Abstract

Mass killings that appear to be approximately synchronous and involve disappearance of biomass at a bedding plane in many sedimentologically independent sections globally suggest a common cause and possible synchroneity. Diversity changes and taxa plots are of dubious value in detecting extinctions and cannot identify an event horizon with any precision. Less common taxa often show a gradual decline before any arbitrary horizon. Survivors after a major biomass disappearance are not uncommon. Mass killings identify a horizon which may be examined for evidence of cause. Geochemical markers may be ephemeral and absence may not be significant. Highly energetic bolide impacts occur and their flux is known. Large paroxysmal volcanic events may also occur over a relatively short time interval (ca. 1/2 Ma.). Large events must leave a trace: biotic, geochemical and sedimentological. There appears to be no reason why ongoing phenomena such as climate and sea-level changes are primary causes of anomalous episodic extinctions.