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Re: Reversed Hallux in Theropods: a query



In a message dated 95-11-24 21:26:06 EST, DSmith0531@aol.com writes:

>>But if you want to see what
>>the retroverted hallux really looked like in the arboreal forms, take a
look
>>at Luis Chiappe's paper on his Patagonian enantiornithan bird specimen
>
>So, is this a purely avian adaptation, or do we find it non-avian theropods
>as well?  If so, which ones?  I would assume that all this talk of arboreal
>theropods (as in, the not-quite-a-bird-but-on-its-way variety) perching in
>trees is backed up by just such a specimen?  Or is it speculation?
>
>

Of course it's speculation. If there were an _actual specimen_, nobody would
be arguing with me about this(!). What I am saying is that the functional
retroverted hallux came first, in a small, arboreal form that employed it for
perching and continued to employ it this way as it evolved toward birdhood;
but that the hallux lost its perching function (separately) in each lineage
of large, cursorial theropods that diverged from the toward-birdhood-evolving
lineage. We happen to have a number of Mesozoic avian specimens that show the
functional retroverted hallux; but we have as yet no "pre-avian" specimens
that show it, mainly because we have no "pre-avian" specimens at all.

If we had those "pre-avian" specimens, BCF would now be a fact, not a
hypothesis (at least verging on a theory, by the way).

The alternative is the BADD theory, in which the hallux loses its proximal
end, shrinks, retroverts for no evident reason to a quite specific site on
the back of metatarsal II, and then sits there with a dangling digit for
about 35 million years on almost every known theropod from ceratosaurians to
tyrannosaurians awaiting future use as a perching organ in the one theropod
lineage that evolved into birds.