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Re: Pterosaur take-off movie on the NG site

 David Peters wrote:

Actually: no. Wild showed patches but no pattern. Unwin and Bakhurina 1994 tried to follow Sharov 1970, but got completely different lines. My 2002 paper showed those lines to be geological faults. Wang et al. sketched what I did in 2002 so we found the same things. They simply described it differently to fall in line with the paradigm. Lu found patches but no pattern. Bakhurina and Unwin 2003 is an abstract without illustration. Frey et al. 2003. traced no wing membrane but concentrated on veins and such details. What they describe near the thigh of the darkwing is thigh material. The tight curve at the anterior femur was ignored. Bennett 2007 ignored his own sketch , which also showed the tight little curve aft of the elbow, when recreating his wing diagram.

In other words, you disagree with the prior interpretations and published a manuscript with your observations, which is great. However, my argument was that the issue was contentious, not that a narrow attachment had to be wrong. I think your contentions above rather demonstrate what I was pointing out.

They might not be ornithocheirids,

They match specific ornithocheirid pedes. Part of my studies. Don't go by the published sketches of either. They both have errors. I can send you revisions by request, but I only did the feet.

Interesting stuff, and I'd be happy to see your reconstructions. However, I can't exactly just ignore the published illustrations based on your good word, either.

If finger 1 provides a complete impression, not just an ungual, as is the case, then the the fingers are being hyperextended 90º. That they all join at the base supports that reconstruction. Yes, there's a roundness medial to digit II. That must be digit IV. Key here is: the fingers all surround digit IV and have plenty of floorspace alotted to them. They are weight-bearing and get pushed into the substrate. They were not raised as your illustrator and animator show.

They bear some weight, but likely not much (see below). I did not suggest that the free digits bore no weight, except in Nyctosaurus, and *perhaps* in ornithocheirids. We used a reconstruction, produced by the AMNH, in which the fingers did not touch the substrate. An alternative reconstruction might be more accurate from an osteological standpoint, and perhaps we will do other versions down the road from a illustrative standpoint. Little to no effect on the calculations of the launch, however.

There is a deepening of the manus impression in some East Coast tracks (as yet unpublished, so I won't say more on the identity), in what appears to be a launch sequence. That's not surprising, as it would amount to about 2 or 3 times body weight, which is a substantial increase and can be see in the ichnite with some clarity.

I'm looking forward to seeing those tracks when they become available. Remember the old snowshoe example. The broader feet don't sink as deep as the narrow ones. The fingers are smaller in area, typically, than the feet are, so they sink in deeper. Simple as that.

This is rather more extreme than simple area relationships are likely to produce. There are even slumps associated with displaced stream- edge material from the manus push.

Not true; see above. You cannot glean as much about weight support from the tracks as you might think, though you can see a fair bit. Again, if MCIV/PhIV was not contacting the ground, then the tracks would have a more pronounced concave crescent.

Why are you turning this argument around? No one said mc IV is not taking weight. I only said it is not taking -all- the weight.

Actually, the exchange was:

DP: Ordinarily, and pterosaur tracks show this, these three fingers take ALL of the weight not taken by the hind limbs. There is no deep track preservation of digit IV. There is no trace at all, typically, of digit IV. If no weight is placed on this joint, then there is no elastic recoil and the hypothesis has no power source for takeoff.

MH: See above. The fourth metacarpal/phalanx joint was likely taking a lot of the weight. Not only is this apparent in trackways, but we did not shorten the fingers; the proportions were based upon reconstruction of AMNH 22555. Because there is some estimation involved, it could be that the fingers were a bit longer in life than we estimated, and could grip the ground to some extent, rather than lying above the surface.... Even presuming, for a moment, that this is the case, I can say with confidence that digits 1-3 did not take much of the weight not accommodated by the hind limbs, as these small digits are not structurally capable of doing so.

DP: Show me some pterosaur tracks in which digits I-III are -not- taking the weight. They -are- structural capable. Ichnites show it.

You were the one that originally argued for a full weight load on the free fingers, which is not structurally feasible for many species, and is not suggested by ichnites. If your argument is, instead, that the free fingers took some weight in many taxa, then I agree. However, they likely did not take a great deal (see below).

It is leaving an impression, and a rather large one, actually.

Actually no. When you subtract digits I-III from the "runway" tracks, you are left with a very slender minority of the print.

I think you may be attributing too much area to digits I-III. One problem with ichnites is that they can be tough to interpret, and are very qualitative when it comes to things like the meaning of impression depth.

As for structural capacity, a quantitative analysis of bending strength is more informative.

No, deep impressions of tracks are more informative because they take in all the factors including the exact weight bearing down from above.

Tracks are informative in some ways, but vague in others. Tracks do not give a clear and precise idea of weight distribution at the level you are suggesting, except in exceptional cases (such as some leap or sprinting tracks where weight distributions are extreme). There is an averaging effect, but there is also a lot of slop. Track interpretation has quite a bit of qualitative and subjective assessment. Not so much that tracks can't be used; they're a great source of information - but exact weight distributions are not easy to get out (Gatesy (2003) discusses this, as I recall. Other experimental designs have also shown the complexity of track production).

