A HALKIERIID-LIKE ACULIFERAN MOLLUSC FROM THE EARLY ORDOVICIAN FEZOUATA BIOTA, ANTI ATLAS REGION, MOROCCO

Considerations of the origin and early evolution of molluscs have usually focused on specific groups, such as the chitons, monoplacophorans and aplacophorans, for various morphological reasons assuming the antiquity of these body plans. The chitons and aplacophorans in particular have received attention as an ancient grade of molluscs. This contrasts with molecular and palaeontological evidence that suggests that these two groups diversified during the Ordovician long after the Cambrian Explosion. Putative stem groups are the halkieriids and other sachitids, which are common in early and mid Cambrian rocks all over the world.

Here we describe a new mollusc from the Lower Ordovician Fezouata biota, which is a dorsoventrally flattened scleritome-bearing organism with an isolated anterior shell plate surrounded by minute sclerites which cover the dorsal and lateral margins. The general anatomy resembles halkieriids in several aspects, but the minute sclerites are more similar to those of chitons and aplacophorans. The molluscan affinities are demonstrated by a well preserved radula, superimposed on the anterior shell plate.


The fossil provides an important morphological link between the halkieriids and the aculiferan molluscs (aplacophorans and chitons). This supports the notion that sachitid molluscs and the halkieriids are stem group aculiferans rather than some other lophotrochozoan as have been suggested previously. Cladistic analyses recover the new fossil together with the Burgess shale Orthrozanclus as a basal branch on the Aculiferan stem, which lends support to the idea that aculiferans evolved from a successive series of bodyplans with first one anterior valve, then a posterior and finally 5-6 intermediate valves.