RINGS AROUND THE RAFINESQUINA: NEW EVIDENCE FOR LIFE POSITION AND ACTIVITY OF AN ORDOVICIAN STROPHOMENID BRACHIOPOD

The thin, concavo-convex shells of Ordovician strophomenid brachiopods such as Rafinesquina and Strophomena have often been considered as optimal morphologies for a “snowshoe” mode of life on soft substrata, with the convex valve downward. However, strophomenids in the Upper Ordovician of the Cincinnati Arch region are rarely found in this presumed life position, but occur predominantly as pavements of imbricated convex-upward shells, presumably produced by storm winnowing of fine sediments and reorientation of shells into hydrodynamically stable positions. Newly reported circum-commissural gutters or “moats” with some Rafinesquina on a pavement from the Maysvillian Corryville Formation of Kentucky suggest that some strophomenids survived in the convex-upward shell orientation. Moats are narrow depressions in the fine-grained matrix of the packstone pavement that flank the shell commissure but not the hingeline. In one case, well-preserved edrioasteroids are attached along the commissure in life position but within a moat, indicating that the moat is a primary sedimentary feature, not the result of compaction. Moated brachiopods appear to be embedded in the substrate. I propose that moats formed as a result of scour by exhalent feeding currents or valve-snapping activity. Petrographic evidence for early marine cementation suggested that this shell-supported packstone was a firmground or incipient hardground. Given such a cohesive substratum, concavo-convex strophomenids could survive in either convex valve-downward or upward orientations. Coupled with the wide stratigraphic distribution of strophomenid shell pavements in the Cincinnatian Series, moat formation provides further evidence for the generalistic ecology of strophomenids, as well as a new mode of brachiopod-substrate interaction.