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Re: Pterosaurs and birds, was Re: birds and pterosaurs



--- Michael Habib <mhabib5@jhmi.edu> wrote:

> Very interesting thought.  However, as was already
> mentioned via 
> Argentavis, there have been some truly large birds
> as well (far larger 
> than extant forms).  Some pseudodontorns, for
> example, had wingspans 
> around 7 meters across.
> 
> I'm not sure what you mean by scaling in the "flesh"
> part of wing; the 
> amount of muscle and connective tissue is indeed
> important to the mass 
> of the wing, but growing a wing especially long
> generally requires 
> longer bones, longer associated muscles, and longer
> feathers (ie. all 
> structures increase in length).  

Exactly what I mean. Bird which needs to fly
consistently cannot have too long flight-feathers,
because feather grows slowly. 

In theory, evolution might bypass the problem in that
bird with extremely large wingspan has long flesh part
of hand, but flight-feathers remain at manageable
size.
However, bases of flight feathers are at the end of
flesh part of the wing and must be robust. Therefore,
whole flesh part of the wing is heavy and this forbids
this evolutionary shortcut.

I know about Argentavis etc. - it apparently existed
in ecosystem with unusually big density of food.
Judging that big pterosaurs are more common than
equally big birds, pterosaurs could grow bigger on
similar amount of food. 

It is not exactly physical constraints (body mass,
wing size etc.). It is ecological constraints
(like most biomechanical problems). Bird which
feathers grow for too long cannot fly "well enough to
feed". What is "well enough to feed" ultimately
depends from food in birds native ecosystem.  

Judging that 7-m wingspan birds are sporadic in fossil
record, they need extremely food-rich conditions to
evolve. 7-m pterosaurs were more common - ergo they
were more efficent, they ecological niche was less
restricted.

Jerzy Dyczkowski

PS. I am looking for more precise data about bird
moult - it seems that feathers grow more like
10mm/day, but full moult of vultures and pelicans
takes 3-4 years.




                
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