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GENUS RICHNESS IN CAMBRIAN-ORDOVICIAN BENTHIC MARINE COMMUNITIES IN NORTH AMERICA

¹ Department of Geological Sciences and Museum of Paleontology, University of Michigan, 1109 Geddes Rd., Ann Arbor, Michigan 48109, USA shananp{at}umich.edu

The causes of global biological diversification and the nature of ecological change during taxonomic radiations are central questions in paleobiology. It has long been recognized, however, that apparent increases in global taxonomic richness need not be mirrored in local communities. Here, using field data and literature sources from Laurentia, it is shown that genus richness in well-preserved subtidal macrobenthic marine communities increased by as much as a factor of two from the Middle Cambrian to the Late Ordovician. Several potential sources of bias have been addressed in this study, including taphonomic effects, water depth, lithology, sample size, and differences in the relative numerical abundance of taxa (evenness). Excluding samples from environments that may have been oxygen stressed and omitting samples that closely follow Cambrian biomere extinction intervals result in much less complete temporal sampling and significantly influence apparent short-term patterns of biodiversity, but do not change fundamentally the overall richness and evenness trajectories. The apparent magnitude and timing of the increases in local richness and evenness, however, is sensitive to such factors. The overall richness trajectory is also sensitive to how evenness is treated quantitatively. If evenness is assumed to represent a biological positive correlate of richness, then rarefaction is most appropriate and suggests an Early Ordovician richness increase. If evenness is treated strictly as a bias in richness estimates, then most of the richness increase appears to have occurred by the late Cambrian (Marjuman), with only a modest increase after the Arenigian.

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