The palaeoecological significance of tubes, ribs, pallial canals and flanges in caprinoid rudist bivalves
Abstract We undertook a palaeoecological evaluation of the development of ribs, flanges, tubes and pallial canals in recumbent rudist bivalves typified by the genus Titanosarcolites. We used an evidence based system to interpret potential functional significance of features. The dorsal position of the ancestral ligament and the antero-ventral-down orientation of recumbent Titanosarcolites indicates that a recumbent lifestyle was only possible after loss of a functional ligament. The presence of ribs on the ventral side of the shell in recumbents is interpreted as a means of lifting the commissure away from the seafloor to prevent fowling by sediments. Pallial canals, which invade the whole inner layer of the shell, contain tabulae, and therefore cannot be functional conduits for fluid flow, nor cavities for photosynthetic symbionts (as only some would receive light), and are best interpreted as features to allow rapid growth of large shells. Tubes in Titanosarcolites, and similar forms, lack tabulae, are open to the sea at the tips of the valves, and were formed by over-roofing and subsequent division; the presence of epibionts and geopetal structures indicates that they were not filled with sediment, and were conduits for water flow containing food. Mathematical modelling indicates that sucking water in through the tubes and expelling through the commissure was the most efficient mechanism. The presence of a posterior ventral groove is interpreted as a potential exhalent siphon and the presence of flanges on the shell is interpreted as a mechanism for keeping inhalant and exhalent waters separate. We suggest that large flanges also served the purpose of keeping competitive rudists at a distance and preventing overturning of recumbents during storm events. These large recumbent rudists seem to have developed their ecological strategies to cope with relatively low-food availability on late Cretaceous carbonate platforms.