Using a field analysis of Upper Ordovician mollusc-rich faunas of the Cincinnati Arch, this study tests whether the large-scale patterns of Ordovician gastropods and bivalves observed in a companion study are maintained at the finer scales of individual strata and localities, and when utilizing abundance data in addition to taxonomic richness. Non-metric multidimensional scaling and several statistical analyses show that the taxonomic richness and abundance of these classes within samples were significantly negatively correlated, such that bivalve-rich settings were only sparsely inhabited by gastropods and vice versa. There also were important environmental differences between these classes. Gastropods were most dominant in shallow, carbonate-rich, and generally low-turbidity settings. Gastropods also occurred in restricted lagoons, where bivalves were only minor elements. In contrast, bivalves were most dominant in deep subtidal, siliciclastic shales with high levels of turbidity. Both in terms of abundance and taxonomic richness, these results strongly support those observed at the larger scales of paleocontinents and the globe. Taken together, these results argue that, despite similar taxonomical diversification patterns of these classes at the global scale and heterogeneous patterns among paleocontinents and among regions within Laurentia, gastropods and bivalves had quite different, yet unchanging, environmental distributions throughout the Ordovician, and that these classes did not co-occur to a significant degree, either in terms of taxonomic richness or abundance.
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Vol. 18 • No. 6