improvement within the activities of the larger clade, a pattern confirmed by the power of incumbency (Rosenzweig and McCord, 1991). Third, as demonstrated by a recent analysis of Cenozoic mollusks from New Zealand, species and genera exhibit a limited interval of peak abundance, followed by a long decline to extinction. In this system at least, the species at greatest risk of extinction are those already in decline (Foote et al., 2007), although this does not appear to hold true during mass extinctions that may truncate ranges (Foote, 2007). Fourth, mass extinction events periodically upset these patterns, and particularly at the end-Permian mass extinction, trigger pervasive changes in patterns of ecological and evolutionary dominance. Thus over evolutionary time, episodic extinctions has been an important driver for evolution.
Understanding the processes controlling long-term changes in diversity requires identifying and correcting for biases in the fossil record that can be introduced by preservation and sampling. Consequently paleontologists have developed new approaches designed to identify and correct for such biases (Smith, 2001; Crampton et al., 2003; Foote, 2003, 2007; Peters, 2005). These techniques have been applied to correct for biases in our record of the end-Ordovician mass extinction (Krug and Patzkowsky, 2004). As discussed by Alroy (Chapter 11, this volume), the diversity patterns produced by this intensive compilation of taxic diversity largely follow those of Sepkoski’s earlier work (Sepkoski, 1982, 1992, 1993). This effort identifies at least three of the five canonical mass extinctions below. However, like other work (Bambach et al., 2002, 2004) it raises questions about the magnitude of other extinction events.
Finally, counting taxa, whether species, genera, or families, assumes that each taxon is equivalent, which is far from true when one considers the differences in diversity or abundance within different groups, much less their evolutionary distinctiveness, morphologic disparity, ecological function, or evolutionary potential.
The two remaining species of tuatara are the sister clade to the ≈6,200 snakes and lizards of the Order Squamata, as the few remaining onycophorans are to the Phylum Arthropoda. Both onycophorans and tuataras are far more evolutionarily distinct than any two members of their sister clade, a fact not captured by a simple taxic approach. A simple exercise illustrates that identical levels of species loss can conceal very different effects on evolutionary history (Fig. 9.1). In each case roughly the same total number of species has been lost. In alternative A, however, there is little loss of the overall structure of the tree, whereas in alternative B, an entire clade has been pruned. Alternative C removes the most basal