Origination, Extinction, and Mass Depletions of Marine Diversity

Paleobiology, 30(4), 2004, pp. 522±542 Origination, extinction, and mass depletions of marine diversity Richard K. Bambach, Andrew H. Knoll, and Steve C. Wang Abstract.ÐIn post-Cambrian time, ®ve eventsÐthe end-Ordovician, end-Frasnian in the Late De- vonian, end-Permian, end-Triassic, and end-CretaceousÐare commonly grouped as the ``big ®ve'' global intervals of mass extinction. Plotted by magnitude, extinction intensities for all Phanerozoic substages show a continuous distribution, with the ®ve traditionally recognized mass extinctions located in the upper tail. Plotted by time, however, proportional extinctions clearly divide the Phan- erozoic Eon into six stratigraphically coherent intervals of alternating high and low extinction intensity. These stratigraphic neighborhoods provide a temporal context for evaluating the intensity of extinction during the ``big ®ve'' events. Compared with other stages and substages in the same neighborhood, only the end-Ordovician, end-Permian, and end-Cretaceous extinction intensities appear as outliers. Moreover, when origination and extinction are considered together, only these three of the ``big ®ve'' events appear to have been generated exclusively by elevated extinction. Low origination contributed more than high extinction to the marked loss of diversity in the late Fras- nian and at the end of the Triassic. Therefore, whereas the ``big ®ve'' events are clearly times when diversity suffered mass depletion, only those at the end of the Ordovician, Permian, and Cretaceous periods unequivocally qualify as globally distinct mass extinctions. Each of the three has a unique pattern of extinction, and the diversity dynamics of these events differ, as well, from the other two major diversity depletions. As mass depletions of diversity have no common effect, common cau- sation seems unlikely. Richard K. Bambach and Andrew H. Knoll. Botanical Museum, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138. E-mail: [email protected] Steve C. Wang. Department of Mathematics and Statistics, Swarthmore College, 500 College Avenue, Swarthmore, Pennsylvania 19081. E-mail: [email protected] Accepted: 15 February 2004 Introduction: ®ve'' mass extinctions. Each had been noted Mass versus ``Background'' Extinction earlier by Newell (1967). Raup and Sepkoski plotted the extinction The idea that mass extinctions stand out as rate (number of families going extinct per mil- a class of events separate from the range of lion years) for each stage in time order and as- ``normal'' or ``background'' extinctions that sumed that apparent outliers on the arithme- characterize most of the geological record tic plot were true statistical outliers (and not originated with the work of Norman Newell aberrations produced by errors in the time- (1962, 1963, 1967) and crystallized through scale), thus identifying a distinct class of mass the quantitative analysis by Raup and Sepko- extinctions. They also calculated a 95% con®- ski (1982). Using Sepkoski's compilation of dence interval around the remaining ``back- stratigraphic ranges temporally resolved to ground'' extinction stages and demonstrated the stage level for marine families of all ani- a secular decrease in extinction rates over mal taxa (Sepkoski 1982), Raup and Sepkoski time. Quinn (1983), however, showed that the identi®ed ®ve mass extinctions: the end-Or- extinction data were highly skewed and cor- dovician (Ashgillian); Late Devonian (includ- rectly pointed out that the assumption of a ing the Frasnian/Famennian boundary); end- normal distribution for ``background'' extinc- Permian (Guadalupian and Djhul®an togeth- tion data was not valid. When log-trans- er); end-Triassic (Late Norian or Rhaetian); formed, the distribution of extinction rates and end-Cretaceous (Maastrichtian). These was approximately normal and did not have a have become known informally as ``the big suite of outliers that could be categorized un- ambiguously as a class of mass extinctions * This paper is dedicated to the memory of Stephen Jay separate from the overall ``background'' dis- Gould (1941±2002). tribution. Bambach and Gilinsky (1986) dem- q 2004 The Paleontological Society. All rights reserved. 