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

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.