Re: Pleistocene & K/T extinctions

[John Alroy <jack@homebrew.uchicago.edu>]

Whoops, accidental flame war. Okay, so my posts have been worded  
strongly. That's what happens when I write first draft - on a second  
draft I take out all the flame baiting. But here I'm going to shoot  
straight from the hip, so go ahead and correct me when I screw up.

1) Europe. I don't know the dates off the top of my head, but I seem  
to recall the Eurasian extinctions are substantially earlier than  
those in North America but occur when Homo sapiens is going through  
its big migration/population increase phase on that continent. The  
Australasian extinctions are another case of extinctions well before  
the North American extinctions, certainly much earlier than 10 kybp.  
The African extinctions are earlier still. I find it implausible that  
the same "climate" factors are responsible for the SELECTIVE  
extinction of completely unrelated mammals in Australasia and the  
rest of the world (marsupials vs. placentals). And then there's  
always New Zealand... and Madagascar... and Hawaii, or any number of  
other Pacific islands... and South America. The only common  
denominator in any of these cases is the presence of H. sapiens.

2) "no directed human invasion" in Europe? Technically speaking this  
may be right if Homo erectus is "human" or (here's a new one) if H.  
erectus not only was directly ancestral to H. sapiens, but gave rise  
to the latter through an agonizingly (shall we say "glacially"?)  
gradualistic process. Then maybe we could say there was no human  
"invasion" of Europe in the late Pleistocene. But I doubt we're going  
to find many paleoanthropologists at this point who really think H.  
erectus was the direct ancestor of H. sapiens; that H. sapiens was  
present in Europe much before 40 - 50 kybp; or that the appearance of  
H. sapiens (whatever caused it) was gradual. At some point there WAS  
a human invasion of Europe, and that invasion DID shortly precede a  
wave of large-mammal extinctions that is completely outside the range  
of mammalian extinction rates during previous intervals of the  
Pleistocene.

3) I'm sorry, but at this point it sure looks to me like "gradualists  
are practicing an obsolete science" (not my phrase, but it will do).  
Standard genetic models (in evolutionary debates) and standard  
geophysical processes (in geological debates) all predict large  
variation in rates of change. Therefore, it's actually strict  
gradualism and not punctuationalism that requires extraordinary  
processes. Just look at population genetic/microevolutionary theory,  
where a host of processes are known that can cause a punk eke pattern  
- stabilizing selection, developmental contraints, peripheral  
isolates speciation, founder effects, genetic drift, you name it.  
There is no sensible MICROevolutionary account of how a lineage could  
evolve slowly and constantly on geological time scales, unless the  
environment also changes slowly and constantly. Talk to any geologist  
and you will get a quick denial that any factor of the abiotic  
environment ever changes that way. The real question isn't WHETHER  
evolutionary or geological rates vary greatly, but exactly how and  
why.

4) U of C. Yes, I'm a recent graduate of the U of Chicago (actually,  
I just defended and I graduate in August). Does that make me some  
kind of a leper? Does being from the U of C make me a mindless  
"product" instead of a human being? No, my advisors (Raup and  
Jablonski) are not "rabid punctuationalists." They're not really  
rabid about anything in particular, being more interested in figuring  
out HOW to answer a question than in upholding dogma, and neither of  
them really gives a damn about the punk eke debate - nor do any of  
the other faculty or students here apart from a few geneticists (who  
will go unnamed) who seem to think there is nothing else going on in  
paleontology. The fact of the matter is that most professional  
paleontologists gave up worrying about punk eke ten years ago or  
more.  


5) "great deal of valid counterpoint to the impact hypothesis"? This  
may have rung true circa 1981 or even 1991, but at this point I think  
the anti-impact people are fooling themselves. It's perfectly  
reasonable that a lot of people in the scientific community who  
aren't in the thick of things might think there is a reasonable  
debate still going on. It's perfectly insane that certain people  
could continue to oppose the impact model after attending symposium  
after symposium where they are confronted directly with the evidence.  
I'm sorry, but I've SEEN the anti-impact people in action at a  
symposium during the last GSA meeting. Their capacity for doublethink  
is amazing - just check out the recent exchange of letters between  
Gerta Keller/Norm MacLeod and Richard Kerr in the 29 April issue of  
Science. I'm sorry if I'm offending anyone who is in the Keller camp,  
but speaking strictly from my own experience, I felt that talk after  
talk in that GSA symposium exemplified desperate special pleading  
against a) the impact and b) the rapid mass extinction. One of the  
least impressive talks was on the 1966 core data from the Chicxulub  
site, later published as the Meyerhoff et al. paper in Geology. The  
data consisted of a well log describing observations on the core as  
it was being drilled, and there was no documentation of the claim  
that microfossils from units above the presumed base of the crater  
are late Cretaceous. Furthermore, there was no discussion of the  
possibility that said fossils (if they are Cretaceous) could have  
been reworked from carbonates disturbed by the impact event. The  
Meyerhoff paper itself summarizes two independent papers presenting  
40Ar/39Ar analyses for samples from this same core that give dates  
indistinguishable from K-T dates on units from across North America,  
only to declare that these dates must be the result of alteration!

6) Stress below the K-T. I think everyone agrees that the world was a  
very interesting place in the late Maastrichtian. Sea levels changed  
rapidly, the reef-building rudist bivalves seem to have gone out well  
before the boundary, and so on. I wouldn't be surprised at all if the  
severity of the K-T event had a lot to do with the world being a  
lousy place for organisms at the time, as Dr. Schwimmer indicates  
(can I get some reprints on this? - I see from a quick check of J  
Paleont that Dr. Schwimmer's earlier papers are largely on Campanian  
fossils, so perhaps the data he mentions are still in preparation).  
However, I think we should remember that stresses like changes in  
ocean circulation patterns, onset or offset of glaciation, changes in  
global temperatures, sea level changes, etc. happen ALL THE TIME and  
fail to cause any detectable extinctions at all. I am speaking here  
from a Tertiary perspective, where there are plenty of test-cases to  
look at. I DO find the "world went to hell" hypothesis intriguing and  
appealing IN COMBINATION with the impact hypothesis, and a grand  
"world went to hell" hypothesis is currently popular among students  
of the Permo-Triassic mass extinction.

7) "Scientific Method." I'm amazed to find myself being accused of  
having no familiarity with the "Scientific Method" (whatever that is;  
I challenge you to find a philosopher of science who thinks there is  
any one "scientific method"), as if to say that siding with the  
majority of paleontologists and geologists who agree with the impact  
hypothesis disqualifies me as a scientist. I'm amazed to find myself  
accused of not having an "open mind" or of believing that any  
gradualist need "be a fool or ignorant." As I've outlined above, I do  
think that accepting gradualist interpretation could result from not  
following the literature too closely, but I do not think that mere  
foolishness would explain continuing to believe in that  
interpretation despite the published evidence against it. That's what  
I would call not having an "open mind" and rejecting the "Scientific  
Method" (again, whatever that is - if you want to argue Popper and  
Kuhn, maybe we should take this elsewhere).