The canal system in sclerites of lower cambrian sinosachites (halkieriidae: Sachitida): Significance for the molluscan affinities of the sachitids

The halkieriids (Sachitida He, 1980) from the Early to Mid Cambrian possess a hollow sclerite with a complex branching canal system. An analysis of the canal system morphology in the halkieriid Sinosachites (Thambetolepis) delicatus (Jell, 1981) from South Australia reveals similarities to the aesthete canal system in the shell plates of chitons, which has been analysed in a number of extant taxa. The compartments, referred to as macro-aesthetes in chitons, and lateral canals in halkieriids, have overlapping diameters and are constrained in morphology by the space of accommodation by maintaining a constant width, whereas length is more variable. Both canal systems are morphologically distinct from shell pores of other lophotrochozoans and known mollusc classes. Similarities in sclerite growth, microstructure and mineralogy further suggest that halkieriids, along with the other sachitids, are molluscs, most likely stem aculiferans (Polyplacophora and Aplacophora).