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

Abstract

A study on the morphological characteristics of helcionelloids and its microstructure from the Nanjiang area (north Sichuan, China) at the northern Yangtze Platform is taken to understand its position in the phylogeny. Helcionelloids fossils from the Kuanchuanpu Formation of the Changtanhe–Maolinzi section include 7 genera and 11 species. There are distinct differences on the degree of shell bending, the apical morphology and apex position, the outline of the apeture, and the height of the shell, which constitute an important identification feature. Though the helcionelloid apices have varying degrees of rotation towards the aperture, they have no any evident distortion. A pair of basally symmetrical muscle scars are discovered on the shells of helcionelloids (such as Bemella simplex), showing distinct difference comparing 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. They are significantly different from the dense layer microstructure found in contemporary molluscs. The microstructures, muscle scars and morphological characteristics demonstrate that the helcionelloid is a stem‐group of molluscs.