0094-8373/04/3004-0003/$1.00 ORIGINATION, EXTINCTION, AND DIVERSITY DEPELETIONS 523 onstrated that this was also the case for several ly into intensities of background extinction. other extinction metrics, including propor- Although continuity of magnitude appears to tional metrics by interval that are not subject have been demonstrated by Quinn (1983) and to time-scale error. For all metrics the distri- Bambach and Gilinsky (1986), and was as- bution of extinction intensity grades smoothly sumed by Raup (1991) in his kill curve anal- from lowest to highest values with no discrete ysis for the Phanerozoic, several factors led us break between ``background'' and presumed to reconsider the status of these intervals. mass extinction intervals. Indeed, Raup (1991) These include (a) the rarity of the largest ex- developed his view of the Phanerozoic kill tinction intensities, (b) the possibility that they curve on the basis of this continuity of extinc- do not share continuity of cause or effect with tion magnitudes. Bambach and Gilinsky all other intervals, and (c) the fact that origi- (1986) did, however, support the ®nding that nation and extinction have rarely been consid- extinction intensities (and origination inten- ered together in examining patterns of diver- sities, as well) declined during the Phanero- sity change. zoic, a conclusion discussed more fully by Gil- In the following sections, we reevaluate the insky (1994). continuity or discontinuity of magnitude and Despite the evidence that intensities of ex- then brie¯y consider whether events that tinction form a continuum, the ®ve events might be regarded as mass extinctions can be speci®ed by Raup and Sepkoski (1982) contin- uni®ed by effect or cause. We test whether any ue to be labeled ``mass extinctions'' (Finney et of the ``big ®ve'' events are differentiable from al. 1999; McGhee 2001; Wignall and Twitchett the distribution of other extinction intensities, 1996; PaÂlfy et al. 2000; MacLeod and Keller explore the role of origination as well as ex- 1996). Current threats to biodiversity have tinction in diversity changes associated with even been labeled ``the sixth extinction'' (Lea- these ®ve intervals, and comment on the dif- key and Lewin 1995). It may be that magni- ferences, as well as similarities, among the in- tude alone is suf®cient to justify the term tervals. The upshot will be that although the ``mass extinction'' because events with such ®ve intervals in questionÐthe end-Ordovi- pronounced loss of diversity are rare, with cian, Late Devonian, end-Permian, end-Trias- waiting times of about 100 million years (Raup sic, and end-CretaceousÐare the ®ve intervals 1991). But is there any reason to think of these with the greatest diversity loss in the Phan- events as a separate class of events, rather than erozoic, they share little else in common. Only as the uncommon upper tail of a continuous three were driven predominantly by extinc- distribution? tion and even they display distinctly different Wang (2003) recently identi®ed three con- patterns of diversity change, implying that cepts that must be considered individually these events are not related by continuity of when we ask whether putative mass extinc- effect or cause. tions grade continuously into the range of background extinction: continuity of cause, Tracking Genus Diversity continuity of effect, and continuity of magnitude. Continuity of cause would be demon- Figure 1 illustrates the history of marine ge- strated if candidate mass extinctions could be nus diversity through Phanerozoic time. The shown to be driven by the same processes that data were compiled by using a computerized are responsible for background extinction, al- sorting routine written by Jack Sepkoski and beit operating at increased intensity or over modi®ed by J. Bret Bennington to tabulate ge- larger areas. Continuity of effect would be es- nus diversity for 107 stages and substages us- tablished if background and mass extinctions ing Jack Sepkoski's unpublished tabulation of exhibited common patterns of selectivity on the stratigraphic ranges of genera as of 1996. taxonomic, functional, ecological, or other Although the Paleontological Research Insti- grounds. And continuity of magnitude would tution has recently published the raw genus exist if the distribution of intensities of mass ranges from a later version (1998) of Sepko- extinctions graded smoothly and continuous- ski's compilation (Sepkoski 2002), this publi- 524 RICHARD K. BAMBACH ET AL. FIGURE 1. Diversity and diversity turnover of marine genera by interval through the Phanerozoic. The ®ve major post-Cambrian diversity depletions are highlighted. The heavy line connects the data on number of genera crossing each interval boundary. The line directly connecting the numbers of boundary-crossing genera follows the path of minimum likely standing diversity, regarded as the minimum diversity because origination and extinction would have to work in exact lock-step to follow that diversity path. The peaked dotted line represents genus turnover within each interval. The rising part of each peak represents all genus originations (®rst occurrences) reported from the interval. The peak records the total number of genera reported from the time interval. The descending part of the peak represents the

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