Global biodiversity dynamics result in large-scale patterns of change in the variety of life on earth. One deceptively simple measure of global biodiversity is the total count of species or supraspecific taxa that have been described from each subinterval of the Phanerozoic record. These tallies usually are obtained from data compilations based on extensive literature searches supplemented with examination of collections in museums (Sepkoski 1993; Alroy et al. 2001). Such measures of global biodiversity dynamics are probably not meaningless, but it is difficult to understand what they mean. The number of taxa alive on Earth at any one time is a function of innumerable ecological factors acting at the full range of temporal and spatial scales (Willis and Whittaker 2002). Changes in global richness might result from increasing local richness as local communities become more complex (Valentine 1969; Bambach 1977). At the regional scale, changing oceanographic conditions, climate, and the relative positions of continents and shallow seas might alter global taxonomic diversity by changing the size and arrangement of biogeographic units (Valentine 1970). Therefore, the best hope for understanding global diversity patterns is to collect information on local assemblages. These hierarchically organized data can then be integrated to test alternate hypothetical mechanisms of change. Data compiled as the endpoints of stratigraphic or geographic ranges are useless for this purpose. This has been known at least since Bambach (1977) explored patterns of change in alpha diversity of level-bottom marine communities.