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Re: Pterosaur size (Was: Great in the air, not so good underwater)
----- Original Message -----
From: "don ohmes" <email@example.com>
To: <firstname.lastname@example.org>; <email@example.com>
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
------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
----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
-----But 35 My is just the blink of an eye.