Into the air!

Mike Habib (right) with Musings favourite Luis Rey

This time out, say hello to Mike Habib’s work on quadrupedal launching. Just how did those giant pterosaurs get into the air?

It has been long recognized that the largest pterosaurs far exceeded the size of the largest known flying birds.  A new paper by Mark Witton (also in the Wellnhofer volume – details here) suggests that the famous giant azhdarchid, Quetzalcoatlus, probably weighed in around 250 kg.  All this business about size naturally raises the question of how pterosaurs managed to launch in the first place.

Take-off is generally the limited step with regards to the size limit of flying animals.  Traditionally, pterosaurs have been assigned a bird-like launch sequence, in which the animals launch off of the hindlimbs in a bipedal fashion.  Oddly enough, no one seems to have ever actually tested this assumption in the literature – apparently it was just intuitive.

Like a lot of intuitive thoughts, however, there are some serious problems with the bipedal launch model for pterosaurs (not the least being the whole giant size thing).  I went about testing this model using structural mechanics.  The overall approach boils down to this: birds, which always launch bipedally, generate over 80-90% of their launch force from the initial push of the hindlimbs.  That is, the initial leap or run gives them most of the power for take-off.  This means that the bones of the hindlimb have to be very strong in large birds.  We call this “positive allometry”: as birds get bigger, their hindlimbs get disproportionately stronger.  By contrast, the forelimb skeletons of big birds are rather gracile and slim.

If pterosaurs actually launched like birds, with the hindlimbs producing most of the force, then the ratio of their bone strength values should be about the same as birds, and big pterosaurs should have slender wing bone elements (especially the proximal elements, like the humerus) and big, robust hindlimb bones.  Well, it turns out that the opposite is actually the case: pterosaurs have whopping huge forelimb bones and very slender hindlimb bones, and this gets more exaggerated in big pterosaurs (image of Zhejiangopterus courtesy of Mark Witton).

My conclusion is that pterosaurs didn’t launch like birds at all.  Instead, it is more likely that they pushed off the ground with all four limbs – what we call a quadrupedal launch.  In this model, the forelimbs provide most of the initial power (requiring beefy arms).  Interestingly enough, it also solves the size problem – a quadrupedal launch allows even the true giants to take off from the ground, while a bipedal launch is actually quite impossible for a 250 kg Quetzalcoatlus (unless it doesn’t mind breaking a leg).  These structural data do not absolutely clinch a quad launch model, of course (we need launch track-ways for that), but it does provide rather compelling evidence for a serious reworking of our models of pterosaur take-off, and our estimates of their maximum size range.

Edit: This work has also broken on the news, so you may have seen this mentioned elsewhere already, including here.