EVENNESS AND DIVERSITY OF LOWER PALEOZOIC TRILOBITE FAUNAS OF LAURENTIAN NORTH AMERICA

Evenness is attracting increasing attention as a parameter in paleoecological studies of diversity. Here, we examine trends in evenness in shelf trilobite faunas of Laurentia through the Lower Paleozoic. This interval includes the Ordovician Radiation, which saw a profound reorganization of marine paleocommunities. The data set is expanded and revised from our previous analyses of trilobites diversity (Westrop and Adrain, 1998, Paleobiology, v. 24, pp. 1-16; Adrain et al., 2000, Paleobiology v. 26, pp. 625-646) and comprises more than 130 collections with a total sample size of more than 35,000 individuals. The collections span an interval from the Late Cambrian (Marjuman) to the Late Silurian (Ludlow). There is no significant difference between Late Cambrian and Ordovician shallow subtidal (between fair-weather and storm wave bases) collections in both evenness and species richness. A small but significant decline occurs in the Silurian, although this is driven largely by collections from the Wenlock and Ludlow. In contrast, deep subtidal (below storm wave base) settings show significant increases in both evenness and richness between the Late Cambrian and Silurian (Wenlock-Ludlow). Cambrian stage (“biomere”) boundary extinctions of trilobites are marked by sharp declines in both evenness and richness that persist into initial phases of recovery. Strongly uneven trilobite assemblages with low species richness also characterize some stressed environments, including peritidal carbonates and deep, dysoxic basinal settings. Overall, trends in evenness parallel those for species richness and provide no support for a major decline in trilobite diversity as a consequence of the Ordovician Radiation.