They are among the earliest animals with mineralisedshells. They are also the oldest known molluscs wth a shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.[1]
The latest word on this group is the classification of Parkhaev, who splits it into three subclasses.[2][3]
↑Peel J.S. 1991. Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca. In Simonetta A.M. & Conway Morris S. The early evolution of metazoa and the significance of problematic taxa. Cambridge University Press, pp. 157–177. ISBN978-0-521-40242-2