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Re: Placental takeover: final comment.



On Mon, May 28, 2001 at 10:28:14PM -0400, John Bois scripsit:
> Again, I'm not saying that I--or anybody else--knows what happened
> back then.  I am saying we don't know.  Further, I have cited
> scientists for whom this question is a life-long concern.  If _they_
> don't know, it's valid to suggest _we_ don't know.  And so, the
> marsupial extinction may not yet be counted among those caused by an
> extraterrestrial event.

You're trying to explain the wrong thing.

There's this great big hole in the Yucatan, precisely dated to the K/T
boundary.

There's direct evidence for global affects in the iridium layer at the
K/T boundary; there's direct evidence of the magnitude of the energy
transfer in crumbled continental shelf sediment fans, the size of the
crater, the distribution of ejecta, and so on.

So we can be reasonably sure a big rock *did* hit, and that it hit in a
lousy place -- more or less on the equator, in rock that enhanced the
acid rain side effects, and on the other side of the world from the
regions affected by flood vulcanism -- and that it hit at a bad *time*
for at least North America -- seaways down, diversity down, so fewer
species present.

One of the things reported for the Permian boundary are river sediments
that revert to braided patterns, from which it may be inferred that there
wasn't enough terrestrial vegetation to prevent surface erosion and
allow stable bank formation by the rivers.  Evidence of that sort from
the K/T would give a further lower bound to the magnitude of the
ecological dislocation, one above the 'no terrestrial vertebrate over 25
kg adult weight' bound we already have.

However, dislocation *did* happen.  An ecosystem already stressed got
smacked by a big rock; global darkness, and general ecological collapse.

What can perhaps be explained is not marsupial extinction, but the
placental radiation; why'd they fill in the new niches first as the
ecology recovered?

This could be a matter of pure dumb luck -- get the absolute numbers of
survivors down low enough and luck dominates -- or it could be a matter
of competitive advantage in the resurgent ecosystems in question.  Given
that the niche displacement happens _throughout_ NorAm, however, luck
driven scenarios are more plausible, since it is easier to explain low
absolute numbers of survivors than it is to explain why marsupials
didn't do well in *any* niche during re-population.

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.