Survival without recovery after mass extinctions

Abstract

Because many survivors of mass extinctions do not participate in postrecovery diversifications, and therefore fall into a pattern that can be termed ''Dead Clade Walking'' (DCW), the effects of mass extinctions extend beyond the losses observed during the event itself. Analyses at two taxonomic levels provide a first-order test of the prevalence of DCWs by using simple and very conservative operational criteria. For four of the Big Five mass extinctions of the Phanerozoic, the marine genera that survived the extinction suffered Ϸ10 -20% attrition in the immediately following geologic stage that was significantly greater than the losses sustained in preextinction stages. The stages immediately following the three Paleozoic mass extinctions also account for 17% of all order-level losses in marine invertebrates over that interval, which is, again, significantly greater than that seen for the other stratigraphic stages (no orders are lost immediately after the end-Triassic or end-Cretaceous mass extinctions). DCWs are not evenly distributed among four regional molluscan time-series following the endCretaceous extinction, demonstrating the importance of spatial patterns in recovery dynamics. Although biotic interactions have been invoked to explain the differential postextinction success of clades, such hypotheses must be tested against alternatives that include stochastic processes in low-diversity lineages-which is evidently not a general explanation for the ordinal DCW patterns, because postextinction fates are not related to the size of extinction bottlenecks in Paleozoic orders-and ongoing physical environmental changes. Paleontologists have long noted that certain clades (monophyletic evolutionary lineages) survive mass extinctions only to remain marginal or decline in the aftermath, whereas other groups diversify. The aim of this paper is to test the generality of this pattern of survival without recovery, which I term ''Dead Clade Walking'' (DCW) in homage to an award-winning film based on ref. 1, by moving beyond anecdotal accounts to global and regional analyses of marine invertebrates. Here, I show that more taxa are lost at both the genus and ordinal levels shortly after extinction events than expected from preextinction intensities, that the ordinal pattern cannot be explained simply in terms of the stochastic consequences of the losses suffered in the mass extinction itself, and that DCW molluscan genera are not evenly distributed geographically after the end-Cretaceous (K-T) extinction. Taken together, these analyses provide further evidence that the evolutionary and ecological roles of mass extinctions are larger than indicated simply by tallies of taxa lost at the crisis horizons or intervals (2-4), and that more attention should be paid to taxa that do not fit the end-member cases of complete extinction or unbridled diversification. Phanerozoic GeneraMethods. One approach to quantifying the frequency and role of DCWs is to test whether the assemblage of taxa surviving a mass extinction is subject to...

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