Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan

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Abstract

Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution1. 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 taxaHalkieria3andOrthrozanclus4(grouped in Sachitida) have been assigned variously to stem-group annelids, brachiopods4,5, stem-group molluscs4or 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 kroegerigen. et sp. nov. from the Fezouata biota of Morocco7,8(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 resolveC. kroegerias a stem-group aculiferan together with other single-plated forms such asMaikhanella(Siphogonuchites) andOrthrozanclus;Halkieriais 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 asHalkieria, to the eight-plated crown-group aculiferans.C. kroegeritherefore 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.

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