Existing reconstructions of the Phanerozoic history of biological diversity are based on occurrences of taxa and do not include abundance data. This study examines trends in the relative abundance of taxa and provides a new perspective on the Phanerozoic history of marine diversity. We compared the abundance versus diversity (AD) structure of early Paleozoic and Cenozoic samples of marine benthic genera compiled from the literature. The sample-standardized results indicate (1) a two-and-a-half–fold increase in sampled alpha diversity but, concurrently, (2) an increase in the evenness of fossil samples through time. The observed evenness trend may be due to taphonomic biases, time-averaging trends, differences in sampling techniques, or real changes in the structure of marine communities. Regardless of the underlying factors, the results indicate that the evenness of fossil samples increased through the Phanerozoic, largely owing to a decrease in the relative abundance of the most common taxa. Because sampling fossil sites of different evenness can result in different estimates of species richness even if their total alpha diversity is the same, the observed changes in evenness may have contributed to the Phanerozoic increase in sampled alpha diversity and, at least in part, to the increase in global diversity. The incorporation of taxon abundance data into future studies should produce a more robust, perhaps even different, interpretation of the Phanerozoic history of diversity.