Exceptionally well‐preserved impressions of two bundles of bristles protrude from the apertures of small, spiral shells of Pelagiella exigua, recovered from the Kinzers Formation (Cambrian, Stage 4, ‘Olenellus Zone’, c. 512 Ma) of Pennsylvania. These impressions are inferred to represent clusters of chitinous chaetae, comparable to those borne by annelid parapodia and some larval brachiopods. They provide an affirmative test in the early metazoan fossil record of the inference, from phylogenetic analyses of living taxa, that chitinous chaetae are a shared early attribute of the Lophotrochozoa. Shells of Pelagiella exhibit logarithmic spiral growth, microstructural fabrics, distinctive external sculptures and muscle scars characteristic of molluscs. Hence, Pelagiella has been regarded as a stem mollusc, a helcionelloid expressing partial torsion, an untorted paragastropod, or a fully torted basal member of the gastropod crown group. The inference that its chaeta‐bearing appendages were anterior–lateral, based on their probable functions, prompts a new reconstruction of the anatomy of Pelagiella, with a mainly anterior mantle cavity. Under this hypothesis, two lateral–dorsal grooves, uniquely preserved in Pelagiella atlantoides, are interpreted as sites of attachment for a long left ctenidium and a short one, anteriorly on the right. The orientation of Pelagiella and the asymmetry of its gills, comparable to features of several living vetigastropods, nominate it as the earliest fossil mollusc known to exhibit evidence of the developmental torsion characteristic of gastropods. This key adaptation facilitated an evolutionary radiation, slow at first and rapid during the Ordovician, that gave rise to the remarkable diversification of the Gastropoda.

中文翻译：

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Pelagiella exigua，一种早熟的寒武纪茎腹足纲，具甲壳纲：沙眼滋养体和壳聚糖的新颖性

刷毛的两束的格外保存完好展示从小，螺旋弹孔突出Pelagiella夜蛾，从Kinzers组（寒，第4阶段，“回收Olenellus区”，Ç宾夕法尼亚。512 MA）。可以推断这些印象代表了几丁质壳壳类的簇，与无节肢动物旁足动物和一些幼体腕足类动物的印象相当。他们从早期活生化石化石记录中得出了肯定的检验，从活的类群的系统发育分析得出，几丁质壳类是恶风体的共同早期特征。贝拉壳菌的壳表现出对数螺旋生长，微结构织物，独特的外部雕塑和软体动物的肌肉疤痕。因此，Pelagiella被认为是茎软体动物，表达部分扭转的半纤维素，无扭曲的腹足纲或腹足纲冠群的完全弯曲的基础成员。基于其可能的功能，其无角附肢是前外侧的推论，促使对Pelagiella的解剖结构进行了新的重建，主要是前地幔腔。在这种假设下，两个横背沟在Pelagiella atlantoides中被唯一保留，被解释为长的左侧骨和短的右侧骨的附着位点。的方向Pelagiella及其its的不对称性可与几种活生的象甲足纲动物的特征相媲美，将其命名为最早的化石软体动物，已知可显示腹足动物的发育扭转特征。这种关键的适应性促进了演化的辐射，最初是缓慢的，而在奥陶纪则是快速的，从而引起了腹足纲动物的显着多样化。