to have survived the extinctions because of their appearance later in the Mesozoic record—a phenomenon Jablonski (1986) has called the Lazarus effect. That is, they “rose again” after apparent extinction.

The Lazarus taxa provide a special challenge for students of the fossil record, because there are two equally plausible explanations for major gaps in the fossil record: species diversity may have been so low that the organisms were not preserved as fossils, or sedimentary environments conducive to fossilization may have been absent. The choice between these two explanations is difficult to make, and no unequivocal case has yet been made for either of them. However, the presence of a few abundant species immediately following the Permian period argues in favor of the lowered diversity theory.

Other consequences of mass extinction are somewhat clearer. Many of the extinction events were followed by major shifts in dominance of biological groups and by the evolutionary radiation of new innovations. A classic example is the diversification of the mammals following the extinction of the dinosaurs. Mammals had been present in moderate numbers throughout most of the time of dinosaur dominance, but it was not until the removal of the dinosaurs during the mass extinction at the end of the Cretaceous period that mammals became truly diversified. It is presumed, though difficult to prove absolutely, that the diversification of mammals, and ultimately the evolution of Homo sapiens, was possible because of the newly available ecological space in terrestrial habitats.

Other examples of replacement resulting from extinction involve tropical reef communities. The builders of reef frameworks, now dominated by stony corals of the Scleractinia order, switched roles repeatedly during Phanerozoic time. Reefs have been built at various times by molluscs, bryozoans, calcareous algae, or coral groups only distantly related to modern corals. It is clear that the extinction-replacement phenomenon has been largely responsible for these changeovers. This is important in broader evolutionary terms, because it suggests that the evolution of communities, and the changing dominance of certain kinds of plants and animals, is not a simple progression based on species-species competition. Rather, the changes may occur simply as a result of the filling of voids left by the demise of previously dominant groups. And the extinctions of the previously dominant groups, if caused by rare conditions of extreme stress, may have little or nothing to do with adaptive level or general efficiency. Thus, there is no reason to believe that the present dominance of scleractinian corals in most tropical reefs implies anything about the fitness of these animals to that environment relative to previous occupants.

One can go further and suggest that without the perturbing effect of the extinction-replacement events, evolution as we know it would have been very different. It is easy to imagine that diversification and innovation in evolution would have come to a stop early in Phanerozoic time, the occupants of most ecological niches or adaptive zones maintaining a stable, steady state. From this viewpoint, extinction, and especially mass extinction, can be seen as a vital ingredient in the evolution of complex life as we know it. This must remain somewhat speculative, of course, because the evolution of life cannot be replayed under different conditions.



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