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



At 12:04 PM 3/20/99 -0500, John Bois wrote:

>Thsi

["This" being recent debates on the origination and radiation rates of
eutherian mammals and neornithine birds - TRH]

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

Yes.  :-)

Okay, to put it another way: eutherian mammals have about the best fossil
record of any group of terrestrial vertebrates.  However, as we've seen, we
have a really lousy record of their basal divergence.  If our "best case" is
this lousy, it is very difficult to see what the pattern (if there is only
one pattern) truly is.  We can make hypothetical arguments for one
particular pattern or another, but it is excedingly difficult to test at the
moment.

>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..."

Okay, it *IS* strictly true, since the modern = now, and the Cretaceous = 65
Ma plus.  What they might mean is that no definite representatives of any of
the modern crown group clades traditionally called "orders" of mammals are
found in the K.  ("Orders" have long been the taxon-of-choice in higher
mammalian studies, although many since McKenna onwards have been recognizing
that there are important clades above and below this arbitrary rank).

If they do mean that no members modern "orders" are present, they might be
correct.  Most of the K eutherians I can think of would be outside the crown
of any order (although obviously within the "crown group" of the next higher
taxon which unites that order with some other living relative).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661