David Jablonski

The fossil record amply shows that the spatial fabric of extinction has profoundly shaped the biosphere; this spatial dimension provides a powerful context for integration of paleontological and neontological approaches. Mass extinctions evidently alter extinction selectivity, with many factors losing effectiveness except for a positive relation between survivorship and geographic range at the clade level (confirmed in reanalyses of end-Cretaceous extinction data). This relation probably also holds during "normal" times, but changes both slope and intercept with increasing extinction. The strong geographical component to clade dynamics can obscure causation in the extinction of a feature or a clade, owing to hitchhiking effects on geographic range, so that multifactorial analyses are needed. Some extinctions are spatially complex, and regional extinctions might either reset a diversity ceiling or create a diversification debt open to further diversification or invasion. Evolutionary recoveries also exhibit spatial dynamics, including regional differences in invasibilty, and expansion of clades from the tropics fuels at least some recoveries, as well as biodiversity dynamics during normal times. Incumbency effects apparently correlate more closely with extinction intensities than with standing diversities, so that regions with higher local and global extinctions are more subject to invasion; the latest Ceno-zoic temperate zones evidently received more invaders than the tropics or poles, but this dynamic could shift dramatically if tropi-

Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637.

190 / David Jablonski cal diversity is strongly depleted. The fossil record can provide valuable insights, and their application to present-day issues will be enhanced by partitioning past and present-day extinctions by driving mechanism rather than emphasizing intensity.

The inventory of life on Earth has always been determined at the most basic level by the difference between origination and extinction. This fundamental macroevolutionary equation, richness = origination - extinction, has been formally applied in many ways and with many elaborations, but takes on special consequence when attempting to evaluate the processes shaping present-day biodiversity, where neither term in the right side of the equation can be observed directly. Some progress has been made in modeling these parameters, but most approaches involve strong assumptions, require very large datasets, and carry large uncertainties [e.g., Paradis (2004), Ricklefs (2007a,b)]. The spatially explicit form of this equation, where richness is a local or regional pool of species or higher taxa, and immigration and emigration terms enter the right side of the equation (Jablonski et al., 2006), is important in many situations, from biotic responses to Pleistocene climate cycles and ongoing climate changes to recoveries from mass extinctions. However, this form is even more difficult to apply rigorously without historical data, and my emphasis here will be on the fossil record. Few would argue against the idea that the spatial fabric of extinction has shaped, and will continue to shape, the biosphere in profound ways, but spatial effects have been neglected relative to temporal patterns (partly because documentation is so challenging). I will argue that the insights beginning to emerge from spatially explicit approaches to ancient extinctions have significant implications for the dynamics of diversity of the past and in the future.

This discussion will encompass a range of extinction intensities and focus on marine systems, where the fossil record is richest: application of these generalizations to terrestrial realms requires more study. Opinions are divided on whether the handful of mass extinctions of the geologic past are a separate class of intensities from the "background" extinction that constitutes the great bulk of geologic time [and the bulk of total extinction; Raup (1994)], but this is a secondary issue that can probably be resolved by factoring out the well-known secular decline in background extinction rates (Bambach et al., 2004; Jablonski, 2005; Stanley, 2007). As discussed below, extinction selectivities evidently shift between episodes of low and high extinction rates, and this selectivity is the more important issue for understanding the role of extinction in shaping past and future biotas. I will corroborate previous evidence for a strong spatial component to survivorship during major extinction events, present a multifactorial analysis of the end-Cretaceous (K-T) mass extinction in which geographic range emerges as the best predictor of survivorship in marine bivalves, and argue that such indirect effects are probably more important than generally appreciated. I will also discuss regional variations in the balance of invasions and local origination in the aftermath of the K-T event, which are somewhat unexpected given that the extinction itself tended to increase biotic homogenization on a global scale by preferentially removing the more localized taxa. Invasions and extinctions are also important during times of "normal" extinction intensities, as I will illustrate with reference to the dynamics of the latitudinal diversity gradient. I will conclude with some implications for integrating insights for past and present-day extinctions and suggest that a powerful approach might involve comparative dissection in extinction patterns according to likely drivers. Throughout I will note gaps in our understanding that would benefit from combined study of modern and ancient systems.

In this chapter, I will focus mainly on marine bivalves such as mussels, scallops, and cockles. Bivalves are becoming a model system for the analysis of large-scale biogeographic and evolutionary patterns (Crame, 2000, 2002; Jablonski et al., 2003b, 2006; Kidwell, 2005; Valentine et al, 2006; Krug et al., 2007) for several reasons. They are taxonomically rich but not unmanageable («3,000 living and fossil genera), and their systematics are increasingly understood, so that taxonomic standardization and phylo-genetic treatment of heterogeneous data are feasible. They have diverse life habits, from filter-feeding to photosymbiosis and chemosymbiosis to carnivory. They occur at all depths from the intertidal zone to deep-sea trenches and from the tropics to the poles. They are abundant and often well preserved as fossils [although not all habitats and clades are equally represented; Valentine et al. (2006)], and they have diverse shell mineralogies and microstructures, which allows analyses to control statistically for, and thus factor out, some, although not all, of the biases in the fossil record (Kidwell, 2005; Valentine et al., 2006). These favorable attributes do not mean that the bivalve fossil record is perfect, and preservation and sampling biases must always be considered in large-scale analyses [see, for example, the variety of approaches in Alroy (2000), Foote (2003), Bush and Bambach (2004), Bush et al. (2004), Jablonski et al. (2006), and Smith (2007)]. However, our growing knowledge of living and fossil bivalves, including the taxonomic, preservational, and geographic factors that can distort their fossil record, makes this group an excellent vehicle for integrating present-day and paleontological diversity dynamics.

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