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.

Methods
The RTI datasets were generated using a Broncolor Scope D50 and Canon R5 EOS camera with a 70mm F2.8 DG MACRO camera lens. Each dataset used a total of 48 images, that were subsequently processed using Relight to create polynomial texture maps (PTMs). Camera settings used for each specimen are available in the supporting information. PTMs were subsequently visualized using RTI viewer using the default rendering, diffuse gain, specular enhancement, and normal visualization options. For specimens with substantial relief, multiple images at different focal planes were stacked using Photoshop, prior to PTM creation in relight.