and a somewhat less substantial drop in diversity (Flügel and Kiessling, 2002). Trace fossils have been studied in different regions, but the impact of the extinction varies between localities, in part because of shifts in the environments of deposition at the same time as the biotic crises. There is, however, evidence of some decrease in the complexity of trace fossil assemblages that cannot be attributed simply to changes in the sedimentary environment in which they were deposited.
The end-Cretaceous mass extinction led to the disappearance of significant numbers of foraminifera and other plankton and a significant drop in primary productivity (D’Hondt et al., 1998). Ammonoids finally disappeared, as did belemnites and rudist bivalves. The loss of rudists was the major loss among reef biota, and Flügel and Kiessling (2002) record few other impacts among reef ecosystems. Studies of the complexity of trace fossils across this interval are relatively few and suggest only moderate impact by the mass extinction (Twitchett and Barras, 2004).
Although not one of the canonical five mass extinction episodes, extinction rates measured by taxic diversity were high during a number of stages of the Cambrian, sorting out the winners and losers among the Cambrian diversification of animals. Indeed by some metrics, particularly morphologic disparity and developmental diversity, these events may have winnowed a greater degree of evolutionary history than any of the subsequent biodiversity crises of the Phanerozoic.
Empty ecological space has long been considered a key factor in evolutionary innovations, as an unexploited opportunity opened by new adaptations, a new geographic region with underexploited resources, or an environment opened up through environmental change. Recoveries from mass extinctions have been viewed as encompassing each of these possibilities, as the removal of previously dominant clades provides opportunities for expansion, including by migration, of minor groups and the origin of new clades, as an increased likelihood for success of adaptations that might have been blocked, and as an instigator of environmental change that might favor new groups. What the economist Joseph Schumpeter described as “creative destruction” is true of evolution: continuing biotic overturn and more comprehensive biodiversity crises have been a normal part of the history of life, and perhaps essential to the success of evolutionary innovations.
Two principal classes of models have been applied to understanding the underlying processes of taxic diversity (Benton and Emerson, 2007). The first class includes global-level correlates of population growth models that invoke logistic growth models and either global carrying