Cambrian ‘sap-sucking’ molluscan radulae among small carbonaceous fossils (SCFs)

Abstract

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.

Dawn of diverse shelled and carbonaceous animal microfossils at ~ 571 Ma

“…The Ediacaran to Cambrian transition (~ 550–530 Ma) is recognized as one of the most transformative moments in Earth history, marked by the ‘explosion’ of animals, including non-biomineralizing 1 and biomineralized organisms 2 , 3 . Small carbonaceous fossils are sub-millimetric fragile non-biomineralized remains, commonly interpreted as the surficial covering of several animals, such as crustaceans, molluscs, annelids and priapulids 4 – 7 , in addition to taxa with uncertain affinities 8 .…”