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

[don ohmes <d_ohmes@yahoo.com>]

I would post this as a correction, but the posting in my "sent" box is 
complete. It is only the posted version that has a critical sentence [the 
sentence immediately after "2)."] truncated-- so i repost that small section 
immediately below. Never seen just the middle of a sentence get truncated 
before. You can compare.

"Consider also--

1). Atmospheric pressure is equivalent to atmospheric mass, and mass is 
constrained by N2, because N2 is 80% of the atmosphere. 
2).
The flux of water vapor (our primary greenhouse gas) into the
atmosphere in response to temperature changes is constrained by
pressure, as is the ocean/atmosphere flux of heat. 
3). The tacit
and universal assumption of steady-state atmospheric N2 mass therefore
colors all evaluations and models of paleo-climate, particularly those
periods of the distant past wherein CO2 levels were much higher. "


----- Original Message ----
From: don ohmes <d_ohmes@yahoo.com>
To: dinosaur@usc.edu; jrccea@bellsouth.net
Sent: Monday, December 11, 2006 11:27:03 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

"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, as ignorant people tend to be. I have no doubt that 
once in the air, the largest (or larger) could fly. I am willing to bow to 
superior knowledge when told that the numbers say they could take-off, and 
landing speeds were, uh, well, non-lethal, at least in theory. I haven't seen 
the numbers yet, but I can wait for the paper (hell, I'll even re-read Marden). 

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. Modern birds nowhere near that size have a lot of 
trouble. I think the mega-volants needed different conditions to find giantism 
advantageous, and increased flight medium density fills the bill nicely.

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

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

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 tect
an 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. 

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, and the acceleration of gravity _increases_ as you approach 
the core/mantle boundary. Also, abiotic fixation of N2 is now known to occur.

Consider also--

1). Atmospheric pressure is equivalent to atmospheric mass, and mass is 
constrained by N2, because N2 is 80% of the atmosphere. 
2). The flux of water vapor (our primary greenhouse gas) into the atmosphere in 
response to temperature changes is 
phere flux of heat. 
3). The tacit and universal assumption of steady-state atmospheric N2 mass 
therefore colors all evaluations and models of paleo-climate, particularly 
those periods of the distant past wherein CO2 levels were much higher. 

I have messing around with this idea for a long time; I only (relatively) 
recently realized that due to reduced N2 atmospheric mass, we may be more 
vulnerable to runaway greenhouse/icehouse scenarios than at any time in 
planetary history. I actually thought the opposite was the case, but "ah now 
thimk ah wuz rong". So I am coming out of the closet, as none of the pros I 
have put this in front of are willing to steal it.

But I digress.  }:D

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

"Jim already pitched in on this for pterosaurs (far better than I 
could).  For birds, it depends a great deal on the planform and launch 
mode involved (which will be related to ecotype, etc).  Using the 
scaling relationships I have for pseudodontorns (and the bone strength 
ratio model I've been using for their mass estimates lately), I would 
cautiously list about 75 kg as the rough limit.  (The largest species 
actually known seems to fall around 60 kg).  For continental, 
convective soarers (ie. Argentavis like forms), the limit in standard 
atmosphere would be substantially higher (probably over 90 kg), but I 
can't say much at the moment."

Thanks, to both you and Jim. 

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. 

I believe one of the reasons the idea of declining air mass was suppressed in 
the early 20th century was they were afraid of a panic. Also, given the state 
of geochemistry at the time, it was scientifically correct to reject it on 
uniformintarian grounds. 

Heh. Did you know there was a movement to ban cars because they "might use up 
all the oxygen"? Ah, for the days when all the outdated books were on the 
shelves, and not in "remote storage".

