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the cause(s) of MASS extinction

With regard to the extinction of non-avian dinosaurs, I see recurring
patterns of thought that I would like to remark on. 

1)  "Dinosaurs were always fluctuating in diversity and were already in
decline at this time." - Before we can talk about what caused this mass
extinction, we have to AGREE that it was a mass extinction, which is to
say, we have to agree that it is something EXCEPTIONAL and not an
everymillienum, run-of-the-mill event.  Once we agree on this point,
presumably we can argue about whether this exceptional event has
exceptional CAUSE(S).

2) "Just because an impact and an extinction event occurred together
doesn't mean one caused the other." - I really don't see how this
contributes to a scientific discussion of this question.  We can say the
same about any question concerning dinosaurs.  If someone has live
dinosaurs to do controlled experiments on, please let me know.  Otherwise
all our inferences about causes will be based on correlations.  Give me an
alternative or give me a break.

3) "Lots of impacts occurred before without causing such extinctions.
Therefore impact cannot be the cause of this one." - This is reminiscent of
people who live in coastal areas, who experience several hurricanes and
lose their fear of them.  It takes much more than a powerful hurricane to
produce truly dramatic devastation.  The storm must be moving perpendicular
to the coast, have high translational velocity, strike at high tide, AND be
powerful.  With this combination of factors, a dramatic storm surge will
produce very effects far greater in intensity and coverage than any
hurricane winds no matter how powerful.  I could enumerate any number of
factors that would modify the effects of the impact of a given object.
Others have already done so.

4)  "Why should a global catastrophe kill all the dinosaurs, big and small,
and not all the birds, mammals, crocodilians, frogs, insects, spiders...?"
Tally up all the species that are known to have gone extinct in the last
100 years.  Look at their size distribution.  Notice anything?  When there
is a general environmental stress, large species are generally the first to
go.  Nevertheless, there is a stochastic component.  There is no magical
body size above which you are doomed and below which you are saved.

Dinosaurs were around for 180 million years.  They had seen many
environmental fluctuations, dramatic floristic changes, and the
introduction of major new animal taxa into their world.  They weathered all
of these, yet disappeared QUITE ABRUPTLY at the K/T boundary.  Pterosaurs
also disappeared, ABRUPTLY.  Lots of other things disappeared, AT THE SAME
TIME.  A large impact occurred, leaving an EXCEPTIONAL iridium signature,

It is for each of us to draw our own conclusions.  Like everything in
science, it is not about "proof."  It is about working hypotheses.  Saying
an impact caused the dinosaur extinction is no different than saying
cigarette smoking causes lung cancer in humans.  It does not mean it is the
only cause, nor is it an oversimplification of a complex process, nor is it
a certainty.  It is merely a working hypothesis that you may take or leave.
 We cannot do controlled experiments on humans to "resolve" the smoking
question, and we cannot do controlled experiments on dinosaurs to "resolve"
the extinction question.  Even if we could, it would not constitute
"proof," since ultimately we would be making inductive inferences and
taking measurements and deriving probabilities of type 1 and type 2 error
and doing all those little things that should remind us that science is not
about certainty.  

Best regards,