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*To*: dinosaur@usc.edu*Subject*: Re: recapitulation & feathers*From*: gator@mail.utexas.edu (christopher brochu)*Date*: Fri, 23 Jan 1998 13:50:37 +0100*Reply-to*: gator@mail.utexas.edu*Sender*: owner-dinosaur@usc.edu

>> 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

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