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Re: Pterosaur size


----- Original Message ----- From: "don ohmes" <d_ohmes@yahoo.com>
To: <dinosaur@usc.edu>
Sent: Thursday, December 14, 2006 9:35 PM
Subject: Re: Pterosaur size



1). What you say about the ptero launch technique seems to make sense
if the image I have taken from context is correct. I am sure that I am
behind on my reading. Do you have a reference that delineates this
launch cycle in detail?

No. John Conway and I have done one poster on it, at the VII International Conference on Vertebrate Morphology in Florida. There may be additional info in the fairly near future (next fall maybe). JimC


That would help a lot. All the descriptions I've read are real vague. 

That's because it's hard to describe orally. JimC


Quantifying launch abilities _from a skeleton(s)_ is a
mechanical problem.

Keep in mind that any quantification is likely to be less than optimal. If the quantification supports the possibility, you can be pretty sure that the animal would have had the potential to be even better at it. JimC


Were mega-volants the only phenomena difficult to explain relative to current conditions I would be much less hard-headed (by the way). --Don

Probably one of our differences is that Mike and I don't find all mega-volants difficult to explain (whether we are right or not, is another matter :-). JimC

2) I agree entirely that limits vary per flightstyle, but there are absolute limits, per medium.

This is true. But the animals that we are talking about, are for the most part not all that close to those limits. JimC

It is not possible to infinitely improve locomotive phenotype.

No, but it is sometimes possible to use to a different motive technique that avoids the problems of the one that is reaching a dead end. Many of our constraints are due more thinking inside the box than to physics. JimC


Further, I posit that, because any living system must engage in multiple processes, theoretical limits for a given process can be approached (rarely), but not achieved. 

That's true.  Living creatures are always a bundle of compromises.

High altitude migratory birds aren't optimized for sealevel flight, 

Swans are, and they are among the highest flying birds as well. JimC

even if they nest there, as high altitude selection obviously occurs.

So does low altitude selection. And selection for a bunch of other stuff as well. JimC

Those examples demonstrate very little about density effects. (Heh. Wind blows like hell up there.

As you know, wind is meaningless to a flying animal that is flying steady-state. It has no effect on their flight at all. Shear and turbulence do have an effect, but not wind. JimC

V^2 and all that.) Don

Sorry, wind don't work that way. Check with one of your aerodynamics text books. I've flown in 60 knot winds that feel identical to a dead calm. JimC


At _any_ weight, the effects of medium density change are NOT easily compensated for if the other critical variables (circulation, temperature, and composition) are controlled.

Are you talking about airfoil circulation, blood circulation, or what? At any weight, in steady-state flight, density changes are very easily compensated for. As you know, the forces are related to indicated airspeed, not true airspeed. And indicated airspeed remains much the same at any density -- it's true airspeed that changes. JimC


The effects are easily observable in lab in both wing kinematics and various metrics of power, especially lift generated per power expended.

There is some difference in efficiency (less difference at high Reynolds numbers), but it is not enough to be critical to flight at large Reynolds numbers. It is important to launch, and to a lesser extent -- landing. JimC

Further, the flight morphologies of birds that are optimized at
5500' are measurably different relative to sealevel birds (per flightstyle/species), 

Quantify a few examples for me.    JimC


ditto w/ insects.

True for insects.  JimC


(See Feinsinger P, Am Nat v 113, #4, 481-497 for Andean hummingbirds).

A very special case, for a hovering animal. We've not been talking about hovering. JimC


As to the relative effect curve, I want to think about that some more. By the way, any testing of that for a given bird can only be done _while controlling for circulation, temp, and composition_.

Please define circulation, temp, and composition for me. I think those terms may have different meanings for different jargons. JimC


And at positive pressures, of course. Testing at negative pressures and extrapolating is distinctly non-empirical in this case. --Don

What's the definition of a negative pressure ?  JimC


aren't limited to elevations less than 5500 MSL."

Sorry. I don't see the relevance to Quetz, Argent, et al. Or any
argument I have ever put forward.

