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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

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


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