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



George wrote:

>This is one reason bipedality should be considered an "unnatural" stance in
>tetrapods.

        At risk of starting this entire vitriolic back-and forth slugfest
all over again:
        What exactly constitutes a "natural" stance in tetrapods.  There
seem to be a lot of animals across a broad spectrum of phylogenetic and
ecological nodes which achieve at least a bipedal stance if not bipedal
locomotion.  Certainly, a quadrapedal sprawling stance seems more stable and
plesiomorphic, but exactly how do you find bipedality to be "unnatural"?
        I find the implication that an behavior or mode of locomotion which
is dangerous is "unnatural" to be unconvincing at best.  We might as easily
say that the Perigrine falcon is engaging in an "unnatural" act because it
crash dives at prey.  What if it didn't pull up in time?  It would die.  Is
its hunting method therefore "unnatural"?  What about Orang-utans?  They
could die if they fell out of a tree (as someone else mentioned earlier).
Are we therefore to presume that their way of life is somehow "unnatural"?
        I find myself especially unconvinced when the example of a very
derived animal like _Tyrannosaurs_ is used.  Bipedality in dinosaurs almost
certainly developed in smaller animals with longer arms, which could almost
certainly catch themselves in a fall and lessen the (small by _rex_
standards) dangers of becoming pizza.
        [Warning:  the following includes some mild speculation]
        Bipedality may confer extra speed certain animals.  I believe this
was suggested by Paul in _PDW_, and certainly sounds plausable, considering
the points Dr. Holtz has made on the list.  Dr. Holtz has already run
through the predilictions towards in bipedal running in certain reptilian
groups.
        Think of it, you are removing the forelimb and the back entirely
from locomotion, simplifying the structure and perhaps saving weight.  It
does not seem unreasonable that it might be more simple to concentrate on
developing the primary locomotor muscles in the hip and tail area, rather
than split the locomotor module between two sets of limbs.  Especially
considering the hindlimb musculature, which, as elucidated by Dr. Holtz,
attatches to the tail.  This might provide the oppurtunity to achieve
greater muscle performance (in terms of weight/power and total power)  to
counteract the omission of the forelimbs and provide faster speeds.  There
would have to be strong selective pressure for speed.  Does anyone know of
any studies on the relative speeds of bipedal and quadrapedal running in the
*same animal*?
        In any case, this hardly sounds "unnatural".  The fact that
bipedality does not arise as a major character of more groups may have to
with differring criteria between the Mesozoic and the Cenozoic, or between
clades.  Extreme specialization for different locomotor modes in exant
cursorial forms (such as the cheetah, which is adapted to achieve great
speed quadrapedally), or perhaps differing ecological modes now (I wonder if
some of the speedy cursorial niches haven't been taken over by birds...).
It has been suggested on this list (and I'm sure elsewhere), that bipedality
in archosaurs may be a product of their "preadaptations", and analogues in
other groups may not apply.  It is cetainly concievale that, at the time,
there may have been a good living to be made by animals which were really fast.
        Thus bipedality might be "unnatural" for *some* tetrapods, but
"natural" for Reptilia, especially for archosaurs during the Triassic.
        Wagner
+--------------------------------------------------------------------------+
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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