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Re: Extinction (was: Religion)
Here we go again. I hope everybody is enjoying this (but I doubt
it!). This is in response to Stan Friesen's latest missive.
Stepped extinctions: I thought we had gone over the dinosaur evidence
enough by now, but I guess not. I've referenced the Sheehan et al.
(1991) paper twice and heard no actual discussion of the data, just
repetition of the claim that dinosaurs started to go extinct before
the K-T, as if this was some kind of widely-known fact (even Ralph
Chapman said something along these lines!!). My own experience with
the Cenozoic mammal record, which is MUCH better understood than the
dinosaur record and involves many more valid taxa, is that
paleontologists routinely assume that little squiggles in their
diversity curves correspond to "step-wise extinction events" when
these squiggles are due to artifacts created by sampling or by the
arbitrary, coarse time scales that are used or by the failure to test
null hypotheses explicitly.
As for brachiopods, I have no idea what data are being discussed
here, but since brachiopods took a huge hit at the Permo-Triassic
boundary and never recovered (see the diversity data in "Phanerozoic
Diversity Patterns" [1985: J. Valentine, ed.], for example), I
suspect Stan is confusing something. Certainly molluscs (clams,
snails, cephalopods) show a huge and decidedly non-stepwise
extinction at the boundary.
"Harmless" impact sites: I know of no Phanerozoic impact crater as
large as the Chicxulub crater. Can anyone cite a counter-example?
Based on Raup (1992: Paleobiology 18,80), the closest runner up is
the Manicouagan crater, the age of which is uncertain and could well
fall at the end-Triassic mass extinction (one of the "big five").
Furthermore, Manicouagan is "merely" 100 km across, much smaller than
the Chicxulub structure, which is at least 150 and possibly 300 km
across (I haven't found the reference but I remember these figures
pretty clearly; can someone find it for us?). According to Raup the
runners-up to Manicouagan are merely 55, 52, 45, and 32 km across,
nowhere near the scale of Chicxulub.
Alternate mechanisms: Ice ages? This must be an error: the late
Cretaceous is a notorious "greenhouse" interval. There is good
paleontological evidence that there were no "ice ages" anywhere in
the world between the Triassic and the earliest Oligocene! What is
the evidence for ocean anoxia? I have seen some similar ideas kicked
around for the Permo-Triassic but know of no such evidence for the
K-T. How does anoxia account for terrestrial vertebrates, plants, and
planktonic forms like marine reptiles, forams, etc.? I guess it
doesn't, but this would be no surprise the essence of a
"multi-causal" model is to find a separate argument for every piece
of evidence. This strikes me as being ad hoc.
I agree that in principle alternatives to the impact model for the
mass extinction could be developed. In practice, I'm not aware of a
well-articulated, published development of such arguments. The
Keller/Officer/etc. camp are trying to show that there was no mass
extinction, not that some alternate model can explain the event. The
anti-extinction camp is just plain wrong (just look at THEIR data, it
shows a sudden mass extinction very clearly - latest issue of
Palaios, for example). I'm sorry I have to be so blunt, but I'm sure
any of you would agree if you looked at the data, if you don't
already (I'm not sure Dr. Schwimmer would agree).
Tsunami deposits: I don't have a full reference on hand, but I can
suggest looking at the Kerr article in Science (11 March 1994, p.
1371-72). A field trip including pro- and anti-impact
sedimentologists plus several "more or less disinterested experts,
among them four ex-presidents of the Society for Sedimentary Geology"
examined a "tsunami" deposit in northeast Mexico, "And the four
ex-presidents agreed" that there was evidence "requiring the whole
deposit to form within days or months... the event most likely to be
energetic enough to lay down the sand bed beneath several hundred
meters of water, concluded the sedimentologists, seems to be an
impact-induced tsunami." Kerr has been following the impact debate
for more than a decade now. I've met the guy myself (I even had
dinner with him at a meeting) and he strikes me as utterly
reasonable, intelligent, informed, and impartial, and furthermore,
deeply unimpressed with the anti-impact literature.
The contradiction between the volcanic theory and the "extreme"
gradual theory is that the former is trying to explain a mass
extinction and the latter denies that one ever took place. There is,
of course, an important distinction between the gradualist and
"semi-gradualist" positions, as Friesen indicates; the latter believe
that the extinctions did occur but took about 10^5 - 10^6 years, or
more. My own view is that the evidence is clearly in favor of much
shorter time scales, 10^4 years or more, but we should argue about
this by looking to particular data sets.
My point about positive evidence and arguments is that the
anti-impact/catastrophe camp has more and more focused its efforts on
attacking particular evidence for the impact or for a rapid
extinction. Of course there could be some who argue in a positive way
for an alternative, volcanism-driven model - again, where are the
I'm amazed that the "2-3 m" issue won't go away. Why are you so
willing to ignore sampling effects and variation in data?
