MODEL OF A BIOTIC HARD SUBSTRATE COMMUNITY: PALEOECOLOGY OF LARGE TREPOSTOME BRYOZOANS FROM THE UPPER ORDOVICIAN (KATIAN) OF THE CINCINNATI REGION, USA

The calcite skeletons of trepostome bryozoan colonies from the Upper Ordovician (Katian) of the Cincinnati region record the diverse interactions and growth responses these colonies experienced. Trepostome specimens from two Cincinnatian strata, the Bellevue Member and the Whitewater Formation, were studied. These strata were deposited in a shallow epicontinental sea environment that was located in the southern subtropics, approximately 20-23°S at the time of deposition. The trepostomes that lived in this environment were often bored by other organisms to create domiciles for the boring organisms. These borings are preserved in the zoaria and filled with sediment and calcite cement. The borings and internal morphology of these trepostomes were investigated by sectioning the colonies and making acetate peels, which were then microscopically examined. Several different sediment generations were found in the borings. The first generation of sediment was deposited while the boring was occupied, filling the space around the boring or nestling organism. After the occupant died, its external mold was preserved by this sediment. A second generation of sediment then partially filled the external mold. Subsequently, the bryozoan overgrew the boring opening, preserving a calcite cast or "ghost" of the occupant. Disturbances like these borings and bioclaustrations (embedment structures) often initiated distinct growth responses in their host bryozoan skeletons. These responses allowed the bryozoan to close the cavity in their skeleton, restore feeding surfaces, and resume normal growth. This was done in one of three ways: (1) Zooids surrounding the cavity growing upwards and angling inwards, creating a “tent” with the cavity closed off; (2) Zooids growing laterally over the cavity opening, sealing it off with a flat “roof”; and (3) Zooids budding down into the cavity then angling upwards, filling in all open space and resuming a feeding surface above. Work continues to interpret budding processes used to repair feeding surfaces as well as the source and chronology of sediments that filled borings and their implications for trepostome life modes.