New data from the southwestern United States on Late Cambrian trilobite biomeres suggest that the non-agnostid trilobite population of each biomere underwent four successive stages of evolution before becoming extinct. The first and stratigraphically lowest stage is characterized by species with considerable morphologic variability and short stratigraphic ranges. The second stage is characterized by species with longer stratigraphic ranges and less individual morphologic variability. The third stage is characterized by high species diversity and species with long stratigraphic ranges and increasingly less morphologic variation. The fourth, stratigraphically highest stage is characterized by low species diversity, coquinoid abundance of at

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least one species, and at least one species of olenid trilobites.

The bounding faunal discontinuities of trilobite biomeres probably resulted from repeated migrations onto the craton of a slowly evolving extracratonic basic stock, each migration replacing the major cratonic nonagnostid trilobites. Following each migration there was an initial burst of adaptive evolution (stage one) as the eugeosynclinal trilobites evolved rapidly under strong selection pressure imposed by their new cratonic environment. Stage two represents the attainment by a few genera of fairly complete adjustment to the environment. Stage three represents maximum adjustment to and utilization of the environment. The extinction of many long-ranging species near the end of stage three and the peculiar but characteristic composition of stage four suggest that stage four represents he last stand of the established trilobites of the biomere prior to their replacement by a new migration of extracratonic trilobites.

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