Biomere boundaries re-examined

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Abstract
Analysis of closely spaced and rich collections of trilobites across the base of the Pterocephaliid biomere shows a distinctive pattern of changes in relative abundance of trilobites and in trilobite diversity. These patterns are repeated at the base and top of the overlying Ptychaspid biomere, but they are near the basal boundaries of, or within, the Irvingella major and Corbinia apopsis Subzones respectively, rather than at the tops of these subzones. Thus, biomere boundaries are not consistently reflected by biostratigraphic boundaries of uniform importance in the current North American trilobite assemblage zone stratigraphy. The positions of the lower and upper boundaries of the Ptychaspid biomere are here revised downward to the bases of the I. major and C. apopsis subzones. As a consequence, they are now consistent with major changes in inarticulate brachiopod and conodont faunas. The evolutionary sequence within a biomere now commences with a brief ‘crisis’ period during which rare elements of the soon-to-be-dominant new fauna are associated with opportunistic bursts of both new and old faunal elements. The takeover and gradual radiation of the new dominants follows this crisis period. Analysis of regional stratigraphy and of estimated radiometric ages for the boundaries of Cambrian Series, permits speculation on the abruptness of biomere boundary events and their frequency of recurrence. The event at the base of the Pterocephaliid biomere probably took place in less than 6,000 years and the next event marking the top of the Pterocephaliid biomere took place between 5 and 7.5 million years later.

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