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Re: BCF& COMMENTS ON IT



A few comments on Matthew Troutman's post:

>feathers (Longisquama) 

On what basis do you call the peculiar structures on Longisquama feathers? 
(and what about the odd imprints in Cosesaurus, for that matter?)  Would it
not
to be better to call them feather-like structures (as are, I suppose, fern
fronds!).

A reversible digit I is 
>not needed for climbing, pamprodactyl (all digits anterior) is by far 
>the best scansorial foot and incidently it is found premanufactured in 
>dinosaurs. 

However, some passerine climbers do rely heavily on the reversed digit I for
climbing, it is large, well-developed and has a massive claw in such birds as
the Wallcreeper Tichodroma muraria and the Nuthatch Vanga Hypositta
corallirostris.  This reaches its apex in the Australian treecreepers
(Climacteridae), in which the digit is long, straight, and lacks the
emargination at the proximal end for the erector of the hind claw found in all
other songbirds I know of (the erector has shifted to the base of the digit
instead - this is from my, alas unpublished, PhD thesis).  The hind claw
is, in
these birds, important for bracing as the tail is not used for this purpose as
in woodpeckers and in the pamprodactyl climbers.  The most extreme
pamprodactyl
foot I know is in swifts, which certainly cling to vertical surfaces but are
not really climbers.

>Phorusrhacoids are the only birds with fully functional forelimbs 
>throughout life, but their forelimbs were modified to a point not seen 
>in any theropod.  So there!

Though I agree with the point here to some extent I would repeat an argument I
made earlier here, namely that the fewer alterations that occur in the
development of a wing or wing-like structure the fewer reversals are required
to lose them.  Thus, though a fully-powered flyer would probably have such
modified arms that re-evolution of ancestral forelimbs might be hard to
imagine, a creature unable to fly but already possessing exaptations some of
its descendants might incorporate into a wing structure could have other
descendants that lost these secondarily, and spotting this would be a real
challenge (or even impossible without cladistic analysis).

The only real "proof" that an animal is secondarily flightless, or has
secondarily lost exaptations for flight, is a phylogenetic analysis showing
that it is derived from an animal that flew or possessed the structures in
question.  So proving or disproving BCF comes down, in the end, to
establishing
phylogeny - anything else is speculation. 
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net