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The database can also be used to help answer a question over which paleontologists have puzzled: Are the giant extinctions fundamentally different from the background extinctions, or do they merely represent the extreme end of a continuum? Figure '6 helps us decide. This chart was constructed by dividing the 600 million years since life began to flourish, at the beginning of the Cambrian period, into intervals of '-million-year duration, and computing for each the number of species still alive at the end as a percentage of those alive at the beginning. The mean is a 25 percent extinction rate per million years (on the average, of '00 species alive at the beginning of a '-million-year period, 75 were still alive at the end). The Big Five lie off on the right tail, but there is no break between them and the lesser extinctions—the distribution appears continuous. Background and mass extinctions therefore do not seem to be qualitatively different, but rather to grade imperceptibly into each other. If all extinctions had a common cause, but one that operated at different intensities at different times, this is the pattern we would expect. We cannot say, however, that some combination of extinctions with different causes might not give the same result.

The great extinctions reached diverse organisms in almost every ecological niche. The K-T extinction wiped out animals as unlike as microscopic foraminifera, intricately coiled ammonites, land plants, and dinosaurs—from the tiniest creatures of the sea to the largest denizens of the mountain slopes. Obviously, such different organisms, in such completely different environmental settings, did not compete in Darwin's sense. Most species that died appear to have been as successful as those that survived. Before their fall, there would have been no reason to predict that they would be the ones to go, yet go they did. The converse is also true. In most cases it is impossible to say why the species that survived did so; certainly it was not because they were more "fit." Thus, Raup concludes, evolution and survival may be more matters of chance than fitness, of good luck than good genes. In his view and that of the Alvarezes, the dinosaurs, and the others that joined them in disappearing at the end of the Cretaceous, were more than anything unlucky enough to be in the wrong place at the wrong time.

t HE f OSSIL r ECORD

When the Alvarez theory broke upon the world, most paleontologists were quite confident that it could immediately be judged by the weight of more than a century's worth of fossil evidence and rejected out of hand. But they were wrong. An obstinate set of problems makes interpreting fossil data so difficult that testing the extinction half of the Alvarez theory proved much harder than anyone could have anticipated. In the testing, however, so much was learned that some paleontologists believe that their field is undergoing a procedural if not a scientific revolution. For the first time, large collections are being established with the specific aim of testing whether extinction near a major geologic boundary was gradual or sudden, and whether species that have always been thought to have gone extinct at a boundary truly did so.

In order to understand how the second half of the Alvarez theory was tested, it is important first to recognize some of the problems inherent in trying to read the fossil record. Common sense tells us that to corroborate the extinction half of the theory, we need to find two kinds of evidence: (a) that prior to the K-T boundary, most species were not already going extinct for some other reason, and (b) that the dinosaurs and others did not survive the K-T impact— that their remains do not lie above the iridium layer. To test both predictions, geologists needed to be able to pin down the exact point in a sequence of rocks at which the extinction of a particular species occurred. Can that be done?

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