THE LIFE ORIENTATION OF RAFINESQUINA AND STROPHOMENA: WHAT WE DO AND DON'T KNOW

Until 10 years ago, concavo-convex brachiopods such as the common Ohio forms Rafinesquina and Strophomena, were believed to live convex-down so that the commissure was elevated above the sediment to keep the shell from becoming fouled. In 1996 I reported that many concavo-convex brachiopods could be demonstrated to have been alive when encrusted by epibionts. These brachiopods had the vast majority of large biovolume encrusters on their convex valves, supporting a convex-up life orientation. This interpretation was bolstered by the presence of cryptic encrusters on concave valves and by extensive taphonomic observations indicating little post-mortem encrustation. Patterns held for Ordovician- Devonian species from 11 collections.

Although convex-up orientation has now been widely accepted and strengthened by new “smoking gun” trace fossils, six critiques have been leveled against the orientation:

1. In flume experiments convex-up shells sink into the mud,

2. Epifauna may have preferred living on cryptic surfaces, 3. Epifauna may have been removed prior to burial,

4. Geniculation of Rafinesquina correlates with mud content,

5. Flattest specimens occur in the most energetic environments, and

6. Strophomenid musculature prohibits movement or valve snapping.

These critiques do not offer an alternative interpretation for the data and are largely ad hoc and/or invalid. Flume experiments on dead shells are more taphonomy experiments than predictors of live host behavior. The 2nd and 3rd critiques conflict with established ecological patterns and taphonomic studies. The 4th and 5th critiques are based on a misleading measurement system that is at odds with published studies and new data on shell curvature and thickness. The final critique, based on an uncertain reconstruction of muscles, ignores the reality that all brachiopods can snap to clear material; this is how they rid themselves of feces! While brachiopod orientation, as all historical science, remains by necessity uncertain, no alternative scenario has yet been proposed to explain the encrustation patterns that are widespread on brachiopods. Any attempt to argue for a convex-down orientation must be consistent with all of the existing epibiont and taphonomic observations.