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Re: More on Birds, dinosaurs, and digits



 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.  

Personally, I'm not convinced that this tracks the (forever hypothetical)
dinosaur embryology, but it makes really good sense.  Digit V is unique
because it grows as a process from the base of IV.  If III is established as
the main axis, this might well suppress IV, and thus V, leaving the observed
I-II-III.  

  --Toby White