Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan.
Exceptionally preserved fossils provide crucial insights into extinct body plans and organismal evolution. Molluscs, one of the most disparate animal phyla, radiated rapidly during the early Cambrian period (approximately 535-520 million years ago (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 and Aplacophora), 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 (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.
School of Biological Sciences, University of Bristol, Life Sciences, Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK.|School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK.
Jakob
J
Vinther
School of Biological Sciences, University of Bristol, Life Sciences, Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK.|Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.
Luke
L
Parry
Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA.|Yale Peabody Museum of Natural History, 170 Whitney Avenue, New Haven, Connecticut 06520, USA.
Derek E G
DE
Briggs
Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA.|Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium.
Peter
P
Van Roy
D060105:Animal Shells / Q000033:anatomy & histology; D000818:Animals; D001499:Bayes Theorem; D005580:Fossils; D008974:Mollusca / Q000033:anatomy & histology* / Q000145:classification*; D009018:Morocco; D010802:Phylogeny; D014070:Tooth / Q000033:anatomy & histology