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rocks below, then the mammals and the channel deposits in which
they are found are Tertiary, not Cretaceous, and the early replace-
ment is an illusion. Here is another difficult puzzle for paleontolo-
gists and sedimentary rock specialists.
SAMPLING EFFECTS
At the first Snowbird conference in 1981, paleontologists Phil Signor
and Jere Lipps presented what has proven to be one of the most
important papers in modern paleontology.
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Like some others that
have had such a result, their paper was short and simple. The authors
showed that sampling can have two separate but related effects on
paleontological evidence, both of which make it harder to draw firm
conclusions.
The first is illustrated by Figure 18, adapted from their paper,
which shows how the diversity of ammonites, the beautifully coiled
and chambered marine fossils that grace natural history museums,
waxed and waned during the Mesozoic era, which includes the Tri-
assic, Jurassic, and Cretaceous periods; and how the extent of sedi-
mentary rocks deposited during the Mesozoic also varied (the Meso-
zoic lasted from 250 million to 65 million years ago). Though the
match is not perfect, the chart shows that the more rock exposed,
the more diverse the ammonites appear to be and the less rock
exposed, the less diverse. For example, ammonite diversity appears
to have declined from the middle to the end of the Cretaceous. But
so does the amount of rock deposited. Thus the apparent decline in
diversity can be explained entirely by the decreasing amount of rock
available to be sampled. Ammonites might have been thriving when,
by coincidence, the amount of rock being deposited and preserving
FIGURE 18
The diversity of
ammonite genera and the area of
sedimentary rock across the
Mesozoic era. Note that the two
roughly correspond. [After Signor
and Lipps.
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