Geoscience Reference
In-Depth Information
their remains was shrinking, causing us to conclude incorrectly that
their true diversity had fallen. Of course, a direct link between the
two might have existed: Whatever caused fewer deposits might also
have caused ammonite diversity to decline, but that we cannot
know. Signor and Lipps concluded that "diversity data cannot be
taken at face value": The availability of rocks to be sampled can con-
trol the apparent abundance of fossils.
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This first effect has relevance to the diversity of dinosaurs. Sev-
eral scientists have noted that there are fewer dinosaur species found
in the last stage of the Cretaceous, called the Maastrichtian, than in
the immediately older stage. (Each section of the geologic column is
named either for a place where it was first recognized, or where it is
thought to be particularly well exposed. In this case, the "type local-
ity" is the Dutch town of Maastricht near the Belgian/German bor-
der.) This decline in species collected suggests that the dinosaurs
were already on the wane by the middle and late Cretaceous, leaving
nothing for meteorite impact but a possible coup de grace. But dino-
saur specialist Dale Russell pointed out that since the Maastrichtian
lasted for only about half as long as the Campanian, we would natu-
rally expect it to produce only about half as many species.
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4
This
conclusion has been disputed, but the ammonite and dinosaur exam-
ples remind us that apparent changes in diversity may simply be arti-
facts of differing sample sizes. To the extent they are, we underesti-
mate the true range of species and conclude they went extinct before
they actually did.
In order to understand the second of the two effects pointed out
by Signor and Lipps, imagine that you have to approximate the
Canada-United States boundary using one of two methods: (1) by
locating the houses of the northernmost residing United States citi-
zens, or (2) by locating the houses of the northernmost residing
members of Congress.
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Obviously, using the abodes of citizens
would give the more accurate result. Using the homes of the more
rare congressional representatives would cause you to place the
boundary further south than it really is. In the same way, the more
rare a fossil species, the less likely we are to find its true geologic level
of extinction. This gives rise to the "Signor-Lipps effect," a concept
with which every paleontologist studying changes in diversity over
time henceforth must wrestle.
Figure 19 represents a hypothetical cross section down through a
formation that contains three fossil species, each marked by a dif-
ferent geometric symbol. The diagram assumes that each species
became extinct at the same time, represented by the top of the draw-
