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Diversity after divergence--pre-K/T modern orders.



A couple of recent articles deal with this issue.  Marshall C.R. Comment,
in Geology Jan 99 on Bleiwess' "Fossil gap analysis supports early
Tertiary origin of trophically diverse avian orders" 1998 geology
26:323-326

Marshall states that analyses of fossils cannot assume a "uniform recovery
potential" since "many higher taxa had long early histories with
relatively low diversities."  Therefore, he says, confidence intervals
must be extended back into the Cretaceous.  Bleiweiss counters with a
couple of arguments (which are complex and don't seem to me to address
Marshalls' basic point) and then goes on to claim: "an assumption of
uniform probability of fossil recovery is the appropriate working null
hypothesis..."

Then Foote et al 1999 Evolutionary and preservational constraints on
origins of biologic groups: Divergence times of eutherian mammals Science
283:1310-1314  argue (I think) that rapid divergence times of early orders
might be the reason for not being able to find fossil evidence of
divergence in the Tertiary.  In other words, because they diverged so
quickly there is not much hope of finding a record of the divergence.  
This is to counter molecular clock evidence that orders arose in the
Cretaceous.  They later give alternate hypotheses (for example, species
may have diverged in the Cretaceous but changed little morhologically,
therefore we can't recognise the divergence).  Then they give and reject
the following alternate hypothesis: "Modern eutherian lineages existed
through the Cretaceous, but their preservation rates were generally lower
than those of species in other mammal groups.  This difference in
preservation rates would have to be more than an order of magnitude, for
which we can offer no support...".

But Marshall's point above re birds would seem to apply to mammals as
well.  That is if it is true that species in their early fossil record
have low,diversity.

Thsi leads me to the question: do species sometimes, always, never, have
low diversity at their first divergence point?

Lastly, Foote et al make the following statement and I ask if it is
strictly true: "..there are no unequivocal modern eutherians in the
Cretaceous..."