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From: guy@slu70.UUCP
Newsgroups: sci.bio,sci.astro,sci.misc
Subject: Re: Mass extinctions
Message-ID: <10@slu70.UUCP>
Date: Mon, 30-Mar-87 09:44:43 EST
Article-I.D.: slu70.10
Posted: Mon Mar 30 09:44:43 1987
Date-Received: Thu, 2-Apr-87 00:44:12 EST
References: <784@scicom.AlphaCDC.COM>
Organization: St. Louis Univ., St. Louis, MO
Lines: 21
Xref: utgpu sci.bio:193 sci.astro:775 sci.misc:233
Summary: periodicity

In article <784@scicom.AlphaCDC.COM>, rwb@scicom.AlphaCDC.COM (Robert Brumley) writes:
> 
> I have become very interested lately in the subject of mass extinctions
> in earth's past.  Apparently, with remarkable regularity, about every
> 26 million years a large percentage of the life forms on our planet
> die out.   
> 
Note that the periodicity proposed by Raup and Sepkowski is still rather
contraversial and is disputed by many scientists, especially paleontologists.
Most of the argument hinges on the definition of an extinction (species
go extinct all the time and defining a *mass* extinction can be somewhat
arbitrary). A second problem is that dating mass extinctions is difficult
as the usual radiometric techniques do not work for sediments (you end
up dating the source rock which may be far removed from the sediment).
I've heard one comment that human civilization may qualify as a mass
extinction agent given the number of species that we have or are in the
process of eliminating.

Your friendly net geophysicist,

                                   Guy M. Smith