Re: Pterosaur size

[David Marjanovic <david.marjanovic@gmx.at>]

-------- Original-Nachricht --------
Datum:  Tue, 12 Dec 2006 01:55:26 -0800 (PST)
Von: don ohmes <d_ohmes@yahoo.com>
Betreff:  Re: Pterosaur size (Was: Great in the air, not so good underwater)

> 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)

> 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...).

Be careful with that.

First of all, what do you mean by "amphibians" and "reptiles"? And what by 
"class" -- clade?

Then, is the first part of your scenario falsifiable? I'm not sure it is. If 
the biggest member of a clade is extinct, you'll say a dwindling has occurred 
since; if it's extant, you'll say the dwindling is yet to come. Right? This 
sounds like you would only accept cases where a maximum size was reached at 
some point in the past _and has been precisely kept ever since_ -- an extremely 
unlikely scenario, even if only for stochastic reasons. Maybe Neosauropoda 
counts (the Morrison supergiants + *Argentinosaurus* + *Puertasaurus*).

The second part -- that the maximum of each "class" is, from the mid-Jurassic 
onwards, below all previous ones -- is of course falsifiable. Let's have a 
look. The biggest known discoglossoid frog was *Latonia* of much of the 
European Cenozoic. The biggest known frog at all is *Conraua* -- the extant 
Goliath frog. One counterexample. The biggest known tyrannosaur is *T. rex*; 
the biggest known bird is *Dromornis*. One more counterexample. The biggest 
known ceratopsians and hadrosaurs were bigger than the biggest known 
stegosaurs. Four counterexamples. The biggest known dinoceratan, 
*Uintatherium*, was smaller than the biggest known perissodactyl, 
*Paraceratherium*. Five counterexamples -- and if I kept searching for 10 
minutes, I'd find lots more, simply because *Paraceratherium* lived later than 
the maxima of a lot of mammal clades, and I can't see a reason why I should 
confine myself to mammals here.

Also, what is "dwindling"? Is it the mere extinction of the maximum? Is it a 
general trend, like the Cope-Depéret rule?

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

Elaborate, please.

> 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.

Don't you think anatomical constraints are to blame here?

> It is an entirely incorrect way of
> determining how much atmospheric N2 is subducting over time.

How should that happen?

> 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,

What do you mean?

> Also, abiotic fixation of N2 is now known to occur.

Other than lightning?

> 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.

So... if the air pressure was greater in the past, we should have had less 
water vapor in the air in those times. Yet, even in the Cenomanian the tropics 
don't seem to have been much hotter than today, while the poles were much 
warmer -- the most important means of heat transport being evaporation in the 
tropics and precipitation at the poles. This suggests that a lot more water 
vapor was in the air than today.

> 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.

Then you have to explain why those models work pretty well.

> 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.

But then, *Limnofregata* is IIRC Danian, and the Danian only lasted for around 
3.8 Ma.

> 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.

In my experience scientists are very bad at keeping secrets. :-)
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