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the case with the most prominent fossil families that we know did
not survive the boundary, including the four that are the focus of
this chapter: the ammonites, plants, forams, and dinosaurs. Were any
of them already well on the way out long before the K-T meteorite
made its appearance?
PREDICTION
2:
Except where reworking has occurred, species that
became extinct at the
K-T
boundary will not be found above the
iridium horizon.
The Alvarez theory maintains that the primary lethal effects of the
impact were immediate and that the secondary ones lasted for at
most a few hundred or a few thousand years. Since on a geologic
time scale these are instantaneous, the time of impact as located by
the iridium horizon and the time of the mass extinction are the
same. In effect, this prediction holds that the mass extinction, the
iridium horizon, the clay layer, and the K-T boundary all are syn-
chronous. Fossils of the major groups that became extinct at the
K-T boundary, such as the ammonites and dinosaurs, or that experi-
enced a major species turnover then, such as the foraminifera and
plants, will not be found above the iridium level (unless they
were
brought there by bioturbation or reworking).
But suppose that a few species thought to have gone extinct at
the boundary were to turn up above the iridium layer, in confirmed
Tertiary rocks: Would that falsify this prediction? In fact, dinosaur
fossils have been claimed from Tertiary rocks in China and Montana,
though the claim naturally depends on exactly where scientists
placed the K-T boundary in each locale and on the assumption that
the fossils there are in their original geological sites. We know from
their absence in the subsequent fossil record that dinosaur survival
into the Tertiary would at best have been rare and temporary. The
finding that a few species made it through, only to become extinct a
short while into the Tertiary, would have no bearing on whether
impact caused the mass extinction and would be nearly immaterial
to broad earth history. If, however, the main extinction horizon for
taxa (a catchall name for an unspecified taxonomic group: species,
genus, family, order, etc.) that have always been regarded as having
gone extinct at the K-T boundary—say the ammonites and dinosaurs
and most plants and forams—were actually found to lie above the
iridium, the case for impact-induced, large-scale extinction would be
weakened. If all were consistently found above the iridium, this pre-
diction would have failed and this half of the theory would be un-
done. We would know that a giant impact occurred at the end of the
