COMMUNITY-LEVEL DIVERSITY/ABUNDANCE TRENDS ACROSS THE LOWER/MIDDLE ORDOVICIAN BOUNDARY IN THE GREAT BASIN: FIELD-BASED APPROACHES TO THE RECOGNITION AND QUANTIFICATION OF PALEOECOLOGICAL CHANGE

The Ordovician period saw a nearly threefold increase in the family-level diversity of marine invertebrates and the rise of the "Paleozoic Fauna", which came to dominate marine ecosystems for the remainder of the Paleozoic. Though taxonomic patterns of diversification are relatively well worked out for this interval, there is little understanding of how this increase in diversity was accommodated at the community level. An analysis of community-level changes must incorporate diversity and abundance data, as well as detailed information on depositional environments. Preliminary analysis of wackestone collections from the Great Basin indicates that there is a dramatic shift in the abundance structure of faunas of the Bathyurid biofacies across the critical Ibexian-Whiterockian boundary. The boundary marks a transition from "Cambrian-type" (trilobite-dominated) faunal associations to "Paleozoic-type" (articulate brachiopod-dominated) associations. Interestingly, this shift does not result in a corresponding change in average evenness: trilobite and brachiopod faunas are characterized by high dominance indices in both upper Ibexian and lower Whiterockian collections. Despite dramatic changes in clade abundance, these communities appear to have similar overall diversity-abundance structures. The measurement of abundance is an important part of modern ecological research, but this information is often omitted or only qualitatively treated in paleoecological studies. There are numerous reasons for this bias: abundance data is generally not available from older literature sources, and field collection of this data can be extremely time-consuming. However, when coupled with alpha and beta diversity data, indices of evenness and dominance provide a potentially powerful tool for unraveling the ecological signature of major biotic transitions. Because ecological patterns are overprinted by physical transport, time-averaging, and taphonomy, and are closely linked to fluctuations in relative sea level, care must be taken in the collection of abundance data. It is essential to collect and analyze samples from like facies.