Don

----- Original Message ----
From: Michael Habib <mhabib5@jhmi.edu>
To: dinosaur@usc.edu
Sent: Monday, December 11, 2006 7:37:08 PM
Subject: Re: Pterosaur size (Was: Great in the air, not so good underwater)


On Monday, December 11, 2006, at 11:34 AM, don ohmes wrote:

> Yep. Stuff gets lost, t
esis calls for a 12-15% minimum increase in 
> _overall_ air density, resulting from an approximate doubling of a 
> paleo-O2 partial pressure of 15%, with a postulated max O2 atmospheric 
> fraction of 35% (if memory serves). I used 10-12% density increase 
> because it is a conservative figure relative to the hypothesis. My 
> remarks were directed only to flight benefits derived from the 
> increase in air density, _not_ to O2 related mass-specific power 
> effects.

Ah, my apologies for the misinterpretation.  12-15% in overall density 
might be enough to matter, but it's still too limited to account for 
the gap in size between birds and pterosaurs.  And again, it does not 
need to given the apparent launch mode differences.  I admit though, 
that 12-15% is enough to make a difference (whereas the 3% I thought 
you were implying would not).  Incidentally, a 35% fraction of O2 
should be a big deal for the insect life around at the time.

> 2). The reason I used the O2 pulse hypothesis is because it 
chemical mechanism for increasing air 
> density at any
> point in post-Archean time that I know of. Note that density is a 
> function of total air mass, and can be independent of composition.

Of course, but in the case at hand I think the fact that it is O2 that 
contributes the most to the density increase is notable, because it 
would have important ramifications for mass-specific power (especially 
in insects).

> 4). "Atmospheric effects" as Habib aptly puts it, do _not_ exclude 
> such things as novel launch systems or climate regimes (see Campbell 
> re Argentavis evolution and takeoff), anymore than the K/Pg event 
> means that dino diversity was not already in decline from other 
> mechanisms. In my opinion, the more extreme animals would have needed 
> all the help they could get, even at 1.2 atms. Now, at 1.5 or 2 
> atms... }: D

I agree that any help would be useful, but I don't think pterosaurs 
required the assistance of an enriched atmosphere.  As Jim already 
pointed out, they would most likely be perfectly viable in the modern 
conditions.  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.

> 5). It has been 10 years since I searched the data relative to birds. 
> My impression at that time was that higher apparent wingloads were the 
> rule in paleo-birds of pigeon-size or larger, not the exception. 
> Limnofregata ( from Storrs Olson) is perhaps the most accessible 
> example of what I mean. Please correct me if I am wrong. Must be a 
> wealth of new material.

I don't think there currently stands a pattern in wing loading of 
extinct birds one way or the other.  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.  
There are plenty of highly loaded and lightly loaded fossil birds.  
They cover about the same range of loadings as modern birds.  Large 
pseudodontorns may have been slightly more heavily loaded than modern 
albatrosses, but not so much for it to be particularly odd or notable 
(beyond the notable fact that they were very large).

> 7). I've seen (Campbell's original) estimates on Argentavis of 120 kg. 
> My impression at the time was that his revision to 76 kg was prompted 
> mainly by 'aerodynamic objections', but I well could be wrong. Re-do's 
> are always good...

Yes, the 120 kg estimate seems very high.  A 120 kg volant bird might 
be possible, but it would depend greatly on the ecotype, launch mode, 
and planform (all of which are interconnected).

> PS-- How big would a ptero or bird have to be to elicit a "OK, these 
> guy's had to have had some help, because there is just no way in 
> standard atmosphere..."? Actually a serious question.

Jim already pitched in on this for pterosaurs (far better than I 
could).  For birds, it depends a great deal on the planform and launch 
mode inv
d to ecotype, etc).  Using the 
scaling relationships I have for pseudodontorns (and the bone strength 
ratio model I've been using for their mass estimates lately), I would 
cautiously list about 75 kg as the rough limit.  (The largest species 
actually known seems to fall around 60 kg).  For continental, 
convective soarers (ie. Argentavis like forms), the limit in standard 
atmosphere would be substantially higher (probably over 90 kg), but I 
can't say much at the moment.

Cheers,

--Mike H.