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Re: snap-action predation



Michael Skrepnick
Paleo Artist
Visit my Website:
http://www.dinosaursinart.com
----- Original Message -----
From: "Jaime A. Headden" <qilongia@yahoo.com>
To: <dinosaur@usc.edu>
Cc: <GMHuman@cs.com>
Sent: Tuesday, September 17, 2002 12:13 PM
Subject: Re: snap-action predation
I see a
> development of a pygostyle as being a biomechanical consideration of
> reduction of external forces affecting (distorting proportions of) the
> distal tail, but tail fans exist in forms without such a structure; this
> suggests to me that tail fans were not dynamically used in balancing
> forces as in gravity versus lift, and maybe also not as in directional
> controls, which would impose torsional effects and also result in a
> pygostylar or other stiffening structure. Thing is, for directional
> control, torsion must exist ... it must permit the distal structure to
> rotate with intertia and stabilize: cheetahs and birds orient their tails
> when maneuvering or banking towards the direction or pivot; it does not
> seem to be a matter of lift production in modern birds, as recent studies
> have suggested: smaller birds have smaller tails when you would expect
> them, and some birds seem unaffected if their tail streamers are removed,
> except when breeding is concerned. They are adaptive to display, or to
> affect torsional stabilization. I suggest the same may be true of
> dromaeosaurids and other still-tailed and short-tailed theropods, which
> likely precludes, as Mike Skrepnik illustrated, a tail fan for *Nominga*
> as a result of the pygostyle.

*** Explain this concept to a peacock, whose visual array doesn't seem to be
predicated on increased physical performance or lack thereof. In "grounded"
dromaeosaurids / pygostylians utilizing their tails as dynamic stabilizers
to whatever degree ( or not), or in other less advanced terrestrial
theropods (with a more conventional caudal series ) sporting a tailfan
array, perhaps locomotory considerations (or impact on same) make little
difference.  When Nomingia was being described, it was of some importance to
Phil Currie to include a tail fan in the reconstruction.  Of course, there
is no physical evidence indicated in the specimen to substantiate the
existence of a caudal array, as there is nothing forward of the ilia on
which to base an image of the front half.  My experiences with Phil have
shown time and time again that he puts a great deal of consideration into
formulating opinions on specimens before putting them to press and always
expresses from a conservative viewpoint in his analysis.  You can ask him as
to his reasoning on how the anatomical features in the Nomingia image were
developed, however, I can tell you that the tail fan didn't simply appear in
the artwork because I didn't have anything better to paint that day  ;o).

Mike Skrepnick