Helcionelloids from the Cambrian Fortunian of the Northern Yangtze Platform, China

A study of the morphological and microstructural characteristics of helcionelloids from the Nanjiang area(north Sichuan, China) at the northern Yangtze Platform was undertaken, in order to understand their phylogenetic position. Helcionelloids fossils from the Kuanchuanpu Formation of the Changtanhe–Maolinzi section include 7 genera and 11 species. There are distinct differences in the degree of shell bending, the apical morphology and apex position, the outline of the aperture and the height of the shell, all of which constitute important identification features. Although helcionelloid apices exhibit varying degrees of rotation towards the aperture, they do not show any signs of evident distortion. A pair of basally-symmetrical muscle scars are discovered on the shells of helcionelloids(such as Bemella simplex), showing a distinct difference when compared with those of the stem-group gastropod(e.g., Pelagiella). The prismatic shell layer of helcionelloids has been observed, which are common components in Cambrian molluscs, but differ significantly from the dense layer microstructure found in contemporary molluscs. The microstructures, muscle scars and morphological characteristics demonstrate that helcionelloids represent a stem-group of molluscs.