Exceptionally preserved fossils provide crucial windows on extinct body plans and organismal evolution. Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian (~535-3520 Ma). The problematic fossil taxa Halkieria and Orthrozanclus (grouped in Sachitida)( have been assigned variously to stem-group annelids, brachiopods, stem-group molluscs or stem-group aculiferans (Polyplacophora + Aplacophora), but their affinities have remained controversial due to a lack of preserved diagnostic characters. Here we describe the youngest known sachitid, Calvapilosa kroegeri gen. et sp. nov., from the Early Ordovician (~478 Ma) Fezouata biota of Morocco. The new taxon is characterized by the presence of a single large anterior shell plate and a polystichous radula bearing a median tooth and several lateral and uncinal teeth in more than 125 rows. Its flattened body is covered by hollow spinose sclerites, and a smooth, ventral girdle flanks an extensive mantle cavity. Phylogenetic analyses resolve C. kroegeri as a stem group aculiferan together with other single plated forms such as Maikhanella Siphogonuchites and Orthrozanclus; Halkieria is recovered closer to the aculiferan crown. These genera document the stepwise evolution of the aculiferan body plan from forms with a single, almost conchiferan-like shell via two plated taxa such as Halkieria, to the eight-plated crown group aculiferans. C. kroegeri therefore provides key evidence concerning the long debate about the crown molluscan affinities of sachitids. This new discovery strongly suggests that the possession of only a single calcareous shell plate and the presence of unmineralised sclerites are plesiomorphic for the molluscan crown.