Darwin recognized, as noted in the epigraph that opens this chapter, one insuperable problem with interpreting the fossil evidence: Erosion has caused the geologic record to be riddled with missing rock units. As shown in Figure 17, a missing unit can lead to the false conclusion that a fossil species became extinct earlier and more suddenly than it actually did. Therefore, unless there is independent evidence that a geologic section contains no gaps, an apparently sudden extinction cannot be taken at face value. This works against the
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 representing 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 catastrophic and gradual. In this so-called stepwise extinction, species appeared to disappear in sets, one after the other, as the K-T boundary 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 impacts 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.
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