THE ORIENTATION AND FUNCTION OF SHELL ORNAMENTATION IN THE RADIOLITID RUDIST BOURNONIA CANCELLATA

Bournonia cancellata is unusual with respect to other radiolitid rudists in that it has a bi-directional pattern of shell ornamentation. Along the dorsal, posterior, and ventral edges of the shell, the primary direction of the ornamentation is perpendicular to the plane of commissure. On the anterior side of the shell, however, the primary direction of the ornamentation is parallel to the plane of commissure.

The anterior side of Bournonia cancellata is also unusual in that it is flattened, giving the organism a semi-circular cross-section, instead of the circular cross-section typically seen in radiolitids. Bournonia cancellata was demonstrably epifaunal, and this flattened side was most likely a resting surface. This inference is supported by statistical analyses of the diversity and distribution of encrusting epibionts on the shell.

The ribs on Bournonia cancellata’s flattened anterior side are asymmetric, and are structurally similar to the terrace sculpturing seen in burrowing bivalves. In fact, these ribs meet all the same criteria for a frictional paradigm as this terrace sculpturing. This fact, combined with the epifaunal life position, and lightweight shell walls of these organisms, suggests that the anterior ribs may have acted as “treads” to prevent slippage in a moving current. If this is true, then it not only suggests a preferential orientation for Bournonia cancellata relative to the current, but it also implies a potential functionality for the ribs along the posterior margin.