Remarkable extinctions of shelf and epicontinental sea trilobite faunas at several times in the Cambrian, separated by periods of several millions of years, provide markers for the boundaries of Biomeres. At several boundaries, the abrupt change from a high diversity fauna with low individual dominances to a low diversity fauna dominated by only one or two species takes place within a few centimeters of section. There is no evidence in the beds below the extinction event of any signs of community deterioration. The best documented change, between the Late Cambrian Crepicephalus and Aphelaspis Zones can be recognized in sections from Nevada to Tennessee. Such a widespread event, involving an extremely short-lived shock to an established and successful community of shallow marine organisms, is difficult to explain by conventional causes. The pattern of extinction is remarkably similar to that of the calcareous plankton at the Cretaceous-Tertiary boundary and may be a response to a similar cause.