FAUNAL COMPOSITION AND DISTRIBUTION ACROSS THE ORDOVICIAN-SILURIAN BOUNDARY IN OHIO AND ONTARIO

Although the global nature of the Late Ordovician mass extinction has been well documented, there remains, with a few notable exceptions, a relative paucity of community-level analyses of faunal transitions across the Ordovician-Silurian boundary. In this study, the composition and distribution of the preserved fauna has been compared on either side of the Ordovician-Silurian boundary in eastern North America, and is interpreted with respect to sequence stratigraphic architecture. In particular, this research seeks to test and describe the change or consistency in faunal dominance, abundance, diversity, and trophic structure from the Upper Ordovician to the Lower Silurian within similar depositional environments in Ohio and Ontario. This study is focused, at high stratigraphic resolution, on the Upper Ordovician (Richmondian) Drakes Formation of Ohio and the Lower Silurian (Llandovery, Rhuddanian) Cabot Head Formation of Ontario. Also included, though to a lesser extent, are the Upper Ordovician Georgian Bay Formation of Ontario and the Lower Silurian Brassfield Formation of Ohio. Each of these units exhibits a shallow marine succession dominated by nearly barren gray shales and packstones varying from modest to high diversity in the lower to mid-portions of the section. Although the degree of paleoenvironmental change varies among sections, each becomes more marginal in the upper portions, with increased dominance of low diversity assemblages in red shales and silty limestones. Preliminary results from multivariate analyses of abundance data from the Drakes Formation and the lower Cabot Head Formation indicate little overall change in faunal composition, with the exception of variation in the relative abundances of orthid versus strophomenid brachiopods, and a relative increase in taxonomic richness from the Drakes to the lower Cabot Head.