PALEOECOLOGY OF AN UPPER ORDOVICIAN SUBMARINE CAVE-DWELLING FAUNA IN NORTHERN KENTUCKY

A bryozoan-dominated fauna that inhabited small caves underneath a carbonate hardground is here described from the Corryville Member of the Grant Lake Formation (Upper Ordovician, Katian) exposed near Washington, Mason County, Kentucky. The dominant bryozoan, Stigmatella personata (a trepostome), is found both growing downwards from the cave ceilings and upwards on the exposed hardground surface. Another trepostome bryozoan, Monticulipora, is a minor component of the cave fauna. There are few discernible anatomical differences between the bryozoan colonies that grew upwards in presumably well-lit waters and those that grew downwards in the gloomy caves. The pendant, cave-dwelling S. personata in some cases appears to have longer zooecial tubes than its exposed equivalent. In both conditions the bryozoans formed rounded mounds of multiple overgrowing colonies, sometimes showing changes in growth direction controlled by microenvironmental effects. The large colonies of S. personata have multiple layers formed by self-overgrowth. The overgrowths in both downward and upward growing forms are marked by thin layers of sediment infilling the upper zooecial chambers in the older portion of the colony. We suggest that biofilms developed on patches of the colony where the zooids had died. Sediment adhered to these surfaces and the colony then overgrew on top. The bryozoan skeletons and the carbonate hardground are extensively bored by the cylindrical ichnogenus Trypanites. There are two forms of Trypanites: a smaller version within one colony overgrowth, and a larger type that penetrates through multiple bryozoans, often passing through into the hardground supporting them. These larger borings are filled with biomicrite and cylindrical calcite-filled tubes similar to previously-described “ghosts” of organic materials possibly left by the boring organisms. Both types of borings preferentially penetrated along zooecial walls, suggesting that the bryozoan skeletons were not yet infilled with calcite cement. Bioclaustrations are present in some of the bryozoan skeletons. This Katian cave fauna is one of few submarine examples known from the Paleozoic. It supports the hypothesis that early cave-dwelling organisms were little differentiated from their exposed counterparts.