Fossil Discovery Sheds Light on Molluscan Skeleton Origins

The early evolution of mollusks has been hard to pin down, but now a newly discovered fossil – of a shell-less, soft-bodied, spiny mollusk from the early Cambrian – provides crucial insights, researchers report. The findings suggest that this fossil, of a creature called Shishania aculeata, is a stem mollusk – representative of an intermediate between early members of the superphylum lophotrochozoans and more derived mollusks. Mollusks are one of the most diverse groups of animals, encompassing various well-known forms such as clams, snails, and octopuses, as well as less familiar aculiferans (i.e., chitons and vermiform aplacophorans). Despite this impressive diversity, understanding the ancestorial traits of mollusks has been difficult due to the limited view provided by existing fossils and currently living forms. Although early mollusk fossils from the Cambrian period have offered some insights, revealing forms that exhibit a combination of biomineralized shells and sclerites, few stem lineage taxa have been described, leaving gaps in the understanding of early molluscan evolution. Here, Guangxu Zhang and colleagues describe S. aculeata, an early Cambrian-age mollusk that displays a combination of features, including a broad foot, mantle cavity, and a back covered in hollow chitinous sclerites, giving it the appearance of a bristly durian fruit. Unlike the biomineralized sclerites of modern aculiferan mollusks, Shishania sclerites are non-mineralized, suggesting a position outside the molluscan crown group. Electron microscopy revealed that the sclerites have a narrow tubular microstructure, similar to the bristles of annelid worms and brachiopods. The findings shed light on the evolutionary transition from simple chitinous bristles to the more complex biomineralized skeletal forms in mollusks.