We are talking about the continued ability to fly at reduced densities. JimC

3). I see the relevance. The point is that there is not an observed change i
g birds across very different atmospheric densities. You suggested previously that a 15% atmospheric difference would be significant for large vertebrate flyers. It turns out that such differences to not have a significant impact. It is important to note that Quetzalcoatlus, being larger and faster than the anserids in question, would probably be affected even less. It is also important to note that Quetzalcoatlus is not at the maximum size for pterosaurs; it's launch performance was almost certainly quite stunning, really, rather than borderline. I get the impression (though I cannot say for sure), that you are thinking of the largest observed flying vertebrates as examples of the size limits for their clades. In fact, none of the mega-volants known are likely to have been at a mechanical size limit, with the possible exception of Argentavis. All others presumably fe
ll at a selection size limit; ie. further increase in size was not particularly advantagous. Quetz., despite being impressively large, was not only below the size maximum, but it was still within what we might consider a "high performance" range for it's particular morphotype. --MH

Well said. JimC

3). I don't. This conversation started out w/ mega-volants. Now, you seem to be saying that because a 27 lb goose can fly in uncontrolled conditions, that is support for a 150 lb whatever flying in standard air.

Why no, that's not at all what I said. Basic physics is support for both flying in standard air. JimC


I am surprised you put forward these 'in vivo' anecdotes. Although it would be interesting to see how a giant albatross does in Denver. Don

As well as anywhere else. You have a lot of shear available just west of Denver -- more than enough to keep an albatross aloft. I know, I've flown through it during an airrace. Keep in mind that albatrosses tend to extract their power from shear processes rather than thermals. JimC

Also-- Controlled conditions, blah, blah (please see comment 2). I haven't read any author that feels that the extant maximals show any potential for doing well or better (as you seem to imply) at larger sizes.

Well no, I didn't imply that at all. Nor did I imply that they would do better at smaller sizes. I spend a lot more time working with Qsp than Qn (4.8 meter span vs. 11), and there isn't a lot of difference between their potential. Qn would appear likely to have a slightly better glide ratio, but not enough difference to write home about. JimC

I grant you, they continue to be volant, but just barely (in still air). Don

I've lost track of which particular animal you are referring to here. If albatrosses, they are superbly volant. JimC

Also: Reference on Quetz. performance, please. (Heh. You sound like you got one in the backyard. Kin I see him? }: D.)

My talk at the Toulouse meetng in 2001. John's and my poster at the 7th. And, you're welcome to look at my computer next time you're in my neighborhood. And no, I don't have one in my back yard. But I do keep a research grade cast of a Quetz left wing and shoulder girdle in my office, and a Q northropi left humerus. It comes in handy from time to time. JimC

As to size limits, please allow me to communicate by re-statement again. I think of the mega-volants as _approaching the practical size limits for their flight morphology at that time_. Don

I know you do. And that is certainly your prerogrative. With regard to Argentavis, I tend to agree -- but that's a launch issue, not a flight issue. JimC


And I feel you should say,
"_theoretical_ mechanical size limits" no matter how much modeling you've done. --Don

I'll say whatever you want me to say, I'm easy. Talk about size limits isn't necessarily based on modeling. Just take a look at the differences between Qsp and Qn (which have no diagnostic differences for species, but do have considerable difference in robustness. -- it is obvious that the big guy still has plenty of room to continue to morph to support greater span and weight. JimC

Fact. Plot maximal volants in timeslice fashion. Stick to birds if you
want. The trend is there,  Don

Don't forget them horsewhips and skirts. I remember studing that some decades ago, when I was still in college :-) JimC

relevance is debatable.  Don

Yup.  JimC

Although the 
correlation on the chart I did 15 years ago is -.75 (past to present),
not -.95. Sorry about that.  Don


No sweat.  The half life on my memory is about 15 minutes.  JimC


4). I'd be careful here; I'm sure that trend looks very distinct, but I don't think it's real. For example, sticking to birds as you suggest, the largest volants form about a plateau from the Eocene through the late Miocene. There's a little peak in the Late Miocene (for Argentavis), then plateaus again until the late Pliocene (loss of pseudodontorns). At that point the max volant size drops to essentially the modern observed. There is a little trend signal there, but it's mostly driven by that last sequence (Miocene, Pliocene, Modern), which is actually two extinction events, and not really a trend. That's without correcting for clade. If you take into account the fact that there are several bird clades involved, and correct for phylogeny (and thus clade-specific effects), then the trend will very likely evaporate altogether. I could actually crunch that phylogeny-corrected trend this weekend, if you want, though the power is going to be very low with such a small number
of samples (same problem with the raw data). --MH

I would also caution against use of the term "maximal volants". The species in question are not maxima; they are the largest observed volant species of their time intervals. Most are not actually near the quantitative mechanical maxima for their particular morphotype. --MH