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Department of Palaeontology, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK N.MacLeod{at}nhm.ac.uk
Two prominent patterns have been recognized in Phanerozoic extinction data: (1) a quasi-periodic distribution of extinction-intensity peaks, and (2) a linear, declining background extinction intensity gradient. Characterization and interpretation of both patterns are necessary to understand Phanerozoic extinction controls. The extinction-intensity peak spectrum has been variously interpreted as a reflection of the time-series of major sea-level regressions, continental flood-basalt province (CFBP) eruptions, and bolide impacts. In order to evaluate the level of association between these time-series and the Phanerozoic marine invertebrate extinction record statistically, a new Monte Carlo simulation strategy is presented. Results of simulation-based tests suggest that the time-series of major, eustatic sea-level regressions and CFBP eruption events have a statistically significant (p
0.05) association with Tatarian-Pliocene, stage-level, extinction intensity peaks. Associations between this peak series and the time-series of crater-producing bolide impacts do not appear significant at this level. A limited multicausal event scenario was also tested using the Monte Carlo method, and recognized the combination of sea-level regression and CFBP volcanism to be significantly associated with the largest extinction intensity peaks of the last 250 Ma. The background extinction-intensity gradient has been interpreted variously as: (1) an indicator of progressive improvement in extinction resistance through selection; (2) the by-product of an invasion of marginal (extinction-resistant) habitats; and (3) as a taxonomic-stratigraphical artefact. Results of subdivided linear trend analyses suggest that the background extinction-intensity gradient is largely confined to the Late Palaeozoic-Cenozoic interval. No statistically significant gradient is present in the most recent compilation of Early-Middle Palaeozoic data on marine, invertebrate extinctions. The timing of gradient initiation and extinction variance analyses suggest that reorganization of global carbon cycles and oceanographical circulation patterns in the Devonian-Early Carboniferous, and the evolutionary appearance of modern phytoplankton groups in the Late Triassic both had dramatic effects on the character of the extinction-intensity gradient.
