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Re: Avian Ancestors, new book on theropods



On 03/01/2013 02:35 AM, Tim Williams wrote:
> Ruben Safir <ruben@mrbrklyn.com> wrote:
>
>
>> I keep reading things like this and to my mind, they make no sense.
>> Having enough lift for flight almost guarantees that they were used for
>> just that .... real flight.  From an engineering POV, flight and lift
>> are a pretty binary things.  As soon as those feather forms developed,
>> you can be sure that they were used for flight, and powered flight.  Not
>> gliding, parachuting, catching dragon flies, but full throttle flight.
>> If anything, one would need to speculate that the powered flight existed
>> even before the asymmetrical feather, and the wing.
>
> Fair point.  But there's a few reasons why this link between
> asymmetrical feathers and powered flight is not so simple.
>
>
> (1) The asymmetry of the feather (i.e., with the vane asymmetrical
> relative to the central rachis, producing a shorter leading edge) is
> not responsible for generating lift.  But if lift forces are being
> generated, then the asymmetry of the feather allows the vane to better
> resist bending and twisting as it's exposed to airflow. 

That is not how I've seen feathers function or read of their function
with regard to their shape.  Obviously, feathers need a force to create
the lift they need, but lift is what they do even with any fluid flow over
them.  You can not move them without them generating lift.  A simple
gust of wind will lift my African Gray Parrot when his/her flight feathers
are attached to the wing, if he doesn't take corrective action.

It would be an enormous drag on the movement of any creature with
these feathers who was not using them specifically for powered flight.


>  So the
> asymmetrical feathers of _Archaeopteryx_ tell us that the wings
> probably had an aerodynamic function - but they do not tell us that
> the feathers (and the wing) were necessarily used for powered flight.

Those wings could fly.  And if they could, they did :)

> The wing could have been used for gliding and/or improved aerial
> maneuverability, which would have utilized some form of airfoil... and
> therefore selected for an asymmetrical feather.

Right, after using powered flight.  I think those feathers and wings had
serious lift.  You line those feathers up, your going to generate serious
lift in the slightest breeze. 

I'm sure you don't need me to, but I can send you a few dozen flight
feathers
just like the ones in the fossil and you can test them yourself :)

>
> (2) The asymmetry of the feather is not responsible for generating
> thrust.  To get powered flight, you need thrust as well as lift.
>
>
> (3) The feather is just one part of the flight apparatus of birds.
> The osteology and the musculature have to be biomechanically competent
> to execute a thrust-generating flight stroke.  On this issue, there is
> some doubt over whether _Archaeopteryx_ could flap its wings.  One
> interpretation is that the shoulder joint orientation precluded
> elevation of the humerus higher than the back - so no recovery stroke,
> no powered flight.  Added to this was the extremely weak pectoral
> musculature (compared to modern birds).  Maybe _Archaeopteryx_ was
> capable of some rudimentary, low-amplitude flight.  

hmm

I'd need to be proved that one needs a complete flap to create lift with
a full set
of flight feathers.  It doesn't make sense to me.  I can so no reason
why these things
couldn't get lift even with a small gust of wind.

It really only makes sense if powered flight was used for, and then the
feathers developed
in an asymmetrical manner.  What possible evolutionary force would
create the reverse?




> In any case, the
> evolution of a powered flapping flight stroke might have been very
> gradual, and it's unclear exactly how critters like _Archaeopteryx_
> feature in this transition.
>
>
>> Also, FWIW, I'm not sure why there is the belief that the asymmetic
>> feathers only developed once, in one family lineage.  Is that not
>> also unlikely?  If there was a lineage of dinosaurs or theropods which
>> had need of and developed powered flight prior to the asymmetic feather,
>> then it would make sense that more than one of those lineages then
>> developed an asymmetric feather.
>
> AFAIK, there is no widely-held belief that the asymmetrical feather
> evolved only once.  On the other hand, it has not been established
> beyond doubt that powered flight came *before* the evolution of
> feather asymmetry.  Small theropods with asymmetrical-feathered wings
> - such as _Archaeopteryx_, _Microraptor_, _Jeholornis_ and
> _Confuciusornis_ - might all have been powered fliers (i.e., thrust +
> lift).  Alternatively, they could have been non-powered gliders that
> used the wings for lift, but not for thrust.  Or, some of these
> asymmetrical-feathered theropods were powered fliers, others just
> gliders.  Any of these scenarios is congruent with a single origin of
> feather asymmetry.
>
>
>
>
>
> Cheers
>
> Tim
>
>