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miss all specimens of the rarest species (fewer than nine are present
to start with) and conclude that it had become extinct even before
this geologic section formed. The apparent level of disappearance of
the other two species would move down in the section, causing you
to place each of their extinctions at a level even further below their
actual occurrence.
Thus the rarer a species and the less perfect the sampling, the
earlier and more gradual its extinction appears. We see, not reality,
but the false, gradual extinction of the Signor-Lipps effect. If each of
the three symbols stands for a dinosaur species and the time period
represented is the late Cretaceous, we would conclude that dinosaur
diversity gradually declined and therefore that they were already
doomed—no meteorite impact is required. But we would be wrong,
victims of the Signor-Lipps effect.
This has been a thought experiment. What about a real one? A
clever geologist named Keith Meldahl went, not back in time, but to
a modern tidal flat in Mexico, where the muds are full of shelled
marine species.
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He imagined that an extinction suddenly occurred
the day he visited the tidal flat, and that it was then preserved and
sampled by some paleontologist far in the future. Meldahl drilled
eight cores into the muds to a depth of about 70 cm and studied the
extracted sediment centimeter by centimeter, making a careful record
of each species and the highest point at which it was found. (Remem-
ber that all the species are alive today.) He located 45 different spe-
cies in all; their positions in the cores are shown in Figure 20.
Even though this imaginary "extinction" was perfectly abrupt—
far more so than any real extinction including that produced by
impact—Meldahl actually observed the false, gradual pattern pre-
dicted by the Signor-Lipps effect. Of the 45 species that were pres-
ent at the tidal flat, 35 appeared to go extinct below the surface,
and this happened even though the cores were crammed full of
"fossils-in-the-making," averaging almost 40 percent shell material
by weight. In other words, the Signor-Lipps effect distorted the
record even for relatively common species. Had the effect been for-
gotten with time, the paleontologist of the future, unaware, would
naturally conclude that three out of four species had gone extinct
gradually.
Sampling problems can never be entirely eliminated, but they
lessen as more samples are collected, which is exactly what paleon-
tologists have been doing since the Signor-Lipps effect was described.
But no matter how exhaustive and exhausting the collecting, the
inexorable mathematics of sampling means that some effect will
always remain. "Gradual extinction patterns prior to a mass extinc-
