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and the characterization of phylogenetic diversity (Faith, 1992a; Faith and Baker, 2006) and evolutionary distinctiveness (Vane-Wright et al., 1991). There is, however, a more important reason for considering the loss of other aspects of evolutionary history, and that is the search for mechanisms underlying patterns of extinction and construction of biodiversity. Ecologists increasingly recognize the importance of a network of interactions in generating biodiversity, including positive feedback relationships among biodiversity, productivity, and stability (Worm and Duffey, 2003; Montoya et al., 2006).

Although paleontologists are aware of the diversity of effects on evolutionary history caused by past extinctions, particularly mass extinctions, we have been slow to develop and apply comparative metrics beyond taxic compilations and estimates of geographic range. Enough work has been done to suggest a range of alternative metrics. Biogeographic structure is an important aspect of evolutionary history that has been considered elsewhere (Jablonski, 2007).

Taxic Diversity

The divisions of the geologic timescale are framed by biotic crises recognized by early geologists as “revolutions” triggering wholesale changes in the biota. Paleontologists have since compiled records of fluctuations in taxonomic diversity for marine taxa (Sepkoski, 1984, 1997), terrestrial plants (McElwain and Punyasena, 2007), vertebrates (Benton, 1989), and various microfossil groups (Rigby and Milsom, 2000). Patterns of extinction and origination have received considerable attention, particularly the decline in “background” extinction rates through the Phanerozoic for marine families and genera (Flessa and Jablonski, 1985) and episodic events of increased extinction. Curiously, as the English geologist John Phillips understood as long ago as the 1840s, extinctions within geologic stages appear pulsed, rather than spread out through the stage (Foote, 2005). Within clades paleontologists have also identified intriguing patterns of replacement where successive subclades replaced earlier clades. For higher-resolution analyses statistical techniques have been developed to account for sampling problems [see Jin et al. (2000) for an application to the end-Permian mass extinction].

Several general lessons emerge from these compilations. First, the persistent decline in extinction rates suggests an increased stability in younger taxa, although this may be a statistical artifact of increased species/genus and species/family ratios (Flessa and Jablonski, 1985). It would be of considerable interest to know whether this apparent increased robustness is real and whether it translates into some of the other metrics described below. Second, patterns of subclade replacement can suggest adaptive



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