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

Abstract

Runnegar, Bruce 1981 12 15: Muscle scars, shell form and torsion in Cambrian and Ordovician univalved molluscs. Lethaia, Vol. 14, pp. 311–322. Oslo. ISSN 0024–1164. Well preserved muscle scars have been discovered on one or more specimens of the early Palaeozoic univalved molluscs 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° 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 ultradextral 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 a new taxon proposed in this work. *Mollusca, Gastropoda, Monoplacophora, anatomy, evolution, Cambrian, Ordovician, Yuwenia n.g.