Escape traces associated with Rafinesquina alternata, an Upper Ordovician strophomenid brachiopod from the Cincinnati Arch Region

Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone bedding surfaces in the Cincinnati Ordovician, are sometimes associated with moats, which are sediment depressions or gutters ∼5-mm wide surrounding the commissure. Moats are interpreted as trace fossils, excavated by water expelled as the valves snapped shut. Other specimens vary from nearly horizontal to nearly vertical with the hinge line down and commissure up. Meniscate backfill beneath the anterior shell margin traces an arcing path formed as the shell rotated upwards around the posterior hinge line. Rotational tracks are interpreted as trace fossils, recording movement from an initial position buried horizontally to an inclined position as the brachiopod tried to escape burial. The traces form a continuum. Specimens lying flat on the bed surface have moats but no rotational tracks. Inclined shells are associated with deeper burial by obrution events and a greater arc of rotation. The moat shape is inconsistent with differential compaction. The precise association between moats and commissures and the independence of these structures on shell azimuth are inconsistent with current scour. If moats formed by rapid expulsion of water during valve snapping, then rotational tracks may have formed by a similar process. These traces are interpreted as fugichnia formed in response to catastrophic burial, but some moats could be equilibrichnia, formed by adjustment to minor sedimentary events. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar tracemaking abilities, then other strophomenates probably shared these abilities.