Xref: utzoo sci.bio:998 sci.misc:1012 rec.birds:479
Checksum: 19732
Lines: 31
Path: utzoo!sq!msb
From: msb@sq.uucp (Mark Brader)
Date: Fri, 18-Mar-88 01:56:44 EST
Message-ID: <1988Mar18.015644.21597@sq.uucp>
Newsgroups: sci.bio,sci.misc,rec.birds
Subject: Re: animals and Earth's magnetic reversal
References: <7387@ihlpa.ATT.COM> <460@amethyst.UUCP> <20@denali.UUCP>
Reply-To: msb@sq.UUCP (Mark Brader)
Organization: SoftQuad Inc., Toronto


> During polarity reversals, which probably take ten thousand years or more,
> the earth's magnetic field is probably weak and irregular, and generally
> useless for navigating.  The dates of ancient extinctions are not known
> precisely enough to allow convincing correlations with magnetic reversals;
> for that matter, the timing of reversals longer ago than ten million years
> is not known too precisely, either.

I happen to have at hand a copy of "The Nemesis Affair", a book* by David M.
Raup, one of the proponents of the theory of periodic extinctions.  He writes:

:  Unlike the paleontological record, the [magnetic] reversal record is
:  fairly clean, at least for the past 165 million years of geologic time.
:  About 300 reversals have been found in this interval, most well dated.

This does not necessarily disagree with the previous posting; Raup admits
that the magnetic/extinction correlation is not convincing, and his idea
(as a paleontologist) or "well dated" may be looser than that of someone
in another field.

The book is well worth reading, by the way, no matter what you think of
the theory.

* W. W. Norton & Co., 1986, ISBN 0-393-30409-4 paperback, $10 in Canada.

Mark Brader, SoftQuad Inc., Toronto, utzoo!sq!msb, msb@sq.com
	Until 3,000 million years ago we can say not a lot happened
	although further study would not come amiss.  Then signs of life
	appeared, including some large reptiles and, very recently, bipeds.
	It is too soon to say whether these bipeds will play an important
	part in the world's story.    -- Colin Morris in "History Today"