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At 05:00 PM 3/28/97 -0500, George Olshevsky wrote (Hi George, didn't expect
to hear from you on this one :) :
>Sure, except that the keeled sternum is >plesiomorphic< to Coeluria, since it
>is known in at least one pre-coelurian theropod (the no-longer-headless
>wonder from Dinosaur National Monument, which also displays a giant furcula).
>The keeled sternum was lost repeatedly, along with the furcula and volant
>wings, in many if not most Cenozoic flightless birds, and it was likewise
Your point being? Just because it happens alot doesn't make the
character transformation any less important. 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.
>lost in most cursorial, flightless Mesozoic non-avialan theropods--which
>never had such strongly keeled sterna as the ancestors of the various
>Cenozoic flightless avian groups--as well. In the non-avialan theropod
>_Mononykus_, the keeled sternum was incompletely lost [...]
>Vestigialization of a body part through functional loss is quite common and
>rapid in vertebrate evolution, so great care must be taken in assessing the
>presence or absence of characters such as a keeled sternum and a reduced
>fibula in cladistic analyses of theropods and birds.
Recall that it would be more parsimonious to accept the inheretance
of a trait, including a vestigialization, from a common ancestor than to
accept its convergence. And yes, vestigialization, commonly occuring or not,
is still a character.
>Nature need not be
>parsimonious at all and can frequently be convergent when it comes to
a) Parsimony and convergence are not antonyms [sic?].
b) Convergence is not un-parsimonious
c) Nature may not be parsimonious, but the methods of reasoning
which form the basis of science are founded on parsimony.
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.
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.
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
Web Page: http://faraday.clas.virginia.edu/~jrw6f