Mode of life of Early Devonian Orthonychia protei (Neritimorpha, Gastropoda) inferred from its post-larval shell ontogeny and muscle scars

Abstract

Rich fossil material of the Early Devonian Orthonychia protei (Oehlert, 1883) from the Saint Céneré Formation, Brittany (France) provides a unique chance to analyze the infraspecific variability of muscle scars and post-larval shell morphology. The position of the muscle scar in this species depends on the shape of the teleoconch, which seems to be influenced by its living position upon the crinoid host. Individuals having a teleoconch aperture with a pentagonal outline and five apertural processes probably lived on top of a crinoid calyx and their muscle was attached to the center of their teleoconchs. However, muscle scars are situated more laterally in individuals with irregular teleoconchs lacking apertural processes. The latter group was probably attached to another part of the crinoid body or even to another type of hard substrate. Juvenile post-larval Orthonychia protei was a mobile animal, but very early in its ontogeny became adapted to a fixed position on the crinoid body and remained immobile for the majority of its post-larval ontogeny. Long-term coexistence of Orthonychia protei with the same crinoid suggests that this species had a long-term advantage from such an ecological strategy. However, the existence of individuals with irregularly shaped teleoconchs suggests that Orthonychia protei could also live successfully outside of the digestive openings of crinoids and, thus, use a food source not closely connected with crinoids. This interpretation can be supported by the fact that at least some Silurian and Devonian platyceratids were adapted to living on small firm objects (like empty cephalopod shells) lying on the sediment surface.