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Re: recapitulation & feathers



>> The appearance of digits in the avian foot is 4-3-2-1,
>> and it ought to be the same in the hand - and the fact that the order of
>> appearance is 4-3-2-1-5 in most (but not all) nonavian tetrapods.
>
>     What are the exceptions, and what digit do they start with?  I
>thought the serial appearance of digits was the most interesting piece of
>evidence in the paper.  I don't recall they even mentioned that there WERE
>exceptions.

I don't think they did, but a large series of papers from the 1980's,
primarily involving Per Alberch and Neil Shubin, demonstrated this sequence
in several tetrapod groups - squamates, crocs, lissamphibians.  But if
nonavian theropods maintain 1-2-3 in the adult hand, some sort of
reorganization must have occurred - either in the sequence of digital
appearance, or in the sequence of digital loss.  Given that birds always
have four digits as embryos, I suspect it's the former.

As for exceptions - there are some known in amphibians.  THe primary
developmental axis passes through the fourth digit in most tetrapods -
hence the argument that in birds, the digits are 2-3-4-5.  But in
salamanders, it passes through the third.  Frogs thus have 2-3-4-5, while
salamanders have 1-2-3-4.  No living amphibian has more than four digits,
and all outgroups to Lissamphibia among temnospondyls have four digits -
based on phylogeny, it looks like this was a reorientation of the axis
rather than independent loss of digits in the frog clade and salamander
clade.


chris


:::::::::::::::::::::
Christopher A. Brochu
Department of Geological Sciences
University of Texas at Austin
Austin, TX 78712

gator@mail.utexas.edu

http://uts.cc.utexas.edu/~brochu/brochuhp.html

(512)471-6088