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Re: Birds have "thumb" genes like alligators: free pdf



Great... PLoS ONE is proud of letting people make comments, but requires them to log in. I don't quite see why I should bother. So I'll comment here.

The paper does nothing to distinguish between the frameshift hypothesis and the possibility that bird fingers really are in positions 1, 2 and 3, with the position that Feduccia, Hinchcliffe etc. call 1 being occupied by the prepollex. The following paper is not cited:

Monique C. M. Welten, Fons J. Verbeek, Annemarie H. Meijer & Michael K. Richardson (2005): Gene expression and digit homology in the chicken embryo wing, Evolution & Development 7(1), 18 -- 28

Likewise not cited is this paper, which does not draw any such conclusions, but invites them by showing interesting figures of chicken embryo wings with extra FGF-4 at the caudal edge -- and one or two small extra digits there:

Neda Nikbakht & John C. McLachlan: Restoring avian wing digits, Proc. R. Soc. Lond. B 266, 1001 -- 1004 (7 July 1999)

Not cited either are whatever Starck used as sources for his vertebrate anatomy textbook in 1979, where he states as a matter of fact that bird embryos have a prepollex _and_ five normal digital rays. Full story (several years old) here:
http://www.unet.univie.ac.at/~a0000265/bird-fingers.htm


(I notice that the figures aren't there. I'll try to find them and upload them back.)

The only evidence Vargas et al. seem to use for how to number the digit positions is the "primary axis": whichever digit lies in the imaginary continuation of the ulna and ulnare has to be number 4. I don't see how this follows. In their very own photos of alligators, this imaginary line lies between positions 3 and 4. In my hands, it lies between 4 and 5. In *Tyrannosaurus*, it goes through 2, and in *Mononykus*, it goes through the lateral half of 1... I submit that where each digital ray lies with respect to that line depends on the size of the hand with respect to the lower limb, and on nothing else. There is no reason why the same ray should always lie on that line even in early embryos; the digital arch curves wherever it needs to.