New Insights into Autecology of the Ordovician Strophomenid Brachiopod Rafinesquina from Ichnology, Epibionts, and Biomechanics

Sedimentary structures associated with Rafinesquina specimens have been documented at several Upper Ordovician horizons and localities in the Cincinnati Arch region. These include ”moats” which are gutter-like depressions in the sediments adjacent to the commissure of flat-lying convex-up individuals, and ”rotational tracks” which are up-arcing spreiten beneath the brachial valves of inclined individuals.

These features are interpreted as escape traces excavated with water expelled by valve snapping during and immediately after catastrophic burial. They are similar to trace fossils associated with Sowerbyella. The association of escape traces with body fossils demonstrates live burial.

Sections of these specimens reveal that epibionts on the concave brachial valves are thinner with lower profiles than epibionts on the convex pedicle valves. Most of the pedicle valve epibionts could not have existed if the brachiopods had lived with the pedicle valve down, mostly occluded by sediment. In contrast, if the brachiopods had lived with the brachial valve down, the concave surface would have provided a cryptic microhabitat, especially between the commissure and geniculation, the region most frequently encrusted.

The maximum gape angle for Rafinesquina, estimated from the muscles and lever system, was 45°-55°. Hinge structures of well-preserved silicified specimens show that the valves would have articulated and stabilized at a gape of about 55°, suggesting that this was a habitual feeding gape. This is consistent with the frequent observation that the anterior margin of the pedicle valve, which would have been elevated during feeding, was preferred by some epibionts.

It is difficult to escape the conclusion that Rafinesquina was capable of clearing its commissure of significant accumulations of sediment, and that it was not only capable of living in a convex-up position, but that most individuals probably did so as they gaped at about 55° for feeding.