Re: Gliders to fliers

["James R. Cunningham" <jrccea@bellsouth.net>]

Rob Meyerson wrote:

> During this discussion on the evolution of powered flight, there is an 
> important question that needs to be asked.  But first, let me state the 
> premises for my argument (I will use pterosaurs to make my point; I propose 
> that the same basic ideas will coorespond to dinobirds).
>
> 1)All powered flight needs to start from a jumping and/or gliding lifestyle.

I would disagree with this.  There is a possibility that birds originally used 
their wings to allow an increase in the zenith angle of the lateral force 
vector their feet could apply to the ground when running.  Also hatchet fish 
seem to be well on their way to establishing powered flight without a jumping 
or gliding life
style.

> 2)The flying squirrel is a good model for the gliding-phase in bat evolution. 
>  The continuous flap of skin from fore to aft in the squirrel can easily 
> adapt to the wings of the pterosaur (I will use the trees-down theory for 
> this line of reasoning).
>
> 4)For powered flight, the forelimbs evolve to take full control of all flight 
> needs: speed, direction, lift, etc.

I would disagree with this.  It didn't happen in birds, and it didn't happen in 
pterodactyls.  Both still use their tail or leg/uropatagium structure to 
provide yaw command authority and some pitch authority.

> My question: how does a critter go from 3 to 4?  What environmental stresses 
> and what selection pressures are present in a glider lifestyle that would 
> "force" a glider to develop fully powered flight?  Could a thinning forest, 
> and larger distances between trees, provide the needed pressure for the 
> transition powered flight?

Why not develop powered flight first and go to gliding later.  I think this 
probably happened in birds, but not in pterosaurs.

All the best,
Jim