Biodiversity has two principal components: richness (the number of taxa) and evenness (the distribution of individuals among taxa). Both of these attributes are critical in defining community composition and structure, but evenness may be particularly important for ecological reasons, especially in the context of a possible evolutionary increase in richness. Here I examine abundance data in well-preserved Cambrian and Ordovician benthic marine assemblages from mixed carbonate-shale and shale lithofacies deposited below normal wave base in North America. Evenness increases significantly from the Cambrian to the Ordovician in these assemblage data. The increase cannot easily be attributed to differences in sample size, lithology, water depth, or other sample characteristics. There is relatively little variation in evenness among stages within the Cambrian and Ordovician periods. Much of the within-period variance in evenness appears to arise from environmental and/or taphonomic differences that may exist between stratigraphic formations, but some variance may also be ascribed to the degree of taxonomic and/or ecological overlap among dominant taxa. Specifically, assemblages that are co-dominated by taxa from the same class or order tend to have lower evenness than assemblages dominated by genera from different higher taxa, but the effect is not strong in these data.
The Cambrian and Paleozoic evolutionary faunas show similar, but out-of-phase, patterns of evenness within assemblages. Both faunas have comparably low evenness early in their history but then increase to similar, higher evenness values. In the case of the Cambrian fauna, the increase occurs in the Late Cambrian or Early Ordovician. In the Paleozoic fauna, the increase appears to occur after the lower Arenigian.