DEPTH-RELATED BRYOZOAN ASSOCIATIONS WITHIN THE C5 SEQUENCE (LATE ORDOVICIAN, RICHMONDIAN) OF THE CINCINNATI ARCH

Trepostome bryozoans constitute a large proportion of type-Richmondian paleocommunities, but quantitative information on their relative abundances in depth-related facies is poorly known. To better understand changes in abundance of trepostome genera along water depth gradients, 233 acetate peels were made on ten collections of ramose, frondose, and bifoliate trepostomes from the C5 sequence in southwestern Ohio and southeastern Indiana, primarily from the South Gate Hill exposure. Diversity and evenness of trepostomes is greater in the offshore Waynesville Formation than in the deep subtidal Liberty Formation and the shallow subtidal Whitewater Formation. Although many genera occur across the entire depth gradient, or nearly so, most are abundant in only a portion of the gradient. The offshore is dominated, in roughly even proportions, by Parvohallopora, Batostomella, Heterotrypa, Dekayia, Homotrypa, and Amplexopora. The deep subtidal is dominated by Homotrypa, but also contains common Parvohallopora and Rhombotrypa. The shallow subtidal is dominated by Parvohallopora and Homotrypa, with individual collections strongly dominated by one genus or the other. These results help to place trepostome bryozoans in context with previous gradient analyses of other invertebrate taxa. Because individual collections from a single facies may vary substantially in the abundance of individual genera (as is true for other invertebrate taxa), more collections will be needed to more firmly understand the distribution of bryozoan genera along environmental gradients. Additional work is also needed on other growth forms, including massive and encrusting forms, which can be locally abundant. Finally, additional work is needed to understand the consistency of these distributions in other depositional sequences of the type Cincinnatian.