random model that assumes that all species extinctions are independent of one other, the probability of a 10% extinction every 1 Myr (on average) is vanishingly small (Raup, 1991a). In fact, a kill curve based on this model could not be plotted at the scale of Figure 3: the curve would be indistinguishable from the horizontal axis. The only available conclusion is that extinctions are nonrandomly clustered in time, and this implies strongly that the K-T extinctions, for example, had a common cause.

Some of the clustering of extinction may be due to the removal of one or a few species that are crucial to the existence of other species. Or clusters may be due to destruction of one important ecosystem or habitat. However, for the larger events, at least, the extinctions are far more pervasive. At the end of the Cretaceous, high levels of species extinction (>50%) are found in all geographic areas and involve organisms as different as burrowing mollusks, planktonic microorganisms, land plants, and dinosaurs. This suggests, among other things, that the big mass extinctions cannot be explained by Darwin's species interactions unless one is willing to postulate an incredible degree of connectedness in the biosphere.

A striking effect of the typical mass extinction is its aftermath. For as long as 5–10 Myr, fossil faunas and floras are impoverished and are often dominated by only one or two species. The longest such interval followed the late Permian extinction (the largest of the Big Five): many major phyla and classes, known to have survived from later occurrences, are absent from the early Triassic assemblages. And about a third of the Triassic is characterized by what has been called the "coal gap," an interval where no coal deposits have been found—either of temperate or of tropical origin (A. M. Ziegler, University of Chicago, pers. comm., 1993).

When full diversity does return, it often has a strikingly different character. A classic example is the history of marine reefs. Reef communities have been wiped out several times in the past 600 Myr, coinciding in four cases with Big Five events. Each time reefs reappear, the principal framework organisms have changed, switching back and forth between calcareous algae, sponges, bryozoans, rudist mollusks, and various corals (Sheehan, 1985; Copper, 1988). The contemporary term "coral reef" describes only the current occupants of that adaptive zone.

Selectivity

Darwin argued that all extinction is selective: species not able to compete with other species die out. In one of the passages quoted above



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