Biomeres are evolutionary packages of continental or larger scale that include several biozones and that are bounded by sharply defined extinction events. They are best developed in the Cambrian rocks of North America. The extinction events certainly affect all trilobites and probably affect all of the Cambrian fauna. Biomere boundaries have been precisely located in several fossiliferous sections to within 1 cm, generally within a bed of uniform lithology. At some localities, this boundary is at a hardground. No geochemical anomalies of the type associated with meteor impacts have been identified with the extinction events. Among earth-bound causes for the extinction events, a rapid rise in the thermocline or a widespread anoxic event are suggested as targets for testing.--Modified journal abstract.