title: Post-cambrian trilobite diversity and evolutionary faunas
abstract: A cluster analysis of the stratigraphic distribution of all Ordovician trilobite families, based on a comprehensive taxonomic database, identified two major faunas with disjunct temporal diversity trends. The Ibex Fauna behaved as a cohort, declining through the Ordovician and disappearing at the end-Ordovician mass extinction. In contrast, the Whiterock Fauna radiated rapidly during the Middle Ordovician and gave rise to all post-Ordovician trilobite diversity. Its pattern of diversification matches that of the Paleozoic Evolutionary Fauna; hence, trilobites were active participants in the great Ordovician radiations. Extinction patterns at the end of the Ordovician are related to clade size: Surviving trilobite families show higher genus diversity than extinguished families.
authors.affiliation: J. M. Adrain and R. A. Fortey, Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK. S. R. Westrop, Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman, OK.
authors.initials: JM
authors.lastname: Adrain
authors.initials: RA
authors.lastname: Fortey
authors.initials: SR
authors.lastname: Westrop