Abstract
In the excitement over the discovery of high levels of the element iridium at the Cretaceous-Paleogene (KPg) boundary and the high likelihood that the KPg mass extinction (66 Ma) was caused by the collision of an extraterrestrial object with Earth (1), many envisioned a general theory of mass extinctions driven by impacts (2). Others favored an endogenous process, identifying correlations between the eruption of massive flood basalts and mass extinctions (3). A general model of mass extinctions and other biodiversity crises seemed to be supported by time-series analyses of compilations of the marine fossil record, which revealed a periodic pattern to extinctions over the past 250 million y (4). Over the past decade or so, evidence has grown for distinctive causes for the Permo-Triassic (PT) and the KPg mass extinctions, belying any general mechanism. However, in PNAS, the Burgess et al. (5) paper regarding very high-resolution radiometric dates for the PT event suggests that the similarities may lie not with the triggering mechanisms but in how the Earth’s biota responds to environmental insults.
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