Biodiversity consists of two components: richness (number of taxa) and evenness (distribution of individuals). In random samples, evenness can be well estimated but richness estimates are confounded by evenness and other parameters. Therefore, questions concerning evenness are quantitatively attractive and may offer new insights into community evolution.

Genus abundance data were collected in a field study of Middle Cambrian to Upper Ordovician subtidal macrobenthic marine assemblages. To date, a total of 22,931 specimens in 65 samples from 37 locations in North America have been analyzed. During the Lower Ordovician, mean community evenness in these samples increases from Cambrian values of ~0.4 to Ordovician values of ~0.7. The difference is statistically significant using several parametric and non-parametric tests (p < 0.001). The increase in evenness cannot be explained by lithology, depth relative to storm wave base, or collecting method. Increase in assemblage-level richness is difficult to evaluate because most of the variance in richness can be explained by sample size and evenness. Thus, an increase in assemblage richness can neither be supported nor ruled out by these data.

The transition between Sepkoski’s Cambrian and Paleozoic evolutionary faunas in the assemblage data exhibits a pattern similar to his global compilation of genus richness. Lower Ordovician assemblages do not resemble those of the Cambrian or later Ordovician and the Cm and Pz faunas each comprise ~50% of the taxa and individuals in the collections. Thus, while evenness may increase abruptly, the transition between the Cm and Pz faunas appears more protracted.