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Re: Atmospheric density



On 03/07/98 Jim Cunningham wrote:

>I have seen several mentions of increased oxygen content in the Jurassic
>and Cretaceous atmosphere, approximately 35% vs our 21%.  This also
>implies an increased carbon dioxide percentage, and both would
>contribute to increased mass in the atmosphere and therefore increased
>density near the surface, thereby lowering the density altitude more
>than the increased temperature would raise it.  So in addition to
>providing for enhanced oxyenation; compared to the modern atmosphere,
>the denser early atmosphere would have allowed flight at reduced
>airspeeds for a given weight and wing area...... Can you or anyone on the
list provide me with suggested references that actually attempt to quantify
the properties of the atmosphere during the periods when pterosaur and bird
flight were
>developing?

I agree entirely.

One excellent summary of this can be found in "A Flying Start - Did an
Oxygen Surge get Birds off the Ground ?" by Nell Boyce(New Scientist No
2129 11 April 1998). 
This in turn relates to an original article by Robert Dudley in The Journal
of Experimental Biology (Vol 201, p 1043)

Being inspired by the article in New Scientist, I wrote them the following
letter which I hope you will find interesting:

I read with great interest Nell Boyce's "A Flying Start" on recent research
linking the evolution of flight in "bats,birds and flying dinosaurs" to an
over-abundance of oxygen.

For years, I have had a similar view . I started trying to answer the
following difficult questions :

How did Archeopterx fly with such puny flight muscles ?
How did Brachiosaurs and Mamenchisaurs pump blood and oxygen to their heads
in the trees ? .....

.... What is certain is that these animals were successful and therefore
well suited to their environment.  Physical limitations bases on experience
of our own environment are clearly false.

I believe atmospheric pressure to be the key factor.  It is likely that
feathers developed in dinosaurs for insulation.  Whether flight evolved as
a development of parachuting, or hopping, or running, less atmospheric
pressure meant that assistance from feathers was much greater than would be
expected under present conditions. Modern birds have through evolution
adapted to current conditions which would probably not have suited early
evolution of flight. 

Similarly,Brachiosaurs make much more sense if atmospheric conditions were
very different. No need for huge hearts, impossible dietary requirements.
They could even submerge themselves in water if they so wished.

The oxygen effect described could have resulted in an atmospheric pressure
perhaps only two-thirds the current level. Oxygen being a relatively
lighter gas compared to carbon dioxide would have caused this.

In my view the KT extinction event, however it was caused, liberated huge
amounts of carbon dioxide into the atmosphere increasing atmospheric
pressure. Extinction of oxygen producing organisms such as plants and
phytoplankton would have amplified this effect. Although, the composition
of gases has improved since the KT event due to the recovery of the oxygen
producing biota, atmospheric pressure is still considerably higher than it
was when birds or their dinosaur precursors began to fly.

Roy Nash