Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution(1). Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian period (approximately 535-520 million years ago (Ma))(2). The problematic fossil taxa Halkieria(3) and Orthrozanclus(4) (grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods(4,5), stem-group molluscs(4) or stem-group aculiferans (Polyplacophora and Aplacophora)(6), but their affinities have remained controversial owing to a lack of preserved diagnostic characters. Here we describe a new early sachitid, Calvapilosa kroegeri gen. et sp. nov. from the Fezouata biota of Morocco(7,8) (Early Ordovician epoch, around 478 Ma). The new taxon is characterized by the presence of a single large anterior shell plate and 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 through 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 (an ancestral trait) for the molluscan crown.