Musculature and shell microstructure of the ancestral bivalve Fordilla (Mollusca) from the lower Cambrian of Greenland (Laurentia)

Shell microstructure and exquisite details of muscle attachment scars are preserved on the surface of internal moulds in the bivalve Fordilla troyensis from the Aftenstjerneso Formation (Cambrian Series 2, Stage 4) of North Greenland (Laurentia). Subsidiary muscles developed in commarginal series that extend laterally from the anterior and posterior adductor muscles probably controlled the centrally emergent foot. Their position close to the ventral margin indicates that only slight withdrawal and limited manipulation of the foot was possible, confirming that Fordilla was not a burrower. The pattern suggests derivation of Fordilla from a clamping mollusc with shell muscles distributed around the shell margin. No such pattern of serially repeated muscles close to the shell margin is currently known in the few available records of helcionelloid muscle scars, but Postacanthella has a commarginal U-shaped muscle scar reflecting its limpet-shaped shell form. Shell microstructure in Fordilla troyensis is closely similar to Pojetaia, with central areas of the valves dominated by a laterally continuous imbricate lamellar structure with a width of 10-20 mu m between successive crests. Imbricate lamellae with about half this spacing form a marginal zone with crests lying perpendicular to the valve edge. The hinge line lacks teeth, although a single tooth is present in each valve of the related Fordilla sibirica and Fordilla germanica. The site of the simple ligament is coarsely striated. Comparative material of Fordilla troyensis is illustrated from New York State, North-East Greenland and Denmark.