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Re: Digit Identity



----- Original Message -----
From: "Matthew Bonnan" <mbonnan@hotmail.com>
Sent: Tuesday, June 17, 2003 7:03 AM

> For some reason, this is not possible in
> reptiles, birds, and mammals -- there are many reasons, of course, but I'm
> not going into detail on them here.

I assume one is the mammalian fear of cancer, which has fixed the number of
7 vertebrae except in species with low metabolism?

> "Now suppose a mutant arises that produces only three digits (II-IV).  If
> nothing else changes, the concentration of the "thumb-determining" protein
> will now be highest in ancestral digit II (since I isn't there at all),
and
> so the digit homologous to ancestral digit II (being the third digit
formed)
> will develop just like ancestral digit I.  Voila`:  frame shift.  Am I
missing
> something?  Why would something like this not be *expected* to happen?"

That's more or less what I tried to explain. (I didn't. :-] )

> Again, the other point of interest to me is that digit identity in birds
is
> being based, in part, on supposed wrist identity and homology.  Also, not
> very many bird embryos have been examined in detail to see how the manus
and
> carpus develop -- there could very well be differences in different birds.

Taking an ostrich with reduced hands may not have been a good idea...

> Whereas I think the frameshift hypothesis offers an interesting
explanation
> for the possible difference in digit identity in birds and dinosaurs,
> another possibility is that birds do indeed have I, II, III (without a
> frameshift) and that these have been mis-identified because of problems
with
> determining wrist homology.

Feduccia always identifies the digit that lies distal to the ulna as IV.
Well. Which digit lies distal to the ulna seems to depend, unsurprisingly,
on digit size. In us, I'd say the line in continuation of the ulna passes
between IV and V. In most tetanurans (and *Herrerasaurus*), it passes
through III. In *Tyrannosaurus*, it passes through II...
        At the same time, Feduccia identifies the digit that develops most
strongly first as IV. My old source, which is based on line drawings from
the 1930s, says that birds have I-II-III, in early embryos all 5 fingers can
be identified, and IV indeed develops most strongly first, but then simply
stops growing, even though it chondrifies "and can be incorporated into the
ossification of metacarpal III" (must look similar to "*Ornitholestes*" and
*Sinraptor*). Looks like they took more consecutive samples than Feduccia.
Of course it's difficult to argue from data from the stone age of staining
methods etc..
        BTW, that same book says the proximal part of V is involved in the
"ulnare" of birds.