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Placental takeover: final comment.

I agree with posts making the point that we just don't have enough
data to choose between various hypotheses for marsupial extinctions.  I've
just re-read: Lillegraven, J.A., and J.L. Eberle. 1999. Vertebrate Faunal
Changes Through Lancian and Puercan Time in Southern Wyoming. _Journal of
Paleontology. 73. Number 4 pp. 691-709.  For an amateur such as myself,
the discussion of stratigraphy is illuminating; I cannot recommend it
strongly enough!  They have spent years in the field examining the very
question we have been throwing around...so, because it bears directly on
the list's discussion, I'll quote the relevant section:
"But did the advent of condylarths within the North American western
interior lead to the decimation of the considerable known diversity of
Lancian native marsupials through direct competition?  That scenario was
favored by Archbald...and indeed may have been the case.  He stated: 'Of
special interest is that the archaic ungulate invaders had dentitions very
similar to their marsupial contemporaries and presumably ate similar
things. It seems more than coincidence that marsupials did well in NA for
about 20 million years only to almost disappear with the arrival of the
ungulate clade'...(However) the true diversity either of condylarths or
marsupials remains unknown precisely at the boundary, and therefore one
cannot evaluate the extensiveness of competition, if any, among species
of the two groups at that time."

They later make a plea: "the most pressing need today is for new and
better paleontological _data_, derived from as yet unknown parts of the
fossil record..."

And here I want to voice my discontent with the scientific complacency of
some on this list.  To claim one knows what happened 65 mya--i.e., that
placentals and marsupials were strictly casualties of an extinction
"event" and that ecological forces were irrelevant--is to claim something
that cannot be known.  Physical scientists may be forgiven for accepting
the revelation of preeminence of their field of study.  For ecologists,
synergistic effects and interspecific relationships must be
considered.  What about paleontologists?  I suppose, sitting as they
do between the two fields, they must arbitrate if they can.  This is why
they are always crying out for new data!!!!  Data are good.  But when you
say you already know what caused species distribution at this time, you
are ineffect saying we don't need more data.  I've been reading Flannery's
"The Eternal Frontier" with its swallowing whole of the bolide
hypothesis as an explanation for everything.  The phrase "would
have" gives away the _just so_ nature of of this book, as in: tsunamis
would have...blah, blah, and firestorms would have...blah, blah.  I mean,
it was just yesterday that everyone was warning everyone else not to make
up stories without having direct evidence.  So how is it that
"adaptationists", "ultra-Darwinists", whatever, must jump through hoops to
suggest, for example, that the clitoris might be valuable to a woman, and,
at the same time, any "scientist" can ramble forever about whatever
killing agent they please, as long as it is recognized as a random
force.  Again, I'm not saying that I--or anybody else--knows what happened
back then.  I am saying we don't know.  Further, I have cited scientists
for whom this question is a life-long concern.
If _they_ don't know, it's valid to suggest _we_ don't know.  And
so, the marsupial extinction may not yet be counted among those caused by
an extraterrestrial event.