Muscle scars in Miaolingian helcionelloids from Laurentia and the diversity of muscle scar patterns in Cambrian univalve molluscs

Two pairs of equidimensional muscle-attachment scars are described on the dorso-lateral surfacesof internal moulds of the helcionelloid molluscs Hensoniconus gen. nov. and Vendrascospira fromthe Henson Gletscher Formation (Cambrian, Miaolingian Series, Wuliuan Stage) of North Greenland(Laurentia). Two patterns of muscle scars are recognized in helcionelloids. In the Hensoniconus–Vendrascospira group, two pairs of sub-equally sized scars are located on the dorso-lateral surfaces.In the Bemella–Figurina group, thin, band-like scars are distributed on the sub-apical surface andalong the dorsal (supra-apical) surface. Comparison of these rare described occurrences of musclescars in bilaterally symmetrical molluscs indicates that the simple shell form may obscure recognitionof distinct lineages within Cambrian univalves that are based on anatomical features, such asmusculature.