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RE: Mass Extinction Statistics

The real point of doing analysis of what a purely random process would do is
to put a bound on what needs to be explained.

It is not that actual process of extinction was unbiased - it probably was -
but it is valuable to see how much of the imbalance can be explained without
invoking special mechanisms.

Given a sufficiently high mortality rate, and given some grouping of taxa
into "families", it is possible to get extinction of those "families" and
survival of others even in a unbiased random mortality.   Exactly how many
can be calculated with the proper assumptions.  Doing the full calclulation
is quite involved and probably not worth debating over email.  However, the
key lesson is that in a severe (90% or above) mass extinction, a large
portion of the survival can be explained without recourse to special
mechanisms for differential survival.

I put families in quotes because there is a certain subjective nature in how
we analyze the results after the facts.  The most famous example is how one
groups dinosaurs and birds - many workers these days would say that
dinosaurs were NOT driven extinct at the K-T boundary, because they count
birds (cladistically) as dinosaurs.  

This subjective factor is non-trivial when it reaches the level of
individual taxa - many of the findings of gradual extinction prior to K-T
depend on subjective judgement as to which taxa disappear.  It is the
"lumper versus splitter" debate, but with higher stakes.  Double blind
studies where people analyze specimens without knowing where they lie with
respect to the K-T boundary have generally concluded that there is no
gradual extinction prior - earlier findings contained the unconsious bias of
people who expected to find a gradual extinction.

Judging whether the extinction was 90% or 99% or higher depends on both the
quality of the fossil record, and on these subjective determinations.  It is
therefore hard to measure accurately.

Raup's book on mass extinctions has many references on this.