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

David Marjanovic wrote:

<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.>

  David, not only are you using highly modified manus and digit reduction here, 
including a functionally monodactylous manus, but you are also usin adult 
conditions. It has always been MY understanding that modification through 
ontogeny is the reason why embryological data is preferred over at-a-glance 
guestimation. Should we use ichthyosaur or temnospondyl polydactyly as the case 
for arguing "standard" embryological digit identities in tetrapods, which is 
what we've been trying to assert here?

I am not going to take issue with Vargas et al. because my understanding of the 
literature is based on what has been done on mice, this gator, and birds. I 
have yet to see a comprehensive study on development in comparison across 
"clades". This would allow us to understand the efficacy of the models used by 
any workers. It would also help should anyone disregard the prepollex idea to 
cite it, but that is a question you CAN ask the authors directly. Perhaps you 
can create a PloSone account and comment there?


Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)