ON THE CONTINUITY OF BACKGROUND AND MASS EXTINCTION

Are mass extinctions a fundamentally distinct phenomenon that cannot be explained by processes responsible for background extinction? Or are they continuous with background extinctions, the right tail of a smooth continuum? I propose three criteria for deciding whether background and mass extinctions are continuous. Continuity of cause occurs when the same processes that are responsible for background extinctions, operating at an increased level or intensity, also cause mass extinctions. Continuity of effect is established when background and mass extinctions exhibit common patterns of selectivity on taxonomic, functional, morphological, geographical, or other criteria. Continuity of magnitude exists when the distribution of intensities of mass extinctions (as measured by the number of extinctions per unit time or some other metric) grade smoothly and continuously into the intensities of background extinctions.

Using Silverman’s Critical Bandwidth Test, a statistical method based on kernel density estimation, I test for continuity of magnitude using per-genus extinction rates at the stage and substage level. The method improves on existing work based on searching for gaps in histograms, since it does not depend on arbitrary choices of parameters (such as bar widths for histograms), and it provides a direct estimate of the significance of continuities or gaps in observed extinction intensities. I conclude that mass and background extinctions appear to be continuous in magnitude over the entire Phanerozoic, but may be discontinuous if only extinctions after the Ordovician radiation are studied.