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.

中文翻译：

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寒武纪带刺茎软体动物与冠轮动物巩膜体的深层同源性


软体动物种类繁多，包括熟悉的蛤和蜗牛，以及不太熟悉的具有复杂多组分骨骼的鳖类（石鳖和蠕形甲壳动物）。古生代化石追踪冠软体动物的形态，表现出生物矿化的贝壳和骨片（例如鳞片、刺和骨针）的组合。我们描述了一种无壳的寒武纪茎软体动物，Shishania aculeata gen。等十一月，具有圆锥形、中空的几丁质骨片和光滑的带、宽足和外套膜腔。骨片具有狭窄管的微观结构，与环节动物和腕足动物中发现的毛皮微绒毛的印象一致。石山菌片在典型的毛壳（几丁质刚毛）和包围矿化体的阿库利弗菌片的外部有机部分之间提供了形态上的垫脚石。这一发现强化了冠轮动物毛壳和生物矿化的阿卡利弗骨片的共同起源，表明软体动物祖先密集地覆盖着中空的几丁质毛壳。