SEDIMENT AVOIDANCE AND ESCAPE TRACES ASSOCIATED WITH RAFINESQUINA ALTERNATA, AN UPPER ORDOVICIAN STROPHOMENID BRACHIOPOD FROM THE CINCINNATI REGION, OHIO AND KENTUCKY

As with other strophomenids, Rafinesquina has been interpreted as living unattached and immobile on its convex valve. Newly discovered sediment avoidance and escape traces challenge this interpretation.

Ditch-like depressions, about 5 mm wide, in the sediments adjacent to the commissure are associated with Rafinesquina specimens that lay flat, convex side up on upper bedding surfaces at several distinct Cincinnatian horizons and localities. These “moats” are interpreted as trace fossils, presumably excavated with water expelled by valve snapping.

Other Rafinesquina specimens are associated with structures termed “rotational tracks”. These brachiopods are inclined from nearly horizontal and convex up, to nearly vertical, hinge line down and commissure up. Spreiten beneath the brachial valve trace an up-arcing path of the anterior shell margin about a fulcrum at the posterior hinge line. Rotational tracks are also interpreted as trace fossils, recording movement from an initial position buried horizontally within skeletal sand.

Orientations of Rafinesquina on a bedding plane demonstrate that these are not sediment-scour structures; while association between moats and commissures is precise, distribution of brachiopod azimuths is pandirectional and loosely bimodal NE-SW, perpendicular to NW flowing currents indicated by cross bedding. Development of moats and rotational structures is independent of azimuth.

These traces form a continuum. Specimens lying flat on the bed surface have moats with no trace of rotation; inclination is associated with deeper burial and a greater arc of rotation. “Moats” are outlined on the surfaces of undisturbed sediments at the base of rotational tracks where brachiopods presumably came to rest during depositional events before initiating rotation.

Continuous form suggests a single cause; if moats formed by valve snapping, then rotational tracks may have formed by a similar process. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar trace-making abilities, then other strophomenates probably shared these abilities. This mode of escape provides a functional role for the concavo-convex shape. Traces also indicate live burial of brachiopods with epibionts, a potential aid to analysis of strophomenid life habits.