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Re: Pterosaur size (Was: Great in the air, not so good underwater)

----- Original Message ----- From: "don ohmes" <d_ohmes@yahoo.com>
To: <dinosaur@usc.edu>; <jrccea@bellsouth.net>
Sent: Monday, December 11, 2006 10:27 PM
Subject: Re: Pterosaur size (Was: Great in the air, not so good underwater)

"...Incidentally, a 35% fraction of O2 should be a big deal for the insect life around at the time."

Heh. Yeah. And for anybody who strikes a match! My friend Alley Oop sez that's when he stopped smoking. }: D

-----Yes, 35% is about enough to spontaneously combust the biosphere. If I remember correctly, the late Cretaceous is reported to have had about 20-25% more oxygen than now, so was perhaps 25-26% of the atmosphere.

"So atmospheric effects probably made a slight difference
in flight speeds and kinematics, but were not a prerequisite for large
size as seen in pterodactyloids."

I'm somewhat skeptical,.......

But I follow the dictum that "organisms that find size to be an advantage, increase in size until _size ceases to be advantageous_". These volants had to _evolve_ to mega-size; they had to carry prey, avoid predation, carry eggs, and cope with bad weather.

-----Yes, and a stretch of that latter seems to be what done 'em in.

Modern birds nowhere near that size have a lot of trouble.

-----This is true. It appears to be related to two things., the first being the launch techniques available to birds, and the second being that membrane wings are capable of maximum steady-state lift coefficients about a third greater than the maximum that birds achieve (bat wings do not do quite as well as birds). For a given speed and wing area, a pterosaur can support about a third more weight than a bird can. Or, for the same weight, he can fly at about 87% of avian speed. This alone offsets the proposed difference in atmospheric density.

I think the mega-volants needed different conditions to find giantism advantageous,

------This may be true.

and increased flight medium density fills the bill nicely.

-------Not really. It's not particularly advantageous for anything except launch. Density of the flight medium affects true airspeed. But animals (and airplanes) fly at indicated airspeed, and indicated airspeed remains much the same at different density altitudes. For example, atmospheric density at the surface of Mars is about the same as it is at 110,000 feet here (near vacuum by the standards of us surface dwellers). An Anhanguera piscator that flies at about 25 mph here on Earth would fly at an indicated airspeed of about 19 mph on Mars (due to the reduced gravity, the wing loading is reduced) even though his true airspeed would be about 180-190 mph. That said, his little pressure suit would need to be a marvel of engineering ingenuity....... :-)

Also, I find the following overall patterns suggestive. Simplified, the sequence goes; 1). a class appears, reaches maximum size, then disappears. If the disappearance is not catastrophic, they dwindle in size. 2). New class appears, reaches maximum size that is _less than the preceding class maximum_, then dwindles gradually in size. This exact pattern is the rule (since the mid-Jurassic, anyway), not the exception, for terrestrial animals. Dwindling giantism, generally, seems to be the rule in the terrestrial record (insects, amphibians, reptiles...).

------Interesting. The very largest pterosaurs were present at the very end of the Cretaceous, so apparently the rule doesn't hold for them.

In contrast, where _the density of the locomotive/thermal medium has definitely not changed_, aquatic animals show no such consistent pattern.

-----I haven't bought into the successive giantism-dwindling hypothesis yet.

Here we have in chronological order the flying vertebrates; pteros
(biggest)=> birds (smaller)=> and bats (smallest). Bats are the
most recent, and (correct me if I am wrong) the largest known bats are
extant. Like I say, this is suggestive, given the consistency of the terrestrial record and the contrast with the aquatic record.

-----Not really. It can be more parsimoniously related to differences in wing mechanics between the three.

The contrasting uber-patterns are hard to explain in parsimonious fashion, in the context of a post-Archean standard atmosphere.

-----Not at all.  See above.

In fact, the list of patterns in the terrestrial record that are a fit for the assumption of an initial (late Archean) larger atmospheric mass combined with a persistent negative flux (ongoing as I write) into the mantle is quite comprehensive (I am working on it), although separate explanations exist for many of these.

-----If I read you right, you are saying that the atmosphere is losing mass due to subduction into the mantle? In other words, the amount of nitrogen is steadily falling too?

Of course, you can't change the density of air very much without changing the mass of N2, as it is 80% of the atmosphere. However, I find that the current geo-chemical argument for steady-state post-Archean N2, a sacrosanct assumption that pre-dates the discovery of plate tectonics, is less than convincing. In particular; upwelling magma, continental lithosphere, ocean crust, oceanic continental sediments have all been analyzed for N2 and other nitrogen content. Last time I checked, the analysis methods used are specifically designed to _remove_ all N2 _of atmospheric origin_ from the sample _prior_ to analysis. This is an excellent method of setting lower bounds on the planetary nitrogen mass. It is an entirely incorrect way of determining how much atmospheric N2 is subducting over time.

-----Yup, that was what you were saying. Should be provable, if true (and worth checking out).

There are other problems, including assumptions relative to 100% efficiency of return mechanisms. For instance, there is a thermal minimum close to, but short of, the atmosphere,

----That went over my head.  Please elaborate a bit.

and the acceleration of gravity _increases_ as you approach the core/mantle boundary.

----But reduces to zero at the center of the core. At what depth is it a maximum (I realize that it varies from place to place)?

" Limnofregata has a higher wing loading than a modern frigatebird, certainly, but that is to be expected: frigates have evolved from a more highly loaded ancestry, and so stem members of their lineage should have intermediate loadings..."

As I am sure you realize if you've read this far, it is precisely the slow high-load to low-load progression I find so enthralling.

-----But the progression generally isn't in that direction. Keep in mind that there is always a biological advantage to higher wing loadings, so long as they ae commensurate with launch requirements.

I defend this by saying that, having cleared such hurdles as evolving feathers and wings, the time logically needed for a volant to optimize wingloading is relatively short.

-----This is true. And optimal wingloading varies hugely with the niche being filled. Which is why we see such a range of wingloadings when different niches are filled.

In my opinion, such a trend through the history of the taxa is indicative of environmental change.

-----Or, a gradual dispersion into previously unoccupied niches.

By the way-- for those who might be concerned, we ain't gonna run out of air anytime soon, even if I am correct. From an extrapolation of Bob Garrels' figures, the atmospheric half-life is ~35 mys; no need to start a count-down.

-----But 35 My is just the blink of an eye.