Muscle scars, shell form and torsion in Cambrian and Ordovician univalved molluscs

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Abstract
Well preserved muscle scars were discovered on 1 or more specimens of the early Paleozoic univalved mollusks Pelagiella, Matherella, Sinuites and ?Bucania. From this information it is argued that the bellerophonts Sinuites and ?Bucania were untorted, that Pelagiella had undergone about 10.degree. of torsion, and that Matherella may have been a fully torted, hyperstrophic descendant of Pelagiella. Other early gastropods such as Aldanella could have been independently derived from Pelagiella and therefore the Class Gastropoda may be at least diphyletic. In general, small dextrally coiled archaeogastropods such as Aldanella and the pelagiellids are the most common asymmetrical univalves of the Early Cambrian, but rare ultradextral (hyperstrophic) forms are also known. Only pelagiellids are known from the Middle Cambrian and ultradextran forms dominate in the Late Cambrian. As a result, slit-bearing archaeogastropods are not common before the Ordovician, YUWENIA bentleyi gen. et sp. nov. is proposed.