Stasis and extinction of Silurian (Llandovery-Wenlock) trilobite associations related to oceanic cyclicity

Silurian trilobites of the central United States belong to a series of temp orally-successive associations which appeared abruptly, maintained taxonomi c stasis for a time, and then disappeared abruptly. Their disappearance res ulted from global perturbations of short-term duration and moderate magnitu de, which caused substantial taxonomic replacement but no reorganization of major ecosystems. The most significant extinction and replacement in Silur ian trilobite associations in the study area occurs near the Llandovery-Wen lock boundary. This turnover in trilobite associations appears to correspon d to Jeppsson's Ireviken Event in his model of oceanic and climatic cyclici ty. Major sea-level changes earlier in the Llandovery did not have a simila r impact on trilobite associations.