A Cambrian spiny stem mollusc and the deep homology of lophotrochozoan scleritomes

<p>Molluscs encompass enormous disparity, including familiar clams and snails alongside aculiferans (chitons and vermiform aplacophorans) with complex multicomponent skeletons. Palaeozoic fossils trace crown molluscs to forms exhibiting a combination of biomineralised shells and sclerites. We describe a shell-less, Cambrian stem mollusc, Shishania aculeata, with conical, hollow organic sclerites and a smooth ventral girdle, broad foot, and mantle cavity. The sclerites have a microstructure of narrow canals consistent with the impressions of chaetal microvilli found in annelids and brachiopods. Shishania sclerites provide a morphological steppingstone between typical chaetae and the external organic part of aculiferan sclerites that encloses a mineralised body. This discovery reinforces a common origin of lophotrochozoan chaetae and the biomineralized aculiferan sclerites, suggesting the mollusc ancestor was densely covered with hollow organic chaetae.</p>