The Judith River Formation of Montana, USA, renowned for its preservation of Late Cretaceous dinosaurs, now yields the oldest-known evidence of trematode parasitism of bivalves. Highly distinctive igloo-shaped traces found on shells of the freshwater bivalve Sphaerium are virtually identical to igloo-shaped traces known from living marine bivalves infected by metacercaria larvae of gymnophallid trematodes (flatworms). This unique record of paleoparasitism provides key insights into the evolution of an important parasite group, reveals the inner workings of cryptic ecological associations, and enriches our understanding of ancient food webs. Our discovery extends the record of trematode–bivalve interaction back to the Late Cretaceous (ca. 76 Ma), and indicates that this parasite–host relationship was established in freshwater ecosystems much earlier than previously surmised. The complex multi-host lifecycles of modern trematodes and the general stability of parasite–host associations suggest that sphaeriid bivalves in the Judith River record likely served as the second intermediate host. Potential candidates for the definitive host range from molluscivorous fish to birds and non-avian predatory dinosaurs. With the history of trematode–bivalve interaction pushed back to the Late Cretaceous, patterns in trematode infection can now be interrogated across major episodes of global change, including the Cretaceous-Paleogene mass extinction and the Paleocene-Eocene thermal maximum.

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