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widespread? Raup answered this question at Snowbird I, concluding
that: "Modern biogeography is too robust for mass extinction to
result from annihilation of life in a single region. ... A global or
near global crisis or environmental deterioration is required."
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Let me rephrase Raup's two prerequisites:
I
. For a species that lives over a wide area to be driven to
extinction, the cause of death not only has to be powerful, it
must also be outside the experience of the species—not of
individuals but species. This means that the cause must be so
rare as to appear no more often than once every few hundred
thousand or few million years.
2. The extinction of over 50 percent of all living species—a mass
extinction—requires killing on a global scale; mass death in a
region or two will not do the job.
Which of the theories summarized by Benton are rapid enough
in their action to be beyond the reach of natural selection (or migra-
tion) and are also global in their reach? Medical problems, competi-
tion, and floral changes are the stuff of natural selection; they also
tend to be regional. Climatic, atmospheric, and oceanic changes are
widespread and therefore appealing, but they tend to occur on geo-
logic time scales. Rather than causing mass extinctions, these grad-
ual changes would give organisms opportunity to evolve or to
migrate in response.
Changes in sea level are worth a special look, for they are the
most-cited cause of dinosaur extinction. Consider the most recent
dramatic change in sea level, the rise that occurred when the last
glacial ice melted. The earth has a fixed amount of water during any
one period of geologic time; the more that is locked up in ice, the
less that is available to fill the oceans. Thus as glaciers form, sea level
drops; when glaciers melt, sea level rises. During the Pleistocene Ice
Ages, which ended (or at least paused) 15,000 years ago, sea level
first fell and then, when much of the ice melted, rose by hundreds
of meters. Although many large mammals became extinct, no mass
extinction resulted. Indeed, sea level has risen and fallen throughout
the history of the earth as glaciers have waxed and waned, sea floors
have spread, continents have collided, and oceans have opened and
closed. The record is shown by the so-called Vail curve of sea level,
developed by researchers at Exxon (Figure 22).
Some of the major shifts in sea level occurred near in time to
major geologic boundaries and extinctions, but many did not. The
much-touted change in sea level during the Cretaceous actually
