An iridium anomaly has been detected in the Upper Devonian of the Canning Basin. The anomaly, which occurs in a 12-cm stromatolite bed with a high iron oxide content, is associated with a global extinction that involved a major decrease of biomass. The formation and preservation of geochemical anomalies in the Paleozoic may be unlikely in many sedimentary environments, while only biotic changes examined at the finest time scale will prove of value in identifying abrupt extinctions. Geochemical anomalies associated with extinctions may provide reliable tests for impact hypotheses. The anomaly may be due, however, to local chemical, physical, or biological processes, or to fallout from a meteorite impact or volcanic event.