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you realize, of course, that this means war




On Thu, 31 Jul 1997 Dinogeorge@aol.com wrote:
> << There are some gross excesses of other kinds, of course. Does the most
> parsimonious explanation have to be the correct one? No. Do those employing a
> cladistic methodology say that it is? Of course not. >>
> 
> Of course they do, character versus character! Otherwise, why are they using
> parsimony in their algorithms? PAUP: Phylogenetic Analysis >Using Parsimony<.
> Unless they're engaged in the pursuit of meaningless cladogram generation.
        They use parsimony because it's most likely to give the right
answer, not because it is guaranteed to. 

> But remember that it is much easier to >lose< digits
> through vestigialization than it is to >regain< lost digits. How many mammals
> have six-digit hands these days, besides pathological cases? Not even
> cetaceans. But how many have four, three, two, and one-digit hands? Most
> ferungulates, for starters. So: digital loss is like an evolutionary one-way
> sign. Happens easily one way, but seldom if ever the other way.
        This might be relevant if segnosaurs had actually lost the digit.
But they didn't, it merely fails to connect to the ankle. And according to
the experts it still does not incorporate into the ankle as in more
primitive dinosaurs. 
        Shades of Dollo's law return to haunt us...

> Consequently,
> the fact that segnosaurs and theropods have tridactyl hands is not nearly as
> important (since it could easily have happened twice by convergence)
        ...because segnosaurs are a parallel to the avians and were
themselves clambering through treetops experimenting with flight??
         We are still waiting to hear why this feature is so godawfully
important in theropods as a flight-related feature but not in Segnosaurs.
It sounds like the "it's importantonly when it supports my preconcieved
notion of how things ought to be" principle is in action here... 


> Why should it have changed in just that
> one lineage, when it doesn't change in thousands of others? More
> specifically, why should the segnosaur foot have changed--in this character
> >and< in others-->to resemble the prosauropod foot<?
        The same reason that Hesperornis should have a complex system of
the joints in the toes that causes them to rotate when pulled back
reducing water resistance, like in grebes- since both are 
foot-propelled divers they see similar selective pressures which may cause
them to evolve in similar directions (they also have congerved in many
other functional details) Same reason sharks, dolphins, and
ichthyosaurs look alike- similar selective pressures. 
        Prosauropods are graviportal herbivores that spend at least some
of their time on two legs. Segnosaurs are graviportal herbivores. Similar
lifestyles, similar selective pressures, similar results. When the
selctive pressures are the similar you will get similar results, that's
why the great auk of the northern hemisphere and the penguins of the
southern hemisphere and the plotopterids all look very similar in the
construction of their wing bones, because they are all using them in
similar ways. It's not fantastic to think that two big slow-moving
herbivores should come to resemble each other in the feet. You'd expect
it.