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Re: Arms into wings



At 01:51 PM 3/3/99 -0500, Tim Williams wrote:

>I think what might be bothering many folks on this list (me included) is
>the transitional phase. Ornithothoracines lost the grasping/predatory
>ability of the wing (hence the carpometacarpal fusion).  Sure,
>_Archaeopteryx_ and other non-ornithothoracines used their forelimbs for
>both grasping and flying.  But how and why did the forelimb take on this
>additional function of flight?  How did it begin?

Those are the big questions, and are still argued about.

>I know this goes all the way back to Darwin and the Evolution of Incipient
>Structures.  What good is half a wing, etc?

49% better than 1% of a wing (as Dawkins would say...). :-)

>I'm not arguing against a
>maniraptoran origin of birds - but for my own benefit what kind of changes
>MAY have occurred in the design of the forelimb (especially externally -
>i.e. feathers) between a non-flying basal "eumaniraptoran" and a flying
>avian like _Archaeopteryx_?  There must have been a "not-quite-flying"
>phase somehere in the middle, so what was the forelimb used for during this
>stage?

And that gets to part of what Ostrom's work and the Ostrom Symposium was about:

What skeletal changes were there in the forelimb between basal maniraptorans
and _Archaeopteryx_?  Damn little: as Ostrom showed back in the 1970s,
Gauthier & Padian & Paul showed in the 1980s, etc., they are morphologically
extraordinarily similar.  Very similar angles of motion possible, very
similar limb proportions, etc.

What external changes were there?  That is what the feathered Chinese guys
are showing: feathers on the arms seem to go back to at least before the
oviraptorosaur-eumaniraptoran split (before the divergence of
oviraptorosaurs from the dromaeosaurid-bird clade).  Any functional scenario
has to reconcile with this: either Greg Paul's scenario, where the earliest
forms were fliers and oviraptorosaurs and droaeosaurids are secondarily
flightless descendants; or a Gauthier-type scenario, where the basal form
was a non-flier doing something else with feathers.

However, you are suggesting that the pattern is: grasping -> ??unknown?? ->
flying.  In fact, what the morphological studies of _Archaeopteryx_ and
_Confuciusornis_ hands suggest instead is: grasping -> grasping + flying ->
flying only (with the "handy birds" Archie & _Confuciusornis_ representing
the middle stage).  In this scenario, there isn't a period of some unknown
use, but rather a period in which both uses (grasping and flying) are at
play, followed by selection for those forms with superior flight (and
consequently less grasping) ability.

(Actually, the above scenario would fit in with Greg Paul's scenario, with
Archie representing a grasping + flying stage and dromaeosaurids etc. being
descendants of Archie-like forms in which the grasping ability was
maintained but the flight ability was lost.  Thus, the above scenario fits
both Paul's and Gauthier's etc. model, as long as what is being grasping
(prey vs. tree vs. prey & tree) isn't specified).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661