RECOGNIZED GLOBALLY, MANIFESTED LOCALLY: SEPKOSKI’S EVOLUTIONARY FAUNAS IN NORTH AMERICAN CAMBRIAN-ORDOVICIAN BENTHIC ASSEMBLAGES

The Cambrian, Paleozoic, and Modern evolutionary faunas are important features of the global Phanerozoic marine fossil record first recognized by Sepkoski as groups of higher taxa with covarying richness patterns. The evolutionary faunas have long been recognized as ecologically relevant aspects of the global fossil record, but relatively little work has quantitatively compared community-level estimates of the relative abundance of the three faunas to those derived from global compilations. Based on field collections from 30 formations in North America and a limited number of literature-derived assemblages, I show that well-preserved, Cambrian-Ordovician benthic assemblages occupying deep subtidal, mixed carbonate-shale and shale lithofacies exhibit faunal proportions that are congruent with relative genus richness in Sepkoski’s compilation. The correlation between local and global faunal proportions is strong regardless of how relative genus richness is tabulated and whether relative faunal abundance in assemblages is calculated as the proportion of genera or the proportion of individuals.

The transition between the Cambrian and Paleozoic faunas appears to occur gradually during the Ordovician. Lower Ordovician assemblages contain approximately equal proportions of Cm and Pz faunal elements in the target environment. In agreement with previous work, these assemblage data support a broad onshore-offshore differentiation of community types in the Ordovician. Shallow-water, nearshore environments exhibit a higher proportion of Modern faunal elements than do deep subtidal assemblages. Corroboration of an onshore-offshore distinction in faunal composition, combined with a strong correlation of evolutionary faunal proportions between deep subtidal assemblages and Sepkoski’s global compilation, suggests that the deep subtidal zone may contain the majority of known Cambrian and Ordovician genera. Alternatively, Sepkoski’s data may preferentially represent this depth zone in North America. Regardless of the reason for the good correspondence, relative genus richness of the evolutionary faunas in Sepkoski’s global compilation is a good predictor of the composition of deep subtidal assemblages in the Cambrian and Ordovician of North America.