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Re: Avian Ancestors, new book on theropods

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.  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.
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.

(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.  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.