Abstract: While variability in body size and form of the brachiopods and bivalves of the Cincinnatian Series has been noted for over a century, few studies have placed this observed variablity into an environmental/stratigraphic framework. The purpose of this study was to link body size and morphological change in two genera of fossil brachiopods and one fossil bivalve genus to the changing facies in the regressive packages of the Cincinnatian Series.

The small orthid brachiopod genus Onniella was the most severely affected by changing environments. It was found in tremendous, rock-forming abundance in its preferred environment: the muddy, quiet, offshore facies of the Kope and Waynesville Formations. However, Onniella is rare or absent from other facies of the Cincinnatian Series.

The large strophomenid brachiopod genus Rafinesquina is responsive to changing environments. While no recognizable trend was observed in shell size, the observed frequency of shell geniculation in Rafinesquina varies by facies. In offshore facies, frequencies of geniculation are very low, while in more energetic, onshore facies, rates of geniculation can be as high as 85%. As previous authors have suggested, this pattern likely represents an ecophenotypic response of Rafinesquina to changing energy regimes.

Quantitative analysis of shell size variation of the epibyssate bivalve Ambonychia collected from the major facies of the Cincinnatian Series shows the most interesting pattern of all. Although previous authors have noted changes in Ambonychia shell size, those studies have focused on changes between two, or at most, three depositional facies of the Cincinnatian Series, and have not examined the overall change of Ambonychia shell size throughout the changing facies of the Cincinnatian Series. This overall pattern of shell size changes correlates very well with published sea level curves for the Cincinnatian Series, strongly implying that the variations in shell size for Ambonychia is either genetically or causally related to changes in sea level.