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Change before the KT boundary



As an invertebrate lurker who has spent altogether too much time looking 
at the KT sections at Brazos, I want to add just two more cents to the 
simmering thread on the end Cretaceous extinction.  A number of comments 
recently have mentioned that there were changes in the fossil 
record of certain groups prior to the extinction and this is usually 
taken to mean that the bolide impact was not the sole factor in that 
groups demise/decline.  At Brazos, we sampled in detail the molluscan record 
through a one million year interval of the latest Cretaceous and the 
first two million years of the Paleocene.  We saw a decline in diversity 
of molluscs shortly (within a hundred thousand years or so) before the 
extinction, part of which is probably due to the Signor-Lipps effect, and 
part of which is not because the taxic composition changed as well as 
numbers of species.  But we also saw such changes further down the 
section.  The entire Cretaceous section is an apparently uniform offshore 
mud that 
contains no sedimentary indications of facies change yet contains several 
intervals of low diversity molluscan assemblages.  Why do these low 
diversity intervals occur?   We don't know, perhaps lack of preservation, 
perhaps slight decreases in the oxygen content of the water, perhaps they 
had a bad day/millenium.  The point is that it is natural in a long 
stratigraphic sequence to see faunas experience ups and downs and the 
latest Cretaceous molluscan decline (immediatelly prior to the KT 
boundary) seems no different from any other, especially when you consider 
that it is still composed of a typical "Cretaceous" assemblage.  
Immediately above the boundary, however, a host of new molluscan species 
appears and the entire ecology of the fauna changes drastically.  The 
Paleocene molluscan assemblages were unstable and apparently stressed for 
over half a million years after the KT event.

Thor Hansen
Western Washington University