Cambrian monoplacophoran molluscs (Class Helcionelloida)

Cambrian monoplacophoran molluscs (Class Helcionelloida) are morphologically diverse, but also show considerable interspecific variation. While usually small (1-2 mm), they may achieve a size of 30 mm or more. Shell form ranges from low limpets to high cones and these may be straight or curved; coiled shells may be bilaterally symmetrical, dextral or sinistral. This diversity provides a fertile ground for speculation about early molluscan evolution and almost all extant classes of Mollusca have been recognised here. Certainly, molluscs can be traced back to the late Vendian (late Precambrian), but not all classes appeared during the Vendian-Cambrian transition. Small-sized late Vendian-Middle Cambrian molluscs (helcionelloids) show little resemblance to crown molluscan groups, including the living monoplacophorans (tryblidiid Tergomya).