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have not figured out how the ice ages spared so many species, though
perhaps the ice advanced slowly enough to allow some to migrate to
warmer climes, while others may have been preadapted for cold or
survived in ecological refuges. Some think that the arrival of skilled
aboriginal hunters on a virginal North American continent has much
to do with the Pleistocene mammal extinction, but many disagree,
pointing to the millennia of coexistence of humans and large mam-
mals in Africa.
If we accept Raup's conclusion that species living over a wide
area can be killed off only by stresses with which they are unfamil-
iar, and that those stresses must occur too rapidly for migration or
adaptation, it follows that rare, sudden, and global catastrophes must
also exist—otherwise there is no way to explain the several mass
extinctions that mark the geologic record. This is worth repeating:
To accept that global mass extinctions have occurred is also to
accept that global catastrophes have occurred, a conclusion that is
the antithesis of strict uniformitarianism.
The five largest mass extinctions in terms of percent of species
killed—the Big Five—are shown in Table 2. Note that the record for
intensity is held not by the K-T but by the end-Permian extinction.
Until recently, paleontologists believed that extinction came in
two forms: a regular, low-level, background extinction, and the much
more destructive mass extinctions. Over the last few years, they have
had the benefit of databases meticulously compiled by such paleon-
tologists as John Sepkoski, also of the University of Chicago. He and
Raup studied the fossil record of over 17,000 extinct genera of
marine animals, and several times that many species. Their database
shows that the mean duration of a genus is about 20 million years
and that of a species is about 4 million years.
T ABLE 2
The Big Five Mass Extinctions
Age (million years
Estimated species
Extinction episode
before present)
extinction %
Cretaceous-Tertiary
65
70
76
Triassic-Jurassic
-202
Permian-Triassic
-250
96
Late Devonian
367
82
Ordovician-Silurian
-438
85
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