Re: Placental takeover: final comment.

[John Bois <jbois@umd5.umd.edu>]

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.

> 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.

> 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.

> 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?

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

Case closed.

> 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.

> 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...".