Relative abundance of Sepkoski's evolutionary faunas in Cambrian–Ordovician deep subtidal environments in North America

Abstract

The relative proportions of Sepkoski's Cambrian, Paleozoic, and Modern evolutionary faunas in Cambrian-Ordovician benthic marine assemblages from mixed carbonate-shale and shale lithofacies deposited below normal wave base (herein, deep subtidal) in North America are strongly positively correlated with global relative genus richness in Sepkoski's global compendium. The correlation between local and global faunal proportions is robust regardless of how propor- tions are calculated, including when local proportions are based on number of specimens. Like the global pattern, the transition between the Cambrian and Paleozoic evolutionary faunas appears to occur gradually, in that Lower Arenigian (Ibexian) deep subtidal assemblages contain approxi- mately equal proportions of Cambrian and Paleozoic faunal elements. In agreement with previous work, an onshore-offshore differentiation of faunas is evident both within Ordovician deep subtidal communities and across a larger environmental gradient. Within the deep subtidal assemblages studied here, the Paleozoic fauna tends to have a greater proportion of individuals for a given proportion of genera than the Cambrian fauna, although both tend to accrue genera at similar rates with increasing relative abundance. The Modern evolutionary fauna appears to accrue genera more rapidly with increasing local relative abundance. The extent to which these differences reflect ecological factors such as biomass, metabolic requirements or larval recruitment patterns, taxonomic practices stemming from variable morphospace saturation, or taphonomy-related counting biases remains unclear, but it suggests the possibility that Sepko- ski's evolutionary faunas may share ecological characteristics that influence both local relative abundance and global rates of taxonomic evolution.