[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: No Hedging.

>I respect your opinion--that genetic change doesn't
>necessarily create morphological change.  But it is just an hypothesis.

Actually, this isn't entirely my opinion.  I think what people are
confusing is divergence date and divergence rate.  The lineages today
including elephants and rodents may have diverged in the Turonian, and they
may even have been morphologically distinct early in their separate
histories (though not necessarily recognizably rodent or elephantine).  The
"burst of radiation" in mammalian diversity during the Paleocene might have
involved an increase in diversification, but the basic lineages were
already in place.  Moreover, many of the lineages present in the Paleocene
are extinct and thus not available for molecular sampling.

Another limitation is that many (or most) Mesozoic mammals are known from
dental remains only.  This will diminish the likelihood that characters
bringing a particular fossil closer to rodents or elephants will be
observed.  The mammal fossils coming from the Mongolian Cretaceous are
important in large measure because they include much more than the teeth,
and hence increase the chances that we can resolve the relationships among
basal placentals.


Christopher Brochu, Ph.D.

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566