Re: Milankovich Cycles

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And here I was worried that PaleoNet was getting a little dull lately!

Being a skeptic about these notions of extraterrestrial forcing, I'd like
to point out (along with Roy Plotnick) that the 26 m.y. extinction
periodicity Tom alludes to has a lot of critics ranging from statisticians
to systematists.  And the dates on many (most?) impact craters are much
more imprecise than the biostrat. ages for the biotic data.  Moreover, the
simple demonstration of an impact having occurred at any particular time is
not sufficient to demonstrate a causal link between the impact event and
elevated levels of extinction intensity.  Even the most cursory examination
of the data reveals many cases in which well-dated impacts are not closely
associated with heightened levels of extinction (e.g., the Late Eocene
impacts; see Prothero and Berggren, 1992) as well of cases in which bona
fide mass extinction events are not associated with unambiguous impact
markers (e.g., the Late Devonian event: BTW the current consensus is that
Ir anomalies are NOT unambiguous evidence of impact occurrence; see
Colodner et al., 1992, Nature; Sawlowicz, 1993, P3; Wang et al., 1993,
Geology).  The assertion that extraterrestrial factors (a polite euphemism
for impacts?) cause global extinctions cannot be tested unless the precise
mechanisms responsible for those extinctions are specified AND the fossil
record examined to determine whether it conforms to the predictions of
those mechanisms.  If you try this for most fossil faunas you run into big
problems in trying to get the predictions of your mechanisms to provide a
detailed and unique fit to the observations (e.g., ammonites gone from the
record prior to the K/T impact debris, fish sailing through the same
"catastrophe" seemingly unaffected). I think Gould summed it up best, if
inadvertently, when he said (in reference to the same gross correlations
that underpin Tom's posting) "my first-class prejudice is not to accept
coincidence on that scale." (originally quoted in Glenn, 1994, p. 266).  It
seems that this question really comes down to a matter of belief,
predisposition, and prejudice (his word, not mine) rather than one of data,
logical inference, or (for that matter) even science.

Finally, even though its been said many times in many places by many people
far better qualified to address this issue than I, let me point out that
dinosaurs didn't go extinct. Birds are dinosaurs.  It's precisely this sort
of lapse that gets us into trouble when we try to analyse associations
between extraterrestrial factors and evolution/mass extinctions.


Norm MacLeod




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Norman MacLeod
Senior Scientific Officer
N.MacLeod@nhm.ac.uk (Internet)
N.MacLeod@uk.ac.nhm (Janet)

Address: Dept. of Palaeontology, The Natural History Museum,
         Cromwell Road, London, SW7 5BD

Office Phone: 071-938-9006
Dept. FAX:  071-938-9277
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