Extinction and migration in Silurian trilobites and conodonts of northwestern Canada

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Abstract
Appearances and disappearances (lineage turnover) of trilobite and conodont evolutionary species’ clades in Silurian shelf marginal sections of the Mackenzie Mountains are concentrated at particular stratigraphical levels. A depth/energy curve generated for this region shows three major and several minor transgressive-regressive cycles in the Llandovery and Wenlock Series. A comparison of this curve with conodont and trilobite distributional data supports the thesis that there are cause/effect relationships between changes in sea level and migration, evolution and extinction events of these marine organisms. The rate, amount and direction of change in depth were important factors in the migration and extinction of clades. The depth/energy curve for the region fits the pattern expected for a passive downdropping shelf responding to global, eustatic sea-level changes. Emigrations and extinctions are associated with rapid transgressive episodes and concentrated at three levels: lower Llandovery, lower Wenlock, and middle or lower part of the upper Wenlock. The early Wenlock event was the most impressive. A further concentration of extinctions and/or migrations, not considered here, occurred later in the Ludlow, associated with a regional regression. Phyletic evolution is most convincingly demonstrated in fossil collections from intervals of gradual regression. A model is proposed that predicts that a similar pattern of extinctions occurred around the Silurian world, in regions of similar palaeotectonic regime, and that global extinction patterns of clades, between episodes of mass extinction, were not random. Episodes of high extinction (turnover pulses) were caused by perturbations of the environment, foremost of which were rapid changes in the depth of the seas on the continental shelves. Because taxonomic loss during these extinction/emigration episodes was generally at low (below generic) levels, resolution of Silurian turnover patterns requires data collected at the evolutionary species level.