[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]


Thanks Dave!

Couple of general comments regarding your concerns, and then more specific replies indicated below:

With regards to clearance issues, bipedal launch actually makes clearance more problematic, not less. Jim has already responded to the specific kinematic issues, and did so just as well as I can, so I'll let that stand. More generally, though, a biped launch would require that the same clearance be generated with less muscle power. This appears to have had substantial influence on planform evolution in birds, with steeper launchers requiring reduced spans in order to get the clearance for early onset of flapping strokes. It feels a bit like the biped launch gives more clearance, because the animal is more erect and all. But, it's the top of the leap that matters for flapping clearance (be it a biped launch or quad launch), and more power means more height.

It's also worth noting that running launch (among birds) is not really a way of "building up" flapping power, either - it's mostly an adaptation to water launching, and occurs in species with posteriorly located hips, shortened hindlimbs, strong femora, high wing loadings, and gracile forelimbs. The only one of these that matches pterosaurs (some) is the high wing loading.

David Peters wrote:

While Mike makes a good case, I was still a little disappointed not to see a step-by-step sequence illustrating this launch sequence in a variety of pterosaurs. In the spirit of 'spirited discussion' I'd like to challenge Mike a little, again.

Well, the paper went in some time ago, and I've been working on illustrations since that time. I also don't want to scoop my Masters student, who is doing a launch animation for her thesis. Still, your point stands: an illustration would be very helpful for lots of folks.

At manus lift off, the metacarpus would, of necessity, still be vertical, having just pushed off the earth like a airborne pole vault. A vertical metacarpus means the wing finger was still in the vertical plane, but no doubt beginning its rotation snap to the flight position. The question is: how long does this take? After passing the horizon, the wing finger would be on a collision course with the earth--unless the proximal wing had rotated laterally sufficiently to enable passage of the entire wing above the substrate. The clearance shrinks with every passing nanosecond.

The positions aren't quite correct, but your basic question is perfectly valid: is there enough time and clearance for a stroke cycle? While we cannot calculate the amplitude and flapping frequency exactly, rather robust estimates can be produced using known scaling trends in flapping flight (that hold across multiple clades). Having run these calculations, I find that clearance distance and time were more than sufficient, even when I use very conservative numbers for pterosaur flapping frequency.

The quad launch of Istiodatylus, with a much smaller scapulocoracoid, tucked wing fingers several times longer and relatively weak legs several times shorter presents quite another scenario. Istiodactylus would have had to leap several times higher, relative to its torso, than Quetz would have on ostensibly weaker landing/launching gear.

It's launching gear isn't as weak as it seems: most of the power is in the forelimbs. Now, that said, azhdarchs do generally have more powerful launching systems (not so much because they have stronger hindlimbs as much as they have more powerful forelimbs). This explains the larger observed max size in azhdarchs, and it makes the launch sequence faster. Istiodactylus ends up with a slower, longer vault phase, but clearance still seems to be fine.

If, on the other hand, you make Istiodactylus launch with the hindlimbs, then it does run into a power and clearance problem: now those weak legs you mentioned are a serious handicap. Again, if clearance and height are the issue, then more power helps.

As a rough comparison to give everyone an idea of how much this helps: a passerine bird just barely clears on its first flight stroke when launching. A vampire bat of roughly the same size, full of a blood meal, quad launches up to three feet vertically.

There is some evidence (which I have not seen) for a bipedal launch in the literature. Unfortunately I don't find it in Mike's reference list.

A New Pterosaur Tracksite from the Jurassic Summerville Formation, Near Ferron, Utah

Authors: Debra Mickelson1; Martin Lockley2; John Bishop3; James Kirkland4

Ichnos, Volume 11, Numbers 1-2, Numbers 1-2/January-June 2004 , pp. 125-142(18)

I probably should have referenced it, but it didn't happen because I haven't actually looked at the tracks myself, and (more importantly) one of the authors indicated to me at SVP a while back that these are probably landing tracks. It's also worth noting here that when looking for trackway evidence of launch sequences, we should keep our eyes open for evidence of leaping (probably more so than running). Even if pterosaurs somehow managed to launch bipedally, it still doesn't suggest a running launch - most birds launch by leaping, for instance.



Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280-0181 habib@jhmi.edu