A REVISED HISTORY OF TRILOBITA

A comprehensive species database of trilobites permits two new descriptions of the evolutionary history of the group. First, a temporally corrected species sampling curve based on 37 Paleozoic stratigraphic intervals reveals several features not previously apparent on curves derived from higher taxa (families and genera). Trilobites reached their peak species diversity much earlier during the Cambrian than previously appreciated. By the Late Cambrian, diversity had declined nearly to levels typical of the remainder of the Early Paleozoic. Further, the post-Cambrian peak in species diversity occurred during the Early Devonian, at which point trilobites were nearly as globally diverse as during the Late Cambrian. Second, hierarchical cluster analysis and ordination of all 165 trilobite families according to their species diversity in each of the 37 intervals reveals associations explained best by extremely tight temporal control, combined with a strong geographic overprint during the Cambrian. Such associations have been recognized in the Late Cambrian of Laurentia as Palmer's "biomeres," but the results of clustering and ordination demonstrate that similarly temporally restricted associations are a global phenomenon during the entire Cambrian, even if they are not mass extinction-bounded as in Laurentia. Previous work parsing post-Cambrian families into an "Ibex Fauna" which largely became extinct during the end-Ordovician mass extinction, and a "Whiterock Fauna" which diversified and participated in the Ordovician radiation, is confirmed. A new association within the Whiterock Fauna includes those families with significant Devonian and Late Paleozoic diversity.