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Re: More on Birds, dinosaurs, and digits



Stanley Friesen wrote:

>At 05:27 PM 11/28/97 -0600, Toby White wrote:
>> Stanley Friesen <sarima@ix.netcom.com> said:
>>
>>...  Yet, it is not found
>>in birds nor in any other group of tetrapods.
>
>On what basis do you make this determination?  At no time in development do
>birds have more than four manual digits.  How does one *tell* that the main
>axis digit is IV not III?  The whole *point* of the article was using the
>main axis to *identify* the digit as IV.  If III is the main axis in
>dinosaurs sensu stricto then what is left to say that the main digit in
>birds is IV?

Imprimis, as I've said before, I'm not sure I buy the concept of a "primary
axis."  At this point I question whether it is either (a) useful to explain
anything or (b) accurate in reflecting the underlying developmental
genetics.  The term implies that IV (or III, as the case may be) is an
organizing factor which causes the undifferentiated mesenchyme of the limb
bud to form the appropriate elements of the digits.  I don't know this to be
the case, although I'm trying to find the time to check it out (says he,
whining a little).

However, putting all that to one side, Burke & Feduccia use several lines of
evidence to identify the outermost fully-formed digit of the avian manus as
IV.  They do not rely solely on arm-waving (sorry, couldn't resist that)
about a primary axis.  The other lines of proof include: (1) Digit V has a
fairly unique ontogeny in that it is not part of the digital arch.  It forms
directly at the base of IV.  This is observed, if faintly, in the chicken
embryos and marks the base of IV.  (2) The chicken pes clearly has five
digits at some points in development.  The pes and manus are analogous.
Since the fully formed digits of the pes are clearly II-III-IV, this
suggests that the same is true for the manus.  (3) the morphological
analogues in the five-digit croc and turtle manus also suggest that IV is
the correct identification.

  --Toby White