Furthermore, what is the EVIDENCE for this "gap"? It consists
entirely of a claim made by proponents of gradual mass extinction
(e.g., Archibald, Clemens, Williams) that such a gap exists. Even if
there WAS real, statistically meaningful evidence for this "gap," it
would show nothing apart from the existence of variation in
taphonomic regimes, which no serious paleontologist would question.
I'm sorry, but I'm getting frustrated with comments like "Williams
results are... pretty convincing... The existence of a *large* drop
in abundance of dinosaur bones (and teeth) in the last few meters of
the Hell Creek Formation is now a thoroughly established fact." THERE
ARE NO DATA IN WILLIAMS, much less a statistical analysis! Sheehan et
al.'s data showed NO CHANGE IN ABUNDANCE and involved a proper
statistical analysis. Take another look at the paper. What would it
now take to "prove" a rapid extinction of dinosaurs? A home video of
the K-T events as they occurred? An eyewitness account directly
quoted from the lips of Jesus Christ? Doesn't anyone realize that
arguing over "2-3 m" is the paleontological equivalent of debating
the last 2-3 seconds of an NBA playoff? The figure of "100,000 to
250,000 years" is simply incorrect. Rogers et al. (1993: Can J Earth
Sci 30,1066) compute an average rate of sedimentation for the Two
Medicine Formation of 7.0 cm per THOUSAND years. The supposed 2-3 gap
would translate, therefore, to no more than 43,000 years, but this is
a very high overestimate because 1) the Two Medicine, despite being
nearby the Hell Creek and not much older (Campanian), was deposited
in a proximal foreland basin - sedimentation rates in the Hell Creek
were probably half of this (Rogers pers. comm.); and 2) this figure
completely ignores variation in sedimentation rate and is an average
based on 372 m (!). Interpolating down to a stratigraphic interval
two orders of magnitude smaller than this MUST result in a massive
underestimate of the rate. Therefore, the top 3 m probably represent
a few thousand years or less. An NBA game last 48 minutes = 2880
seconds; 3 seconds is 0.1% of this; the Maastrichtian lasted about 10
million years, and 0.1% of that is 10,000 years.
Chance alone: Sorry, I'm really getting steamed at this point. Take a
look at the Raup paper I just cited. As I said, there simply ARE no
other craters on the scale of Chicxulub. I am not going to argue
particulars about the evidence for Chicxulub being an impact crater
because I feel that this has been dealt with quite adequately by bona
fide geologists (I'm a biologist). This "one coincidental bolide
impact" occurred within 100,000 years of the boundary - actually, the
argon dates for the crater and various boundary sections are all
within error of each other, but let's grant a tiny offset based on
the supposed (but spurious) gap in the record, and on a doubled
figure for the maximal duration of this gap (see above). The
Phanerozoic is about 535 m.y. long; there has been exactly one impact
on the scale of Chicxulub; that amounts to the largest known crater
on the planet falling in a window lasting 1/5,350th of the
Phanerozoic and corresponding with one of the five biggest
extinctions in that interval!
I give up. If you don't find this convincing, there's no point in
continuing a flame war.
Causal theories: "all known instances of a particular effect have
been shown to be correlated with the postulated cause." This is not
terribly generous - once again, I feel like I am being asked for a
videotape of the K-T. I suppose I could ask for the same criterion to
be applied to the volcanism story, but why bother?
Non-selectivity: Check the paper by Raup and Jablonski (1993: Science
260,971), which I believe I already have cited. There is no
geographic pattern to the extinction of molluscs, NOR is there any
relationship between various ecological characteristics (body size,
infaunal/epifaunal, etc.) and extinction probabilities. This DOES
match with a catastrophic event because only a catastrophic event
could so thoroughly change "the rules of the game"; a drawn-out
volcanic episode doesn't.
I don't have a reference for the Texas "tsunami" beds. Can somebody
else supply us with one?
Lancian mammals: The "lowest" channels certainly do have Lancian
mammals in them, namely, reworked Lancian mammals. This, like the
dinosaurs, has been gone over endlessly by the VP'ers and all I can
say at this point is that I have never in my life seen such an
unambiguous case of a single pattern (mass extinction) being obscured
in a very minor way (a few meters) by sampling effects (reworking and
Friesen throws in a few more choice quotes, such as claiming that the
extinctions started "100,000 years" before the boundary (what, based
on the gap again?) and that 10,000 years of ecological havoc aren't
enough to cause a mass extinction (?!?) and that Williams presents a
statistical analysis of some kind (he simply does not) and that mass
extinctions are somehow "normal" (this has been disproved on
statistical grounds; see numerous papers by Raup, Sepkoski,
Jablonski, assorted students), but I'm too exhausted at this point to
argue about it. Sorry about stuffing more mailboxes with this junk -
I'm going to try hard to just give up and let the other guys have the