Re: Milankovich Cycles

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Of course clade membership per se has no bearing whatsoever on extinction
suseptibility (though that membership usually carries with it a large
amount of morphological, ecological, behavioral, etc. information that does
bear in the question of extinction suseptibility).  Nevertheless,
statements such as "the dinosaurs went extinct at the K-T boundary" imply
that neither the dinosaurs nor any of their direct descendents are to be
found in Tertiary sediments.  This is simply false.  The problem is of
importance to extinction studies because one cannot talk of the
"extinction" of higher taxonomic groups unless one has a unambiguous
definition of such groups.  While there are many ways to define groups of
organisms, definitions that refer to common ancestrty have tradiationally
been preferred.  By this definition, the dinosaurs did not go extinct.

The operative word in Stefan's scenario is "suddenly."  My reading of the
K-T fossil record suggests that the event did not result in a faunal
turnover that lasted weeks to a year or so at most.  If you are out of that
range, then you are either out of the timespan assigned to the impact
scenario by most of the simulations, or into a never-never land of saying
that the somehow the evidence of the fossil evidence is irrelevant to
explaning one of its most prominent features.

To broaden the discussion out a little, the same problem appies to attemtps
to relate Milankovich cycles to fossil data.  Since the faunas obviously
are not responding to the tilt of the Earth's axis, any effect that
Milankovich cycles may have on ancient faunas must be transmitted though
other terrestrially-based processes.  Unless those processes can be
specified and their predictions tested against the data of the fossil
record (as Roy Plotnick is trying to do), the attribution of biotic
phenomena to Milacovitch cyclicity explains very little.  Because of their
lack of detailed testability, such statements are articles of philosophy,
or scenarios to a much larger extent than they are verifiable/refutable
scientific hypotheses.

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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