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Re: More on Birds, dinosaurs, and digits



At 05:27 PM 11/28/97 -0600, Toby White wrote:
> Stanley Friesen <sarima@ix.netcom.com> said:
>
><snip>
>>previously.  Looking at the pictures they show of hand development in
>>crocodilians, I saw something interesting.  While the "main axis" is fairly
>>clearly through digit IV, this main axis is *curved* laterally through the
>>metatarsals!  This leaves digit III as the one that is actually colinear
>>with the humerus.  So asked myself, what would the result be if digit IV
>>reduced in size, and perhaps digit III increased in size, to the point
>>where distal metatarsal IV was substantially smaller than distal metatarsal
>>III?  From the looks of things, such a process would, with *no* *further*
>>*changes*, be sufficient to shift digit III onto the "main axis".  No need
>>for subtle changes in proportions in the adult.
>
>All of which strengthens their argument.  That is, the change needed to
>establish III as the main axis, starting with bassal crocs, is relatively
>slight.  That change explains what we see in dinosaurs, and is widely
>distributed in dinosaurs.  In fact, I just ran across a reference to a
>I-II-III manus in ceratopsians in Dawson's book.  Given its closeness to
>crocs and wide distribution in dinosaurs, the posited III-based main axis
>presumably arose quite early in the dinosaur lineage.  Yet, it is not found
>in birds nor in any other group of tetrapods.  The natural conclusion is
>that birds did not arise from dinosaurs.  

   Actually, I think you've drawn the wrong conclusion.  If only a slight
change is needed in croc embryos to shift the main axis to digit III, then
one can easily see the main axis shifting to digit III in bird embryos, the
crocs' closest living relative.  It is more likely, given the scenarios in
the above three paragraphs, that avian digits are really I-II-III and the
observations of Feduccia and Burke are incorrect because the main axis
shifted to digit III long ago.

   Now throw into the pot that the main axis in some amphibian embryos is
apparently digit III ... still looking for that ref.

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