Evenness, richness and the Cambrian-Paleozoic faunal transition in North America: An assemblage-level perspective (Utah).

摘要

Biodiversity has two components: richness and evenness. Although both components of biodiversity are important indicators of a variety of evolutionary and ecological processes, relatively little is known about patterns of evenness in the fossil record. Here, I examine evenness and richness in well-preserved Cambrian and Ordovician benthic marine assemblages from mixed carbonate-siliciclastic and fine-grained siliciclastic lithofacies deposited below normal wave base in North America.; In Chapter 1, I explore the sampling properties of evenness measures and propose a new measure of evenness, Ess. This measure is based on taxon proportions and can be estimated with considerable accuracy on the basis of incomplete samples.; In Chapter 2, I present a taphonomic and taxonomic treatment of the Weeks Formation (Marjuman) of Utah. Three new genera and two new trilobite species are described.; In Chapter 3, I compare relative genus richness of the evolutionary faunas in Sepkoski's global compilation to their relative genus richness and relative abundance in local assemblages. There is strong agreement between the proportions of the three faunas globally and locally and the overall temporal pattern of the transition between the Cambrian and Paleozoic faunas is also consistent.; In Chapter 4, I document a significant increase in the evenness of benthic marine communities from the Cambrian to the Ordovician. There is relatively little age-to-age variation in mean assemblage evenness within the same period but there is substantial variance among assemblages. Most of this can be attributed to environmental and/or taphonomic differences between formations. The Cambrian and Paleozoic faunas have similar temporal patterns of evenness but they are out of phase with one another. The Cambrian fauna is the first to achieve Ordovician-like evenness values in the Lower Ordovician.; In chapter 5, I find benthic community richness to increase approximately three-fold from the Cambrian to the Ordovician in raw and rarefied richness data. When the mutual correlations between richness, evenness, sample size, and time are considered in a partial correlation analysis, there is strong evidence for a true increase in assemblage-level richness. However, the magnitude of the increase is substantially smaller than suggested by richness estimates derived from rarefaction.