ABSTRACT
Functional hypotheses for strophomenid shape were evaluated, inclusive of potential taphonomic bias. Results from experimental biomechanical experiments (e. g. flume measurement of flow-induced forces; sediment scour around shells on mobile, sandy substrates; stability of muds with varying pore-water content; wind-tunnel pressure mapping over shells) were used to predict paleoenvironmental occurrence, and then corroborated in the field. Rafinesquina alternata, a ubiquitous Upper Ordovician strophomenid from the Illinois Basin, was used as a standard for similarly shaped brachiopods. Three presumed functional hypotheses were tested: (1) strophomenids lived semi-infaunally with the convex valve down; (2) geniculation placed the commissure above the sediment-water interface to prevent fouling; and (3) flow through the valves for suspension feeding was not lateral-medially separated. Geniculated brachiopods with convex-down orientations generated greater drag, induced greater scour, and experienced larger pressure differentials than arcuate or convex-up specimens, supporting the claim that convex-up orientations should predominate in shell beds. A convex-up life position appears inconsistent with results from muddy substrate experiments; the commissure quickly sank below the sediment-water interface, ostensibly rendering the lophophore ineffective. Geniculate convex-down specimens probably maintained their entire commissure above the substrate. These patterns were tested on substrates with 25% and 50% water contents. On sandy substrates, arcuate, convex-down brachiopods induced little scour and were less likely to become buried than geniculate forms. The paleoenvironmental distribution of R. alternata in Cincinnatian (Ashgill) strata from the Illinois Basin appears predictable, given these interpretations for life position and shell shape. Wind tunnel pressure measurements along the commissure of convex-down Rafinesquina were inconsistent with a laterally incurrent, medially excurrent flow, suggesting that Rafinesquina may not have had a spirolophe.
