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Re: (extinction)

HP John Bois: "The current fav. hypo demands no such explanation:
whatever happened, happened because it happened.  This might be OK if
there were not reasonable ecological explanations for the phenomena
(e.g., competition from placentals, competition/predation from
neornithines, etc.).  Until these have been ruled out, they remain
viable and, I claim, interesting."

I still do not see how competition from placentals, predation from
neornithines, and the like are viable explanations (in a direct sense,
at least).

Reason: these seem very unlikely to act as global events, nor are they
patterns to which the dinosaurian (and other) taxa had not been
exposed.  LOCAL extinctions from competition are common.  A GLOBAL
mass extinction requires a NOVEL effect across the entire range of
several groups severe enough to reduce ALL viable populations.

The extinction pattern at the K-T does seem to be targeted to some
degree, but I'm a bit fuzzy on how this argues against bolide effects
for the extinctions.  I would argue that such a massive impact would
almost certainly be targeted: perhaps large-bodied species would do
worse, perhaps hibernation would be important, etc.

In a sense, this IS a competition model, and this point is (as far as
I can tell) often overlooked.  That is, a bolide isn't expected to
cause targeted mass extinctions simply because it kills everything
directly (though it will do this to some), many of the extinctions are
expected to result from the sudden shift in competition dynamics. 
Suddenly, for example, a species that can hibernate for 10 months
might be doing much better than the species that hibernates for only 4
months.  The advantage of either may have been negligible before, now
it counts.  Or, slightly different, food or shelter might be very rare
in a short time-frame, making competition effects shoot through the
roof.  So the bolide could make new predation/competition factors
important very rapidly.  These are global, novel effects AND they are
targeted based on biology/ecology: some groups do better because they
compete well under post-bolide conditions.  

Just a thought,
--Mike Habib