What are mass extinctions

Conventional wisdom holds that mass extinctions stand out in the fossil record as times when the extinction rate runs far faster than the background or normal extinction rate. They are relatively rare, some 99.99 per cent of all extinctions being normal extinctions. The fossil record points to a continuum of extinction events that range from everyday background levels to mass extinctions. However, it also suggests that mass extinctions are not the chance coincidence of independent extinction events, but regular episodes of mass killings.

A fundamental question is whether mass extinctions are simply the end-members of a continuum of biospheric disaster that ranges from moderately benign with limited loss of species to planet-wide trauma with a huge loss of species (Conway Morris 1998). On the basis of Jack Sepkoski's compilation of the stratigraphic ranges of all marine animal species, resolved to stage level, David M. Raup and Sepkoski (1982) recognized five big mass extinctions - the end-Ordovician (Ashgillian), the Late Devonian (including the Frasnian-Famennian boundary), the end-Permian (Guadalupian and Djhulfian together), the end-Triassic (Late Norian or Rhaetian), and the end-Cretaceous (Maastrichtian). However, are these 'big five' events, shown on Figure 7.1(a), distinctly different from other extinction events, or are they simply the rare upper tail of a continuous distribution of extinction magnitude? Raup's analysis of 'kill curves' suggests that extinction magnitude should follow exhibit a continuous distribution, with legion small magnitude events, a few rare large magnitude events, and all grades in between (Figure 7.2). A recent study explored this crucial question by examining proportional diversity change (Bambach et al. 2004). Proportional diversity change assesses the importance of an extinction event in the context of its time, rather than simply stating how many taxa were involved. Figure 7.1(b)

5_ 1 Late Ashgillian

(end-Ordovician)

5_ 1 Late Ashgillian

(end-Ordovician)

300 1 200

; (million years)

300 1 200

; (million years)

Figure 7.1 (a) Diversity and diversity turnover of marine genera by interval through the Phanerozoic. Note the 'big five' mass extinctions. The heavy line joins data on the number of genera crossing each boundary interval and follows the 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 species turnover within each interval: the rising part of each peak represents all genus originations (first occurrences) reported from the interval; the peak records the total number of genera reported in the interval; the descending part represents the number of extinctions (last records) of genera in the interval. The magnitude of the peaks compared with the minimum standing diversity at interval boundaries represent the degree of faunal turnover in the intervals. (b) Proportion of gain or loss of genus diversity from the Caradoc to the Plio-Pleistocene. The 'big five' mass extinctions have diversity depletions in excess of 20 per cent. The lines drawn at ±13.5 per cent define the range of change that might be regarded as 'background' fluctuations in diversity. Notice that intervals with diversity increases above 13.5 per cent are common only after major depletions of diversity. Source: Reprinted by permission from R K. Bambach, A. H. Knoll and S. C. Wang (2004) Origination, extinction, and mass depletions of marine diversity. Paleobiology, 30, 522-42. Copyright © The Paleontological Society.

5 10 Bolide diameter, d (km)

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