LIVING ON THE EDGE: EPIZOAN ENCRUSTATION AND ALTERNATIVE LIFE ORIENTATIONS OF THE UPPER ORDOVICIAN STROPHOMENID BRACHIOPOD RAFINESQUINA FROM THE CINCINNATI ARCH REGION

Three species of edrioasteroid echinoderms encrusted a shell pavement of the concavo-convex strophomenid Rafinesquina alternata from the Grant Lake Formation (Maysvillian) of Boone County, northern Kentucky. The most common edrioasteroid, Isorophus cincinnatiensis, ranges in size from 3-34 mm diameter, and predominantly encrusted convex-upward, unabraded pedicle exterior valves of Rafinesquina. Smaller Isorophus are preferentially clustered along the pedicle valve near the commissure, but larger individuals are located closer to the center of the valve.

Isorophus on abraded pedicle exteriors show no preferential location regardless of size. Because unabraded shells are also articulated we postulate these were living at the time of burial, along with the edrioasteroids. Was edrioasteroid clustering a response to the life position or behavior of the brachiopod? If the Rafinesquina lived pedicle-valve-down, then small edrioasteroids might have sheltered along the shell margin projecting above the sediment.

An alternative interpretation, inspired by numerous cases where a “moat” flanks the commissure, is that Rafinesquina habitually lived with the convex pedicle valve up, creating the moat by valve-snapping to clear sediment from the commissure. If this interpretation is correct, then the pedicle valve margin would have provided a sediment-free attachment site. If the brachiopod maintained a wide gape, elevation of the shell margin position could have been favorable for epibionts. The convex-up alternative is also favored by edrioasteroids and bryozoan encrusters located on the pedicle valve away from the commissure, closer to the hinge, which could not survive had the pedicle valve rested on the substratum.

Encrusters on the brachial valve exterior, revealed by thin sections and peels, are generally thinner with lower profiles than encrusters on the convex pedicle valves. Obviously these encrusters could have lived on valves in a concave-up life position. However, in contrast to the pedicle valve, the concave brachial valve would not necessarily have been occluded with sediment, even if the brachiopod rested on that valve. Rather, it would have provided a cryptic microhabitat, especially between the commissure and geniculation, the region that is most frequently encrusted.