Quantitative analysis of epizoans on Silurian stromatoporoids within the Brassfield Formation

Stromatoporoids from the Brassfield Formation (Early Silurian, Llandovery) near Fairborn, Ohio, provided substrata for a diverse epizoic community. The stromatoporoids were colonized by at least 28 taxa, including bryozoans, cnidarians, echinoderms, annelids, and endolithic organisms. Analysis of the occurrence, diversity, distribution, and coverage of the epizoans recognized patterns of site-selective attachment on the surface of the stromatoporoids. The upper surface of the stromatoporoids had a slightly higher total epizoan coverage than the lower surface. Concentric zones outlined from the margin also had differences in area and percent coverage. On the upper and lower surfaces, epizoan occurrence decreased from the margin to the interior. Epizoan coverage for the lower surface also decreased inward. On the upper surface, the middle zone had the highest coverage followed by the outer and polar zones, respectively. Among the major invertebrate phyla, only the echinoderms were randomly distributed. Bryozoans, cnidarians, and miscellaneous taxa including borers, cornulitids, and spirorbids were distributed nonrandomly on the surface of the stromatoporoids. Over 95 percent of the stromatoporoid area analyzed is not covered by epizoans. However, epizoans are commonly clumped or in direct contact with one another. These physical interactions are the result of site-selective attachment by epizoans in close proximity rather than competition for available space.