Silurian trilobite alpha diversity and the end-Ordovician mass extinction

Following the end-Ordovician extinction, global clade diversity of Silurian trilobites dropped to about half of Ordovician levels. Although clade diversity failed to recover, this extinc-tion had surprisingly little long-term impact on the number of trilobite species that occupied local habitats (alpha diversity). A new compilation of data from Laurentia and other continents indicates that Silurian trilobite alpha diversities in all major environments were comparable to those of the Late Cambrian and Ordovician; shallow subtidal diversity reached an all-time high during the Late Ordovician. The profound differences in patterns at local and global levels demonstrate the neces-sity for a hierarchical approach to analyses of diversity. Factors governing global clade diversity are lodged at hierarchical levels beyond those controlling local species richness and must be sought in studies of between-habitat (beta) or geographic (gamma) diversity.