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| Shishania aculeata |
Abstract
Mollusks encompass enormous disparity, including familiar clams and snails alongside less familiar aculiferans (chitons and vermiform aplacophorans) with complex multicomponent skeletons. Paleozoic fossils trace crown mollusks to forms exhibiting a combination of biomineralized shells and sclerites (e.g., scales, spines, and spicules). We describe a shell-less, Cambrian stem mollusk, Shishania aculeata gen. et sp. nov., with conical, hollow chitinous sclerites and a smooth 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 stepping stone between typical chaetae (chitinous bristles) and the external organic part of aculiferan sclerites that encloses a mineralized body. This discovery reinforces a common origin of lophotrochozoan chaetae and the biomineralized aculiferan sclerites, suggesting that the mollusk ancestor was densely covered with hollow chitinous chaetae.
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| Complete specimen of Shishania aculeata seen from the dorsal (top) side (left). Spines covering the body of Shishania aculeata (right). Credit: G Zhang/L Parry. |
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| Artist’s reconstruction of Shishania aculeata as it would have appeared in life as viewed from the top, side and bottom (left to right). Reconstruction by M. Cawthorne. |
Shishania aculeata gen. et sp. nov.
Guangxu Zhang, Luke A. Parry, Jakob Vinther and Xiaoya Ma. 2024. A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes. Science. 385(6708); 528-532
Half a billion-year-old spiny slug reveals the origins of molluscs
x.com/OxUniEarthSci/status/1819281981483442461
Editor’s summary: Mollusca is one of the most diverse extant phyla, with forms varying widely from bivalves such as clams to snaily gastropods and highly complex cephalopods such as octopuses. Zhang et al. describe a new fossil, Shishania aculeata, from the Cambrian Period that displays basal molluscan traits, such as a foot and mantle, as well as traits that are more characteristic of other members of their superphylum, Lophotrochozoa. The authors place this taxon as a stem mollusk covered in sclerites, suggesting that it was intermediate in form between members of Lophotrochozoa and the soon to develop and diversify crown mollusks. —Sacha Vignieri
