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Another school of thought would suggest that none of these mechanisms
in themselves is enough to produce a major global mass extinction - that
each produces more minor ones that we see (and sometimes not) in the record.
Instead, the mass extinctions may only occur when a group of different
mechanisms manage to happen within a critical threshold period of time
and major taxa by the buckets full just finally say the hell with it
and croak. Certainly there's a thread of this in Bakker's disease plus
other factors theory for the K-T. That is not to say that the different
mechanisms are necessarily unrelated. An impact that occurs in the middle
of massive volanic activity (Deccan series?) may not only compound the
effect of both, but the former might make the latter go even stronger for
a while. Many of us have talked about such convergences of mechanisms before
and they fit right into gambling theory - which is a useful model for
diversity curves anyway. This does have the effect of giving lots of
evidence for all sorts of proponents of individual mechanisms and makes
for plenty of interesting discussion. By the way, most of us can manage
to do it quite civilly and have great times doing it - even at scientific
meetings. It doesn't always or all that often lead to shouting and
personal attacks - but any number is too often.

Ralph Chapman, NMNH