Geoscience Reference
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FIGURE I 9 The Signor-
Lipps effect. [After Michael
Williams. 16 ]
ing. Suppose that we sample every portion of this imaginary section
of rock, missing nothing. What do we conclude? That the rarest fos-
sil, shown by the large white circles, went extinct first, below its
actual extinction level as represented by the top of the drawing.
Because this species is rare, our chances of finding it anywhere, much
less at its true level of extinction, are small. The next rarest species,
shown by the small filled circles, appears to have become extinct a
little higher up. The most common, marked by the small diamonds, is
found right up to the "true" extinction boundary. Thus the "diversity"
of species in this formation over time—the number preserved at each
level in the rock—appears to have steadily decreased upward toward
the extinction boundary, where all three actually disappeared. We
conclude that extinction was gradual—no sudden disappearances
here—but we are dead wrong: All three species lived right up to the
boundary.
Now, factor in our inability to ever collect more than a fraction
of the fossils present in a rock formation, by imagining that our sam-
pling catches 10 percent of each species present. Mentally strike
out, at random, nine of ten of the symbols representing the three
fossil types and see what conclusion you would draw. You would
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