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yet more K-T drivel
First I will comment on Schwimmer and then Friesen.
Tornillo: I haven't seen the Lehman article (nor do I know what the
reference is) so I will wait to comment. However, I want to point out
that I wasn't claiming there was an uncomformity at the boundary; I
just said there was a 35 m interval devoid of terrestrial vertebrates
that was bracketed an unambiguous Paleocene mammal fauna above and a
Duration of the "2 m gap": I will grant that it is dangerous to
interpolate sedimentation rates based on the known age of an entire
formation. However, the point here is that the best estimate for the
interval is 40,000 years and not 100,000 - 250,000 years (the figure
I contested in the first place), unless hiatuses in that interval are
substantially greater than average for the formation and similar
formations elsewhere. If the 2 m represent a single depostitional
package, as is likely, they easily could represent a depositional
event on the scale of a few thousand years.
I did earlier discuss this with Ray Rogers, who has published
extensively on the stratigraphy, sedimentology, and taphonomy of the
Two Medicine and Judith River formations. He urged me to make the
above argument that the 2 m zone could easily represent a very short
Polls: I agree that science isn't a democracy. However, my point was
that the anti-impact camp was already a minority by 1984 (based on
Raup's book) and has now become a very small one (based on McLean's
data), which agrees with my essentially sociological model for the
acceptance of new theories. Schwimmer has a good point about VP'ers:
I think in general there has been a startling lack of agreement
between geologists and marine paleontologists, on the one hand, and
terrestrial VP'ers, on the other. Because the gradualist VP'er
arguments center on things like the "2 m gap" and the now-falsified
claim for gradual dinosaur extinction through a substantial stretch
of the Maastricthian, this looks to me like a strange sociological
artifact more than anything. I suspect if you took a poll now the
VP'ers would be much more inclined to accept the idea that an impact
did occur. The Rapid City meeting was indeed in 1985. I wasn't there
but I have attended all eight meetings since then, and I have the
impression that anti-impact feelings are no longer strong in the SVP.
Manson: I think it was pretty clear back in the mid-80's that Manson
was just too small to cause a global catastrophe; that's what I meant
by a "decent" candidate crater. I realize that literature as recent
as 1992 (a paper by Anderson and Hartung I haven't seen yet) argue
the contrary. However, I'm not clear what Dr. Schwimmer means by
saying Manson was "subsequently discredited." I seem to recall that
the latest radioisotopic dates for the crater are still within error
of many other K-T boundary dates, but again I haven't seen a
reference yet (apparently Hartung et al. 1990 GSA Special Paper 247
reported dates of this kind).
Meyerhoff: I've dug through some literature and it's complicated. The
original Chicxulub paper (Hildebrand et al. 1991: Geology 19,867) did
address the available 1960's drillhole data and saw no contradiction
of the impact model in these data. Meyerhoff et al. simply repeat
data for a well (Y-6) that was published many years before and
figured in the Hildebrand paper. I'm not competent to judge their
geological arguments that the lower part of this well is
volcanogenic, but Hildebrand et al. don't think so and went over the
same data. I really think we've got a problem here that needs more
research. However, when it comes to the faunal data - the supposed
Late Cretaceous age for material within the crater - I think
Meyerhoff et al. have a very weak argument. They describe exactly one
level in one core as including "interbedded limestone with Late
Cretaceous microfossils, volcanic breccias, and andesite." That's ALL
they say about the supposed Cretaceous fauna, which comes from a
rather deep level in the drillhole (1590 m out of about 1640).
Hildebrand et al. discuss other fossil material from the same
drillhole but a higher level (1000 m) that was said to be Late
Cretaceous in the original writeup of the drillhole. However, Keller
and Sliter are then cited as having reidentified the fauna as late
Paleocene! This indicates to me that no conclusions should be drawn
from the supposed Late Cretaceous fauna until it is restudied. As for
radioisotopic dates, none greater than the age of the K-T have been
reported despite multiple efforts, and several of the available dates
are indistinguishable from the age of the K-T (e.g., Swisher et al.
1992: Science 257,954). These dates are higher than the 1590 m fauna,
of course, but my point is that Meyerhoff et al.'s argument is
completely equivocal from a paleontological point of view.
Regressions: My point was that RELATIVELY equally severe and rapid
regressions occurred several times during the Cenozoic, not that
North America had a large inland sea for a zillion years that
repeatedly regressed (it did not). I know full well about the age and
extent of the Cannonball Sea. I see no reason why a regression
eliminating an inland sea should have any greater or lesser effect on
TERRESTRIAL biotas than a regression of comparable magnitude coming
at a time when there was no inland sea. For example, I think we can
agree that the regression at the Eocene-Oligocene boundary (or in the
earliest Oligocene, depending on who you talk to) was just as or more
extreme than the K-T event and had virtually no impact on North
American mammal faunas, which are quite abundantly represented in the
record in that interval (the White River Group straddles the event).
This actually does raise an interesting point - there is a huge (50%
at the genus level) mass extinction of mammals in Europe at the
Eocene-Oligocene boundary. I myself am convinced this is a
biogeographic effect - a bunch of Asian mammals crossed a short-lived
land bridge that came up at this time and all hell broke loose.
Nothing similar happened in Asia and North America because these
faunas were already full-scale continental faunas (Europe was an
island archipelago) that were "used" to inter-continental migrants.
If this story is right, and if the K-T regression opened up land
bridges that resulted in a lot of migration, why is there basically
no important migration of mammals into North America at that time?
The early Paleocene mammal fauna of North America can mostly be
Friesen: Most of this is agreeable.
Periodicity: I really think this is the wrong place to get into a
debate about periodicity. For one thing, it has basically nothing to
do with dinosaurs except insofar as the K-T fits into the periodic
model. However, my own feeling about this is that the statistical
data were equivocal. I'm surprised that Friesen feels so strongly
that the statistics were flawed, and that a genus-level analysis
wouldn't help matters, but we can discuss that elsewhere. I agree
whole-heartedly that an improved time scale would make a huge
difference; getting the time scale straight for my North American
mammal data has made a huge difference in my ability to get
statistically meaningful results.
Tsunamis: I'm not sure we disagree about anything here. I agree that
the Deccan Traps and the regressions and so on may have had a
negative impact on Maastrichtian faunas and may have set the world up
for a "coup de grace." This requires more research.
Faunal lists: This is just a misunderstanding. I wasn't talking about
the "2 m gap" in this passage, I was talking about the reliability of
the apparently unpublished faunal data supporting a Cretaceous age
for the Y-6 drillhole (see above).
Plants: My point was that plants in North America show a severe and
rapid extinction at the boundary. As far as I know there is some
evidence that the extinction wasn't as bad elsewhere, but I seriously
doubt that any study as large as Johnson's for a good K-T section has
been performed elsewhere in the world. It actually HELPS the impact
model for extinction if plants took a harder hit in North America
than, say, New Zealand.
Mammals and regression: Mammals did go through a huge extinction at
the K-T boundary, as I'm sure Stan knows. For those in doubt, take a
look at the Bug Creek data I distributed yesterday. Given that large
body size is correlated with low intrinsic population growth rates
and low population densities, it is a straightforward prediction of
ecological theory that very large animals will do very poorly during
major environmental perturbations of any kind. Therefore, I think the
mammals vs. dinosaur issue is of no help at all in determining just
what happened at the K-T. Archibald thinks otherwise, but he isn't
here to speak for himself (at least not as far as I know). Maybe
someone should ask him to join the list (what's his address?).