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Re: "The X Digit"...a Pteroid bone??



Fri, Jan 22 , 1999 Cait Kiernan wrote:

>Larry Febo wrote:

>>The whole development of the Supracoracoideus structure >seems (to me)
much
>>too complex to have developed twice >independantly in two separate
lineages.

>Why?

>No structure (at least no structural analog) is too complex, in theory, to
be
>reproduced through convergence and/or parallelism.

I suppose not (nothing is impossible) but what is the likelyhood?? I
personally, openly admit (once again), I am a minimalist when it comes to
Convergence. When it involves  the use of different morphological structures
fine, but when the same skeletal constituants are involved I become
extremely doubtful, especially when the types being compared are close
together in phylogeny. (in this case being both archosaurs).

>Certainly the development
>of the supracoracoideus in avians is no more complex than their evolution
of
>endothermy, which, assuming there isn't an avain/mammal connection hidden
in
>the woodpile, would require that endothermy was developed independantly in
>each group.

Well, that`s another pet theory of mine. I do believe that endothermy to the
point of egg incubation evolved only once, and most likely began in the
early Anapsid forms whence it either was improved upon by later mammals and
birds and relinquished by other forms that assumed a condition of secondary
ectothermy, such as squamates, turtles, crocodiles.


>Another example, cranial kinesis. It appears not only in avians
>(and possibly nonavian theropods), but, independantly, in squamates, and
>surely required a much more complex series of evolutionary steps than would
>the independant development of superficially similar flight musculature in
>birds and pterosaurs.

Well, I believe that cranial kinesis involves different skeletal structure
and joints in avians vs squamates, so if you want to lable that as some kind
of "convergence" OK.

As to its being more "complex" a development than the redirection of force
produced by the supracoracoideus via development of acrocoracoid
process,...I don`t know how to make such an arguement either way.

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