Paleoecology of an Upper Ordovician submarine cave-dwelling bryozoan fauna and its exposed equivalents in northern Kentucky, USA

摘要

A bryozoan-dominated fauna that inhabited small caves underneath a carbonate hardground is here described from the Corryville Formation (Upper Ordovician, Katian) exposed near Washington, Mason County, Kentucky, USA. The dominant bryozoan, Stigmatella personata (a trepostome), is found both growing downwards from the cave ceilings and upwards on the exposed hardground surface above. Another trepostome, 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. The colonies of S. personata are rounded mounds with 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 the patches, trapping sediment within the skeleton. The bryozoan zoaria and the carbonate hardground are extensively bored by the cylindrical ichnogenus Trypanites that occasionally contain cylindrical calcite-filled tubes similar to “ghosts” of organic materials. Bioclaustrations are present in some of the bryozoan skeletons. This 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.