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Re: Mononykus



In a message dated 97-03-28 18:24:46 EST, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes:

<< Indeed, if the loss of an
 ossified sternal keel is so prevalent, one must doubt the character polarity
 you propose. It seems likely that, based on your information, the ossified
 sternal keel was lost in the common coelurosarian ancestor, and redeveloped
 by avians. If it was also redeveloped by _Mononykus_, that's still two steps
 if it's convergent, one if it's synapomorphic. >>

Wrong-o. It was >not< lost in the common coelurian ancestor and then
redeveloped (whatta convoluted history that would be). Keeled sternum
developed once, perhaps somewhere among primitively flying ceratosaurians or
early flying tetanurans, was retained by all flying tetanurans (and became
massively developed in later forms) but was reduced/lost in most flightless
tetanurans throughout the Mesozoic and Cenozoic. It was present in the common
coelurian ancestor (a flying animal). It's still there (retained) in
_Mononykus_ and in _Archaeopteryx bavarica_, too. Not to mention in lots of
flying avialans.

<< And yes, vestigialization, commonly occuring or not,
 is still a character. >>

But one subject to massive convergence and therefore liable to greatly
confuse character analysis.

<< You're argument seems to always boil down to nature doing something
 that we cannot discover using the character data we have. This is assuredly
 possible. The answer is not to abandon our methods, but to seek better data
 which might guard against this. >>

Not quite. Something like 95% of the time, we >can< discover what nature has
done using the character data we have. It's the other 5% of the
time--signaled by a manifestly absurd result, such as segnosaurs in Theropoda
and Mononykus in Avialae--that we need to take a closer look at methodology.

<< d) Further, you have yet to demonstrate that nature, when viewed in
 toto, is not parsimonious. If we had every scrap of data we could ever hope
 to have (down to DNA and living animals), I should be very surprized if the
 theory which required the least number of unsupportable assuptions and the
 least number of untestable suppositions were not the best model of nature.
>>

Alas, this assumption is completely untestable. But there is a much, much,
much larger number of ways for nature to be >unparsimonious< than for nature
to be parsimonious. I'd go with the numbers.