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Where are the fossils?
I pulled this off sci.bio.evolution (I hope that is OK to do).
> Disclaimer: I am a colleague of Drs Kumar and Hedges and I read
> previous versions of their MS as well as the final Nature paper.
> K&H's dates agree well with the fossil record except
> for the early divergence of placental mammals.
> Resolving the discrepancy requires either: simultaneous
> speedups and slowdowns in substitution rate across a
> very large number of genes that otherwise seem
> 'well-behaved'; or a lack of cretaceous fossils.
> Ingrid Jakobsen
> Institute for Molecular Evolutionary Genetics
> Pennsylvania State University
I have previously suggested that there may be undiscovered mammal fossils
from before the K/T. Jim Kirkland almost categorically replied that if
there were any to be found they would have already found them. Should
this recent data cause any reconsideration of that position.
Or is a third possibility likely: that molecular change is not necessarily
accompanied by morphological change?
Or is a fourth possibility worth considering: tertiary-aspect mammals were
around before the end of the Cretaceous. However, relative to populations
of the Teriary they were quite rare. As such they are much less likely to
be discovered. This is a sort of Signor/Lipps effect in reverse.
In any case, from the data--if it is as promising as it appears--it seems
that mammals diversified and then dinosaurs became extinct, rather than