Anatomy of an assemblage zone
The upper Lower Devonian pinyonensis Zone, named by Merriam in central Nevada, is an assemblage zone based on brachiopods. It encompasses a high diversity fauna, most of whose members migrated into central Nevada shortly after the distinctive and favorable pinyonensis Zone environment came into existence. The pinyonensis Zone coincides in space and time with the equally distinctive Bartine Limestone lithofacies, which is a member of the McColley Canyon Formation. The assemblage zone comprises several communities that existed side by side during all or part of pinyonensis Zone time. The brachiopods of these communities migrated in regular, nonrandom patterns within the pinyonensis Zone environment in response to changes in water depths, energy levels, and circulation. The assemblage as a whole ceased to exist when its favorable environment disappeared, judging from the coincidence of the pinyonensis Zone with the Bartine lithofacies. The fauna of the pinyonensis Zone represents an indivisible aggregate, or naturally “packaged” group of coexisting species that underwent little evolution once they were assembled in a favorable environment. The formation of the distinctive fauna and its demise can both be attributed to geological events that altered the environment. It appears that assemblage zones form when and where geological events cause pronounced environmental changes and that they tend not to form in environments in which changes are small and continual. Loss of the environment of an assemblage zone, or of a large percentage of its total area, evidently causes large-scale speciation and (or) extinction of the fauna which previously had attained a relative stability and which is the reason that an assemblage zone, when fully known, is a viable chronostratigraphic unit.