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Re: Reversed Hallux in Theropods: a query



>Fair enough. However, I have yet to encounter in the literature a compelling
>function for the retroverted hallux that does not involve perching. Either it
>evolved for that function, or it evolved for some other function that nobody
>has been able to figure out yet.

Agreed.

 I suppose the feature could have evolved as
>an aid for walking on lily pads (an example from your earlier postings), but
>that strikes me as less likely, because the presence of lily pads in the
>environment for the millions of years it would take for a hallux to retrovert
>is improbable. Most likely, the lily-pad adaptation occurred because the
>retroverted hallux was already present, and all it had to do was grow a
>little bigger and change its shape a little.

Suggseting, again, that once you have one of these things there may be an
evolutionarily valid use for it even if you are not in trees.  As I said in
my longer message, I have no problem with considering it probable (though
not established) that it evolved as a perching mechanism at some point in
the ancestry of theropods and birds - something I see as consistent with
both the BCF and BADD theories.

However, your theory seems to assume that the condition would not have
survived in theropod lineages if it were not being continuously perpetuated
as new lineages sprung from a primarily arboreal line, and this I find
unconvincing for the simple reason that there is no one-to-one correlation
between terrestriality and hallucal reduction in living birds.  Further,
several bird lineages have lost the hallux altogether (eg ostriches,
buttonquails, some plovers and sandpipers) indicating to me that the feature
can be lost  rapidly (I assume that the Sanderling, say, is closer related
to other sandpipers than T. rex was to any small perching dino or
dino-bird);  if it is retained in something like T. rex I assume  at the
least that there as no selection pressure to lose it, and this situation
could have persisted for millenia.  Again this works for both BCF and BADD.
>
>Incidentally, don't evolutionary theorists have some kind of specialized
>vocabulary to describe a "forced" adaptation, an "opportunistic" adaptation,
>and so forth?
>
 Could be, but I forget 'em and am too lazy to look them up right now...
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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