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Re: Signor-Lipps effect?



At 09:17 AM 8/10/98 +0200, Jarno Peschier wrote:
>... Could you please elaborate
>on this Signor-Lips effect?

It is the name for a particular sort of sampling artifact caused by the
relative rarity of (most sorts of) fossils.  Basically, with sparse
temporal sampling, as one gets with larger organisms, the last appearance
datum will almost certainly be *earlier* than the actual extinction of the
species.   Furthermore, the expected relative distribution of last
appearance data has been shown to mimic the form of gradual disappearances;
that is the times are distributed in a more or less exponential
distribution prior to the actual species terminus.

The net result is that it is VERY difficult to distinguish true gradual
disappearances over time from an set of simultaneous ones.

The best approach I have seen, at least in my opinion, is one based on
inter-basin correlations in *order* of disappearances.  The Signor-Lipps
distribution should not be *correlated* between independent basins, since
it is a random sampling error.  Some of the articles I have seen have used
this method to demonstrate what appears to be a *very* strong case for
stepped disappearances, at least in certain marine plankton.

--------------
May the peace of God be with you.         sarima@ix.netcom.com
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