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Re: Airbagged(was Dive!Dive!Dive!)



At 10:51 PM 11/6/96 -0500, George O. wrote:

>The only point I was trying to make here was that
>bipedality is a character state or condition peculiar enough among tetrapods
>to justify requiring an explanation.

        True.  This is tangential to the point you were trying to make,
though.  In the example provided, no explanation is required for this
condition in _T. rex_, as bipedality is a primitive for dinosaurs.

>I wasn't even trying to push the BCF explanation.

        :)

>This very clearly describes the heightened danger
>faced by a large bipedal cursorial animal--something that, I think, evolution
>would "naturally" select against rather than for. Since we know that bipedal
>tetrapods >have< evolved, however, we can infer that there did exist
>circumstances or lifestyles, likely rare and unusual, in which the evolution
>of bipedality is selected for, in spite of the danger of having only two legs
>instead of all four for running.

        It would seem, at first glance, that this is a somewhat moot point
as to origin of bipedality, since we can always make the argument that
bipedality was developed and or perfected in smaller organisms which could
take a few falls at first.
        Your origional point, as retranslated, still confuses the issue
slightly, as you are mixing the order in which derived states are presumed
to have developed.  It may be best to restate your point as something like this:

        "There would be selective pressures against the enlargement of
bipedal cursors, or for that matter the secondary redevelopment of
cursorality in large bipedal animals."

        When stated this way, with bipedality as, as you pointed out, a
derived character within Tetrapoda, but a plesiomorphy for Theropoda, we can
see that the issue is not whether or not it is "unnatural" to be bipedal in
this case, but whether it is "natural" to be large and remain a bipedal
cursor (which, as Tom Holtz pointed out, the bones say _T. rex_ is).

>Can you now see why I chose to use the word "unnatural" in this context?

        I guess, but I still say that evolutionary statements should use
evolutinoary terms...
        :)
        Wagner
+--------------------------------------------------------------------------+
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
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|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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