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

[Michael Habib <mhabib5@jhmi.edu>]

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

> Yep. Stuff gets lost, too. So, some correction and re-iteration.
>
> 1). The O2 pulse hypothesis 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 is the only
> published, peer-reviewed geochemical 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 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.

Cheers,

--Mike H.