an extreme pogo-stick using an air spring to reach the heights required for forelimb launch in pterosaurs. This is a device designed to work under extreme pre-loading.
Using the pogo-stick analogy, it is difficult to envision the evolution of such a behavior in basal pterosaurs, already so well adapted to leaping and running with their hind limbs. As the wings got stronger, they provided more of the thrust so the hind limbs could shrink, which they did in all derived pterosaurs.
So maybe the forelimb launch hypothesis is NOT the answer.
If the humerus vs. femur rules do not apply to all pterosaurs, we might ask, what do you get when you have an inflated humerus (as demonstrated by most, but not all pterosaurs)?
There is no doubt a big fat pterosaur humerus is full of air. That makes it more buoyant when sitting on the water. It is interesting to see how really inflated some forelimbs (includes the ulna, humerus and wing metacarpal) of Pteranodon got. Check them out here. The question is, do the big humeri match to marine pterosaurs, the ones most likely to use floating as a behavior? Maybe so. But tiny Nemicolopterus also had a big humerus. There might be something to this. It would be worth investigating beyond this blog. Most Germanodactylus specimens had a big humerus, but the one I imagined was built for diving had a very small diameter humerus. Hmm.
A big fat humerus also provides more area for muscle attachment. The big Quetzalcoatlus humerus is much more robust than the smaller one. So this makes sense on that scale. The question then becomes, is the big humerus necessary for flight? For leaping? Or for walking around on the ground, supporting that weight and the superstructure of the elongated neck and giant skull? The big Q. does mimic the giraffe in certain aspects (Fig. 2), with its short humerus, long forelimbs, long neck and relatively smaller torso and overall size. Maybe terrestrial locomotion and some floating made the big Q humerus so big.
This is what makes science fascinating. Exposing problems in various hypotheses are just part of the process. On the other hand, whenever someone comes up with a forelimb launch track site, I will be among the first to announce it. I’ll even get excited the first time a wing metacarpal gets impressed into the matrix.
I do hope these presentations help Mark Witton to change his book before publication. It would be a shame to promote this bad hypothesis as valid.
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.
Habib M 2008. Comparative evidence for quadrupedal launch in pterosaurs. Pp. 161-168 in Buffetaut E, and DWE Hone, eds. Wellnhofer Pterosaur Meeting: Zitteliana B28