Well, I've given a plausible alternative. Actually, there is really nothing in any pterosaur suggestive of a gooney-bird or pelican-like launch. We have no reason to assume such a gait, and it turns out to be impossible for all large pterodactyloids. In fact, all of the morphological traits we would presume to exist in a bipedal, running-launching species are not only absent in large pterodactyloids, they trend in the *opposite* direction of what we see in water-launching birds. The femora in large pterosaurs are proportional more gracile than in small species, not more robust. The feet in many of the largest pterosaurs are small, not large. The proximal forelimb is robust, not gracile. The list goes on for a while.

Understood. But notice you're talking about increasingly derived taxa. They are solving various locomotion and mass problems in several ways and different from birds. I just found a pterosaur with a pelvis half the torso length. Now that pterosaur has issues!

I am talking about derived taxa, for the most part. However, we don't have any reason to assume a bipedal launch for more basal taxa, either. Remember, there is no "default" - to get the answer, a quantitative analysis is needed. I know of no quantitative analysis of launch ability in basal pterosaurs, but I have completed a launch analysis for several, relatively distant, pterodactyloids. The most parsimonious conclusion, at present, is therefore that the quad launch is the basal state. However, as someone that rather distrusts such off-handed bracketing, I'll simply analyze the basal forms to check. I have done some anurognathids, incidentally, and they are still consistent with quad launching, though they are less constrained. In lieu of any evidence *for* a biped launch, the best explanation is that they quad launched, as well.

Yes, the limbs extend and the thumb is last to leave the ground, but the leap is not frog-like, either, which was the gist of my earlier comment. In any case, much of the leverage in Desmodus actually comes from the thumb.

Then let's get those fingers involved in pterosaurs, too, at least to the extent that they are able.

They are not very able for most taxa. Pterosaur fingers are not much like Desmodus fingers. Digit III might add some power in a few of the little guys, like anurognathids. Emphasis on might. The finger that could, structurally, push off the substrate is actually digit IV. I expect that only the flexor tubercle actually did any pushing, but others may find evidence that more of the wing finger touched during launch.

There is some from the elbow and wrist, too, of course. While the longer humerus does increase the excursion length, it also limits the maximum stress that the humerus can take. The shorter humerus and antebrachium of pterosaurs gives them much greater power limits about the elbow. The excursion length of the limb is then increased by the long MCIV (relative to the bat). So, in both cases, there is a shorter, stouter element that sees a lot of stress, and some longer elements that help add excursion length. It's just that which elements are which differ between the bat and the pterosaurs. So once again, no trouble.

Mike, I don't have any trouble with pterosaurs leaping with their forelimbs. I only have trouble with the height that can be achieved when I see relatively little angulation (storage of energy) between the elements of the wrist. And I have trouble with the opening of the wing finger when its initial trajectory is straight down toward the earth at take-off.

The elbow provides most of the power; MCIV provides mostly lever length. However, the wrist can provide a larger moment than you are probably given credit for, even with limited excursion. I haven't found anything to suggest that the forelimb power was insufficient for a quad launch in any taxa, so far. I welcome quantitative evidence to the contrary.

As for the wing opening, we discussed this previously: the wing finger does not open until later in the launch. Just let it stay mostly closed until well into the first upstroke. That's all it needs to do. Remember, wing clearance is *improved* by quad launching. A biped launch has a lot of clearance issues for large species, while quad launching does not. There is no problem with clearance - the cost comes in timing. To be effective the quad launch needs to add enough power to make up for the extra half-stroke timing penalty. This is a relatively easy calculation to run, with a few basic assumptions about forelimb power, and the resounding answer is that the extra power more than makes up for the small timing cost. Not to mention that for many species, no other launch will work.

Incidentally, it is also not true that anything forcefully extended becomes ballistic in nature and goes to the limit of joint mobility. Actually, it almost never happens - antagonists usually fire even in powerful motions. For example, a person throwing a kick does not (generally) fully lock their knee. For more technical treatments, look at the myological literature - there are a lot of nice electromyograph papers out there.

Come up with an example that is similar to leaping from the substrate. Kicking is like eye-gouging or punching. Not similar.

Well, I wouldn't consider any of the above particular similar kinematically, I was just making a general point about antagonists and figured it was a fun example. If we really wanted to get into the mechanics of striking then we should be comparing circular kicks with long-arm strikes and thrust kicks with jabs, etc. But I think that's a different sort of forum. :-)

See above. Again, there are multiple ways this can work, which are all feasible. By contrast, I have yet to see any positive evidence for a biped launch, and I can supply a lot of negative evidence. The femur snapping in large species is a pretty good one, just for starters.

Femur snap? Well, that's not good. Surrounded by a massive thigh in perfect coordination with two of the limbs taking the weight off the femur? and it can't run? I'm getting a Whitfield and Bramwell deja vu. Remember, they said Pteranodon could not even walk on its hind limbs and would have been relegated to a small wheeled wagon, ala Porgy and Bess.

No, it's not good. I would not consider the thighs of most large pterodactyloids to be "massive", but subjective usage aside: no, the thigh musculature does not make it better. Actually, muscles usually generate much larger input forces than output, because they work with a negative advantage lever arm. As such, the added musculature further decreases the safety factor for the femur. Muscles don't load as rigid elements in compression or bending, so they do not take weight off the femur - they do, however, impart substantial forces upon it. And no, based on bone strength analysis, most of the larger taxa probably could not run bipedally. Many could gallop or canter quadrupedally, I suspect. I actually agree that Pteranodon would have a hard time walking on its hind limbs, though not because they would fail. The limbs would fail under the required launch loads, however, if they were the only (or primary) propulsive unit.


-- Mike

Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
(443) 280-0181