Geoscience Reference
In-Depth Information
pro-impactors, who hope to use a pattern of sudden extinctions as
evidence of catastrophe; they must first establish that the section
they are studying is free of gaps.
Sometimes gaps are obvious (there's a spot in the Shoshone
Gorge in Wyoming where you can put your finger on one repre-
senting 2 billion years, for example). But especially in rocks formed
in the deep sea—limestones and mudstones, for example—gaps may
be nearly impossible to detect. Due to the intense study the K-T
part of the geologic column has received since the Alvarez discovery
broke, more gaps have been discovered there than had ever been
imagined.
Some geologists studying the K-T boundary found what they
thought was a pattern of extinction intermediate between cata-
strophic and gradual. In this so-called stepwise extinction, species
appeared to disappear in sets, one after the other, as the K-T bound-
ary was approached. What could explain stepwise extinction? Some
proposed that a cluster of meteorites had fallen one after another,
each wreaking its own bit of havoc and each causing an extinction.
A large comet might have broken into pieces that then went into
orbit, and these pieces might subsequently have fallen to earth one
after another, like the Shoemaker-Levy 9 "string-of-pearls," though
over a much longer period of time. If stretched a bit, this idea could
accommodate everyone: Impact had occurred, not once but several
times, satisfying the pro-impactors; the sequence of impacts gave
rise to a kind of gradual extinction, pleasing the paleontologists. Not
a bang, but something more than a whimper. And because volcan-
ism tends to occur in pulses over geologic periods of time, stepwise
extinctions also had a natural appeal for the volcanists. Everyone
could be happy] Distinguished scientists from various sides of the
debate came together to co-author papers proposing multiple im-
pacts and stepwise extinctions. Gaps can also produce a false step-
wise extinction pattern, however. As the evidence has accumulated
favoring a single impact, interest in the stepwise variation on the
original Alvarez theory has waned.
MIGRATION AND DISSOLUTION
Another process that makes interpreting the fossil record difficult is
that species, rather than going extinct, sometimes simply abandon an
area in favor of another one nearby. If today we were to search only
locally, we might mistakenly conclude that the species had become
extinct. Yet a broader search finds it on a nearby island or in an adja-
cent region. It did not expire, it moved. Migration causes us to under-
