COLLABORATIVE RESEARCH: The Ecology of the Ordovician Radiation: Evidence from the Basin and Range Province of the Western U.S.A.

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9706736 Sheehan The Ordovician radiation was one of the largest radiations in the history of life. Current understanding of the Ordovician radiation is based largely on taxonomic studies of 'individual groups and on synthetic literature compilations. This project will be the first field-based examination of the ecology of the Ordovician radiation. During the Ordovician radiation the Cambrian Evolutionary Fauna was replaced by the Paleozoic Evolutionary Fauna. Groups, such as brachiopods, bryozoans, rugose and tabulate corals, echinoderms, stromatoporaids, and cephalopods rose to dominance. Marine family diversity tripled, reflecting vastly increased morphological diversity in the oceans. Community complexity increased significantly as many new guilds were established. In this study, four basic questions will be addressed: 1) How do the communities develop? Are community level changes gradual, sudden (geologically), stepped, or some combination? 2) Do different clades expand in concert within communities or do they just appear to be related when viewed in compilations on large-time scales? 3 ) Is the Cambrian fauna actually displaced during community development? Or is it still present m the communities and simply diluted (Westrop et al., 1995)? 4) How did community complexity increase? For example, are new communities added to the ecosystem or are clades simply added to existing communities? This project will examine carbonate platform deposits preserved in the Great Basin. This region has one of the finest records of shallow marine faunas in the Ordovician anywhere in the world, making it an ideal locale to conduct this study. The Pls have conducted preliminary work for this proposal and also have extensive experience studying Ordovician rocks and faunas of this region Field work will be conducted in the Cherry Creek Range, the southern Egan Range, and the southern Hot Creek Range in Nevada and 'm the Bear River Range and the Ibex region in Utah. T'he focus will be on shallow-marine soft-substrate communities preserved in wackestone and limemudstone facies. This environment has been chosen for several reasons. Reworking of shells is minimal in these facies. By confining the study to one general environment, rather than a complete bathymetric transect, the complications that would result from onshore-offshore changes that took place during the radiation can be avoided. Preliminary work suggests that communities were reorganized at least 5 times during the Ordovician. Six stratigraphic intervals, lower, middle and upper Ibexian, lower and middle Whiterockian, and Mohawkian-Cincinnatian appear to have distinctive community-types that are increasingly diverse and complex. Preliminary evidence also suggests that the reorganizations of communities may have occurred quickly at the beginning of each interval. If reorganizations were rapid, communities may have been stable with little change occurring during the bulk of these five intervals.