A Kill Curve For Phanerozoic Marine Species David M. Raup ...

KILL CURVE FOR MARINE SPECIESSPECIES KILL PER E'JENT(i:)FIGURE7. Simulation of a"typicalr' Phanerozoic time interval of 600 m.y. using the kill curve to show the proportionsof total species extinction involved in small and large extinction events. Mass extinctions (tail on right) includeonly a small proportion of all species extinctions.The kill curve for Phanerozoic species saysnothing directly about the causes of extinction.It is only a description of the extinctionhistory as this history is expressed in the fossilrecord. The curve does not even speak tothe question of how many mechanisms of extinctionhave operated in the Phanerozoic.The curve does have some implications formechanisms of extinction, however. The actionof any cause or causes of extinction mustbe distributed in time in a manner compatiblewith the kill curve. This can serve to constrainour choices of mechanisms and may be usefulto eliminate some candidates. For example,various authors have proposed that Phanerozoicextinctions are dominated by a singlefactor, such as sea-level lowering, climaticcooling, or meteorite impact. These proposalscan be tested in the following manner:1. Construct a cumulative frequency curvefor the proposed mechanism; for sea-levellowering, the coordinates would be the magnitudeof sea-level drop (in short spans ofgeologic time) and the average waiting timeobserved between events of a given magnitude.2. Combine this curve with the kill curvein Fig. 1, eliminating the common variable,waiting time. This yields a curve that predictsthe average species kill for any given magnitudeof sea-level lowering (or whatevercausal factor is being considered). This stepis based on the working hypothesis-beingtested-that the proposed causal factor explainsall or most extinctions.3. Evaluate the plausibility of the new curveby inspecting specific cases where magnitude(e.g., of sea-level lowering) and extinction areboth known with reasonable accuracy.The procedure just described should workwell for any situation in which the Phanerozoichistory of the candidate mechanism iswell known. For the three examples givenabove, one needs only the chronologies ofsea-level fluctuations, temperature changes,or meteorite impacts, respectively. If one wereto postulate extinction caused by a combinationof two or more factors, the factorswould have, to be weighted and combinedand the testing would become far more complex.The kill curve also helps explain why pa-