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.