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Re: Placental takeover: final comment.



On Tue, May 29, 2001 at 08:52:13AM -0400, John Bois scripsit:
> On Mon, 28 May 2001, Graydon wrote:
> > There's this great big hole in the Yucatan, precisely dated to the K/T
> > boundary.
> 
> There are many other things that existed then.  The problem is using the
> big hole to explain the changes in speciation.

That's still backwards -- we've got good evidence of a global disaster.
The thing to explain becomes *survival*, not differential sucess.  The
default in that scenario ought to be 'all die, and O! the
embarrassment'.

> > 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.
> 
> Not saying that it didn't have disasterous effects--just that it doesn't
> do a good job of explaining the fossil record.

Why not?

> > 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
> 
> Flannery, who regards the bolide as the causative agent of species
> changes, says this could not be true because of the lack of changes among
> amphibians.

Buffered ponds; ambibians whose habitat happened to be in ponds (or
caves!) in limestones would avoid the direct affects of acid rain.  This
might well suffice to get them through the boundary with minimal
visible-to-the-fossil-record loses.

> > 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.
> 
> So why attribute the prime cause to the bolide?

Because the information available indicates that the magnitude of the
disaster was such that the thing to explain is not differential success,
but survivial.

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

Well, no -- that's *data*.  We've got multiple lines of evidence of the
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?
> 
> But these answers may also work for marsupial extinction.

Those answers aren't to that question; that approach is looking at
differential speed of re-radiation, not differential success in
niche-holding in a stable ecology.

> > 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.
> 
> Yes.  And this is a great question for research.  But notice the way you
> are phrasing it: "this _could_ be a matter of dumb luck...".  I prefer
> this formulation to "this _would have_ been a matter of dumb luck...".

It's very difficult to come up with a scenario consistent with known
data on the magnitude of the disaster in NorAm that *doesn't* come down
to luck for primary survival.

Secondary survival and niche reclamation *could* explain some portion of
the extinctions, if a remenant population gets niche displaced and was
the *only* remenant population, but we're very unlikely to get data good
enough to support that sort of analysis from fossils.

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