Geoscience Reference
In-Depth Information
for the all-important dinosaurs, the question had already been an-
swered: Evidence that they had collected over more than a century,
and especially in the Montana dinosaur beds in the 1970s and early
1980s, they interpreted to show that the dinosaurs had gone extinct
gradually, not instantaneously. This point of view infuriated the al-
ready irascible Luis Alvarez: "I simply do not understand why some
paleontologists—who are really the people that told us all about the
extinctions and without whose efforts we would never have seen any
dinosaurs in museums—now seem to deny that there ever was a cat-
astrophic extinction. When we come along and say, 'Here is how we
think the extinction took place,' some of them say, 'What extinction?
We don't think there was any sudden extinction at all. The dinosaurs
just died away for reasons unconnected with your asteroid.'"
2
R
ETURN
OF THE
P
TERODACTYL?
The founders of geology and biology were not much interested in
extinction. Lyell thought that extinction was so impermanent that
the vanished pterodactyl might return to flit through a forest once
again primeval. Darwin thought that gradual change was the essence
of natural selection: "Species and groups of species gradually dis-
appear, one after another, first from one spot, then from another,
and finally from the world."
3
"Extinction and natural selection . . .
go hand in hand."
4
He did occasionally make exceptions: "In some
cases . . . the extermination of whole groups of beings, as of the am-
monites towards the close of the secondary period, has been wonder-
fully sudden."
5
(Scientists of Darwin's day thought there were four
main geologic periods: primary, secondary, tertiary, and quaternary.)
Extinction was simply the natural end of every species and there-
fore unremarkable. Biologists were much more interested in specia-
tion, the process by which an evolutionary lineage divides, giving rise
to two species where only one existed. Reflecting the increased inter-
est in extinction since the Alvarez theory appeared, paleontologist
David Raup of the University of Chicago and the Field Museum has
written a fine topic on the subject,
Extinction: Bad Genes or Bad
Luck?
6
Raup's most fundamental conclusion about mass extinction,
drawn from a lifetime of study, is that, because species typically are
well adapted to the normal vicissitudes of life, "for geographically
widespread species, extinction is likely only if the killing stress is one
so rare as to be beyond the experience of the species, and thus out-
side the reach of natural selection."
7
His conclusion is key to under-
standing the role of meteorite impact in earth history.
