Mobergellans from the Lower Cambrian of Mongolia, Sweden, and the United States: molluscs or opercula of incertae sedis?

Abstract

Mobergellans are a distinctive group of Lower Cambrian breviconic shells with a prominent set of internal scars, presumably representing sites of former muscle attachment. Shells grew by accretion and appear to have secreted primary phosphate. Here a redescription is given of mobergellans, assigned to Hippoklosma new genus, from the Khairkhan Formation (Botomian-age equivalent) of southwest Mongolia.Hippoklosma mongolica(Missarzhevsky) is bilaterally symmetrical, with five pairs of radiating structures. These are presumably homologous with the muscle scars of other mobergellans, but in contrast consist of a series of disks (herein fibroplacodes) each containing a characteristic fibrous structure capped externally by a smooth lid. The apical region of the shell is domelike with transverse segmentation. A review of other mobergellans, including new data on Mobergella holsti from Sweden and Discinella micans from New York, and possibly related taxa concludes that although their affinities are problematic, a relationship with the Mollusca may be the most plausible hypothesis on existing evidence.