Geoscience Reference
In-Depth Information
far. Only a short distance away, at Hendaye, they showed up in abun-
dance. In 1994, Ward summed up: "My decade-long study ... in
Spain and France ultimately showed that the ammonites had
remained abundant and diverse right up until the end of the era; the
last ammonites . . . were recovered just beneath K-T boundary clay
layers."
1
2
Ward's latest approach to the problem of ammonite extinction
is to apply statistics. Considering the difficulties inherent in trying to
locate the true extinction horizon of a species, using statistics not
only makes sense, it is essential. At best, for a given horizon in time,
scientists have access to only a tiny fraction of the geologic record.
Combine that with the Signor-Lipps effect, and you can see that the
chances are vanishingly small that the last surviving individual of
any fossil species—say, the last ammonite to have lived and be fos-
silized—will ever be found. The time of the true, last survivor will
always lie above the horizon of the highest specimen recovered. One
way around this difficulty is to collect large numbers of samples and
then to apply statistical techniques, as Ward and Charles Marshall
of UCLA have done for the ammonites at Zumaya.
1
3
This allowed
them to define a range over which a given ammonite species most
probably became extinct. They found that a few ammonite species
disappear all the time in a kind of background extinction, and that,
prior to the K-T boundary, a drop in sea level apparently had killed
off a few others, though this happened gradually. They concluded
that 50 percent of the Zumaya ammonite species had undergone a
sudden extinction right at the boundary.
What lessons can we learn from the story of the Biscay ammo-
nites? Most important, and contrary to the initial impression of
Ward and others, half the ammonite species lived right up to the
K-T boundary, when they suffered an extinction that was both sud-
den and catastrophic, corroborating prediction 1. Ammonites are
not found above the iridium, corroborating prediction 2. Although
their numbers waxed and waned throughout the Cretaceous, as
they had done for the preceding 330 million years, as nearly as we
can tell given the vicissitudes of the fossil record, the final demise of
the ammonites is fully consistent with the Alvarez theory.
PLANTS
The accumulation of knowledge has meant that scientists today
must practice in finer and finer subspecializations: One is not a pale-
ontologist, one is an invertebrate paleontologist specializing in am-
monites; or one is a palynologist, an expert not just in fossil plants,
