Department of Earth Sciences, Palaeobiology, Uppsala University, 752 36 Uppsala, Sweden.
Proc Biol Sci. 2023 Mar 29;290(1995):20230257. doi: 10.1098/rspb.2023.0257.
Molluscs have produced an extensive fossil record, owing to the prevalence of robust biomineralized shells among this clade. By contrast, most other components of molluscan anatomy are seldom preserved. Importantly, little is known of the evolutionary history of the unique molluscan feeding apparatus-the radula. A scarcity of fossil radulae has hampered our understanding of the ancestral condition, and of the dietary ecology of early molluscs. The handful of known fossil radulae all point to early molluscs as simple deposit feeders that obtained food via rasping or scraping. This study reports microscopic radulae preserved as 'small carbonaceous fossils' (SCFs) from Cambrian (Stage 4-Wuliuan, approximately 514-504.5 Ma) strata of Sweden. These rare fossil radulae offer novel insights into the feeding anatomy and ecology of early molluscs. Each radula comprises a uniseriate arc of (≤10) blade-shaped teeth, fringed by a slicing keel. This distinctive morphology is strikingly convergent with the radulae of extant sacoglossan heterobranch gastropods-such radulae are specially adapted for piercing the cell walls of green algal tissues to enable suctorial feeding on the cytoplasm contents. Discovery of analogous Cambrian radulae demonstrates this specialized form of herbivory had already evolved among molluscs more than half a billion years ago.
软体动物产生了广泛的化石记录，这要归功于这个分支中坚固的生物矿化壳的普遍存在。相比之下，软体动物解剖结构的大多数其他组成部分很少被保存下来。重要的是，人们对独特的软体动物取食器官——齿舌的进化历史知之甚少。化石齿舌的稀缺阻碍了我们对祖先状况的理解，也阻碍了对早期软体动物饮食生态的理解。少数已知的化石齿舌都表明，早期软体动物是简单的滤食动物，通过刮擦或刮削来获取食物。本研究报告了来自瑞典寒武纪（第 4 阶段-五峰期，约 5.14-5.045 亿年前）地层的以“小碳质化石”（SCF）形式保存的微观齿舌。这些罕见的化石齿舌为早期软体动物的取食解剖结构和生态学提供了新的见解。每个齿舌由（≤10）个刀片状牙齿的单弧组成，边缘有一个切片龙骨。这种独特的形态与现存的有肺目腹足动物的齿舌惊人地相似——这种齿舌专门用于刺穿绿藻组织的细胞壁，以便通过吮吸细胞质内容物进行取食。类似寒武纪齿舌的发现表明，这种专门的草食形式在 5 亿多年前的软体动物中就已经进化出来了。
