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Re: If Dinosaurs Could Fly



        Ooh, ooh, something besides "Dinos In Space"!
Terry D. Jones writes:
>The feathers from which down feathers are derived functioned in flight, not
>insulation.
        But were insulatory feathers *always* down feathers? "Tetrapods"
started with swimming fins, then they evolved legs. Then ichthyosaurs
returned to the sea, and evolved flippers. Does this mean the ichthyosaur
flipper represents the primitive state of the organ?

>There is a logical evolutionary progression
>from scales (in ectothemic avian ancestors) to flight/contour feathers (in
>ectothermic early birds) and finally to downy feathers (in later, en[d]othermic
>[ornithurine] birds).
        How about another "logical progression": scales in ectothermic avian
ancestors to "protofeathers" in close pre-avians
(?ectothermic/"intermediate"), perhaps persisting for millions orf years
doing something else before flight evolved, to flight/contour feathers (in
?ectothermic or "intermediate" early birds), to downy feathers (in
endothermic ornithurine birds). Same number of steps (when you consider that
your sequence left out protofeathers (of whatever origin)). :)

>In order to have insulatory feathers in the earliest
>birds they would have to appear intact out of the blue!!!
        In order for feathers to come about as you hypothesize, they too
must appear, with full aerodynamic function, "out of the blue". In either
case, one must postulate some sort of intermediate (of whatever function).
The only difference in the arguments is that you appear to believe that, if
feathers evolved for insulatory purposes, then modern insulatory feathers
should represent the ancestral feather condition. Perhaps this intermediate
evolved for flight, perhaps for insulation, whatever. But it was probably
not identical to a modern down feather.
        All modern feathered reptiles are either flighted, or are
secondarily flightless, and are descended from a series of common ancestors
which were doubtlessly acomplished fliers. Why shouldn't their feathers be
derived from those with aeordynamic functions? I would imagine that millenia
of selection pressure for aerodynamic function on every aspect of avian form
would affect all feathers on the body. Selection might have initially
favored the spread of those feathers which were first adapted for flight
over the body versus less derived insulatory (or whatever) protofeathers,
accounting for all of the modern feathers having a flight origin. This holds
especially if "flight-derived" feathers make better insulators. Perhaps your
low-metabolism early bird simply did not need that much insulation...
        To head off another possible avenue of debate, why should feathers
revert to the primitive feather state when their function is only
insulatory? They have undergone countless generations of selection for
aerodynamic function. The ichthyosaur's flipper does not look like
_Ichthyostega_'s fin (to push an analogy too far...). 

        Terry Jones' arguments remind me alot of Feduccia's feather-origin
argument. I hate to do this, but since I no longer have this posted on a web
page, I'd like to re-present an earlier post on that argument, in the hopes
that it might spur the debate.
------------------------------------------------------------------------------
The Origin of Feathers
by Jonathan R. Wagner

Again revisiting questions brought up during the interminable
bird origins debates, my roommate, bless his heart, dug up _The Beginnings
of Birds_, the aforereferenced papers of the 1984 Eichstat conference.  One
of the papers in this volume is by Alan Feduccia, and concerns the origin of
feathers.

Feduccia, A. 1985: On Why the Dinosaur lacked Feathers, in _The 
        Beginnings of Birds_ (Bronner and Drentler, Eichstatt, 1985)

        Feduccia states that contour feathers are very similar to flight
feathers.  He says, referencing Parkes, 1966, that they are practically
"miniature flight feathers".  He continues, "If feathers had evolved
initially as thermoregulatory structures, then the body or contour feathers
[of flighted birds] should be essentially similar to the original
feather..."  He highlights the "hairlike" degenerate feathers of flightless
birds, attempting to show that feathers which "have value only in a
thermoregulatory context" do not require all of the specializations seen in
contour and flight feathers.  Feduccia quotes Lowe 1928, discussing the
unaerodynamic properties of the feathers of flightless birds, in that
discontinuities in their form and arrangement cause a "loss of [air]
resistance leading to an inability to fly."

        Feduccia does not appear to be aware of the concept of selection
pressure.  It seems clear to me that the contour feathers of flightless
birds are experiencing strong selection pressure to retain their aerodynamic
form (metaphorically speaking, of course).  The high selective advantage of
this aerodynamic form may be gleaned from the Lowe quote.  Once a bird
lineage becomes flightless, these pressures are no longer applied, and the
feathers may degenerate to whatever state will still perform the
thermoregulatory/insulatory function.

        Can anyone think of a selective advantage which might be gleaned
from reducing the feathers?

        Feduccia's declaration that contour feathers must represent the
primitive state of feathers used for thermoregulation is of doubtful
validity.  The selective pressure which causes aerodynamic shape in feathers
need not have been present in an insulatory structure, as Feduccia points
out.  These early feathers may have existed merely as "hairlike" structures,
later developing the structure we now see in flighted birds, as a result of
the selection pressure which currently keeps them aerodynamic.  That the
contour feathers may resemble the flight feathers so closely may be due to
convergence, or the pressures on all aerodynamic feathers may be similar
enough that a common solution was reached for all.  Or the similarity could
be the result of [exaptation] of feathers from some former use (such
as shade from sunlight, or display).

        I find it astonishing that a complete counter argument can be made
merely from the evidence that the arguer presents, and a knowledge of the
basics of evolution.

        (Refs for papers cited by Feduccia available on request.  I do not
list them, as the relevant material is quoted)
----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
        "Arrogance and stupidity in one convenient package,
                how very efficent of you..." - L. Molari