Downloaded from geology.gsapubs.org on August 18, 2012 Long-term origination rates are reset only at mass extinctions Andrew Z. Krug and David Jablonski Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA ABSTRACT time series of supports for each infl ection point Diversifi cation during recovery intervals is rapid relative to background rates, but the (Fig. 1). The most strongly supported infl ec- impact of recovery dynamics on long-term evolutionary patterns is poorly understood. The tion points stand out as peaks in this spectrum. age distributions for cohorts of marine bivalves show that intrinsic origination rates tend Survivorship curves suffer from two poten- to remain constant, shifting only during intervals of high biotic turnover, particularly mass tial biases that may confound the ability to esti- extinction events. Genera originating in high-turnover intervals have longer stratigraphic mate shifts in origination rates around infl ection durations than genera arising at other intervals, and drive the magnitude of the shift following points. (1) Genus-level BSCs are potentially the Cretaceous–Paleogene (K-Pg) extinction. Species richness and geographic range promote age-dependent, meaning that the elapsed time survivorship and potentially control rates through ecospace utilization, and both richness and since the origination of the genus can alter the range have been observed to expand more rapidly in recovery versus background states. Post- probability that a branching event occurs. If this Paleozoic origination rates, then, are directly tied to recovery dynamics following each mass effect is large, extinction intensity may factor extinction event. into the slopes of BSCs (Foote, 2001), produc- ing infl ection points even when the origination INTRODUCTION resents the net rate of accumulation through rate is constant. (2) Because genus ranges can In the fossil record, mass extinctions are con- time of all genera coexisting at a time win- span multiple stage boundaries, the origination sistently followed by episodes of rapid diver- dow, which makes this method preferable to rates estimated for cohorts are not entirely inde- sifi cation (Erwin, 2001, 2008), but the lasting stage-by-stage rates, which describe origina- pendent. To account for both issues, stage-level impact of these events on evolutionary dynam- tions within a stage but not their contribution per-taxon rates (Foote, 2000) were calculated to ics in the much longer intervals between mass to diversity in future time planes. A constant corroborate rate shifts discussed below. These extinctions is poorly understood. Peaks and val- slope indicates a constant per-taxon origina- origination rates are estimated only from gen- leys in evolutionary rates through time can be tion rate of that cohort (likely underlain by era originating within each stage, so that each interpreted in terms of environmental stresses constancy in the factors governing the origi- estimate is independent of the others. They are inferred from the geological record, but contain nation rate), whereas signifi cant infl ection also independent of the extinction rate within no direct information on the lasting infl uence points in the BSC refl ect prolonged shifts in the stage, eliminating any bias introduced by of those events on faunas in subsequent times. the probability of origination. To identify and age-dependent dynamics. Stages within the Such interval-to-interval variations can occur evaluate the statistical support for potential recovery interval following the end-Cretaceous stochastically even under evolutionary models infl ection points within each BSC over a single (K-Pg) extinction (defi ned here as extending to where the probability of origination or extinc- exponential function, one-parameter (single the Paleocene-Eocene boundary at 55.8 Ma) tion is held constant (Raup et al., 1973; Nee, exponential probability function with slope were excluded from the analysis. See the Data 2006), and may also be infl uenced by variations λ), three-parameter (two exponential functions Repository for analytical details. in sampling and preservation, making it diffi cult separated by an infl ection point tcrit), and fi ve- to identify long-term shifts in evolutionary rates parameter probability functions (three expo- RESULTS and, therefore, the processes that govern them. nential functions separated by two infl ection Additionally, while signifi cant effort has gone points; see the GSA Data Repository1) were fi t Infl ection Points and Shifts in Origination into estimating and interpreting extinction rates, to the data using the optim function in the soft- Rates origination rates have received less attention. ware package R (R Developmental Core Team, For all cohorts from the Early Cretaceous to 2008). The corrected Akaike Information Cri- the Pleistocene, only a few infl ection points and DATA AND METHODS terion was used to determine the