Keeping a lid on it: muscle scars and the mystery of the Mobergellidae

Mobergellans were one of the first Cambrian skeletal groups to be recognized yet have long remained one of the most problematic in terms of biological function and affinity. Characterized by a disc-shaped, phosphatic sclerite, the most distinctive character of the group is a prominent set of internal scars, interpreted as representing sites of former muscle attachment. Predominantly based on muscle scar distribution, mobergellans have been compared to brachiopods, bivalves and monoplacophorans; however, a recurring theory that the sclerites acted as an operculum remains untested. Rather than correlate the number of muscle scars between taxa, here we focus on the percentage of the inner surface shell area that the scars constitute. We investigate two mobergellan species, Mobergella holsti and Discinella micans, and compare the Cambrian taxa with the muscle scars of a variety of extant and fossil marine invertebrate taxa to test whether the mobergellan muscle attachment area is compatible with an interpretation as operculum. The only skeletal elements in our study with a comparable muscle attachment percentage are gastropod opercula. Complemented with additional morphological information, our analysis supports the theory that mobergellan sclerites acted as an operculum presumably from a tube-living organism. The paucity of tubes co-occurring with mobergellan sclerites could be explained by the transportation and sorting of detached opercula, while the corresponding tube remained attached to substrata in shallower water. The operculum perhaps performed a similar role to that seen in serpulid annelids and in neritid gastropods sealing the living chamber of the organism to avoid desiccation or for protection.