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From: dmr@alice.UUCP
Newsgroups: sci.bio,sci.astro
Subject: Re: Mass extinctions
Message-ID: <6760@alice.uUCp>
Date: Mon, 30-Mar-87 00:56:48 EST
Article-I.D.: alice.6760
Posted: Mon Mar 30 00:56:48 1987
Date-Received: Tue, 31-Mar-87 04:44:57 EST
Organization: AT&T Bell Laboratories, Murray Hill NJ
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Xref: utgpu sci.bio:187 sci.astro:772

The volcanic hypothesis is not dead yet; a recent Nature (376 #6109;
12 March 1987) prints a long review article by Officer, Hallam, Drake,
and Devine entitled "Late Cretaceous and paroxysmal Cretaceous/Tertiary
extinctions" espousing it.  Officer et al. do take care to say
"We wish to emphasize that this article predominantly advocates
a particular point of view and may be criticized in giving less
attention to alternative models."

They do not discuss osmium at all (even though the abstract mentions
"iridium and other associated elements.")  I take this to be
a defect.

There are at least three ideas attending the work of the Alvarezes
and others, and that following it:

1) A large meteor/comet struck the earth at the K/T boundary 65
million years ago, as suggested by the iridium-rich layer
and perhaps other things.

2) This event was responsible for the mass extinctions around that
time ("killed the dinosaurs").

3) Similar events occurred periodically before that.

One doesn't have to buy these as a package, even though they're
frequently offered that way.  Moreover, they are all still in
controversy.  From what I've been able to gather, the meteor
idea is reasonably well supported but by no means certain.

The second is not especially well supported in any direct way;
many paleontologists believe that the extinctions were occurring
for quite a while both before and after the "instant" of the
putative meteor.

The third seems to be on slightly shaky ground after a strong start.
The reality of the periodicity is questionable, and all of the
proposed mechanisms (e.g. the Nemesis planet or dark star,
the oscillations of the Solar system through the galactic
central plane) have been attacked on physical grounds.

I'm most emphatically a non-expert in these issues and can't
really judge them.  The new ideas are most exciting, but I
don't think they have yet carried the day.


	Dennis Ritchie