best-supported shifts in origination rates were supported (Fig. 1; Here, we analyze the distribution of infl ec- model. Only cohorts containing >100 genera Table DR1 in the Data Repository), consis- tion points within backward survivorship were analyzed to enhance statistical power tently positioned at the same geological events. curves (BSCs; Raup, 1978; Foote, 2001) for in determining infl ection points, limiting the Maximal support for an infl ection point in all a succession of cohorts (defi ned as the set of analysis to cohorts that cross stage boundaries Cenozoic BSCs occurs at the base of the Maas- genera whose stratigraphic ranges cross a between the Pleistocene and the Middle Juras- trichtian (ca. 70.6 Ma); the difference in support stage boundary) of marine bivalve genera—a sic (Aalenian-Bajocian boundary), all together between the base of the Maastrichtian and the well-characterized model system whose tem- spanning ~170 m.y. Every stage boundary K-Pg boundary (ca. 65.5 Ma) is equivocal, and poral and spatial dynamics mirror those of the within the stratigraphic ranges of genera within the slight offset may also refl ect less intense marine biota as a whole (Krug et al., 2009b). the cohort was analyzed (excluding the oldest study in the 75–100 Ma interval, which would BSCs, which plot the number of taxa within 1% of the genera), the rates surrounding the artifi cially concentrate originations in the last a cohort that originated prior to a window of infl ection point determined, and the support 10 m.y. of the Cretaceous (Foote, 2003; Krug observation (Foote, 2001), allow for robust cal- for the model assessed. This analysis yielded a et al., 2009a). Origination rates consistently culations of origination rates and for inferences increase from ~0.015 (Fig. 2A, black points) to into the processes affecting those rates. Assum- 1GSA Data Repository item 2012202, description ~0.032 (gray points) genera/genus/m.y. around ing rates are time-specifi c and taxonomically of data and methods, Tables DR1 and DR2, Figures this infl ection point. Because the infl ection point homogeneous, BSCs defi ne an exponential DR1–DR4, and additional references, is available is consistently at the K-Pg boundary, the recov- online at www.geosociety.org/pubs/ft2012.htm, or probability function whose slope is governed on request from [email protected] or Docu- ery interval is counted toward the Cenozoic by the origination rate of the cohort (Foote, ments Secretary, GSA, P.O. Box 9140, Boulder, CO rate, causing slightly higher estimated Cenozoic 2001). This intrinsic origination rate (λ) rep- 80301, USA. rates for cohorts nearer the Paleocene recovery GEOLOGY, August 2012; v. 40; no. 8; p. 731–734; doi:10.1130/G33091.1; 4 fi gures; Data Repository item 2012202. | Published online 29 June 2012. GEOLOGY© 2012 Geological | August Society 2012 of | America.www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 731 Downloaded from geology.gsapubs.org on August 18, 2012 A: Cenozoic cohorts B: Mesozoic cohorts (Fig. 2B), likely representing the recovery inter- val following the Late Permian mass extinction. Rates for this interval vary due to small sample 3 sizes, and steps in this curve occur with the addi- tion of as few as two genera (e.g., the decrease from 89.3 to 93.5 Ma). Following the recovery 1 2 interval, origination rates decrease into the Tri- assic (0.027) and then again into the Jurassic (0.020; Fig. 2B), coincident with the Late Trias- Proportion of originations Proportion of originations 0.01 0.05 0.2 1.0 0.01 0.05 0.2 1.0 sic mass extinction event. The decrease follow- ing the Late Triassic extinction stands in contrast to the increase following the K-Pg extinction. Although Triassic and Jurassic 95% confi dence intervals overlap slightly owing to small sample sizes, the decrease in origination rates is signifi - cant if the Middle Triassic recovery genera (i.e., Support / max Support / max genera that originate within the recovery inter- 1.0 0.99 0.98 1.0 0.996 0.992 val) are excluded and a three-parameter model 0 50 100 150 200 100 150 200 250 is fi t to the data. Time (Ma) Time (Ma) The only infl ection points supported for all cohorts in this analysis occur within one stage Figure 1. Backward survivorship curves (BSCs) (top panels) and support for potential boundary of a mass extinction and/or recov- infl ection points (tcrit) within BSCs (bottom panels) of marine bivalves. A: Cenozoic cohorts. B: Mesozoic cohorts. Support values for bottom panels correspond to stages ery interval. However, other infl ection points within survivorship curves, plotted as the negative of the support returned by the R are statistically supported for smaller groups function optim, so that the lowest values represent the strongest support for an infl ec- of cohorts (Fig. 1; Fig. DR1) and also occur tion point. Support values are normalized to the largest support value for an infl ection point in that cohort.