THE WORM’S TURN: CONSTRAINING THE DEEP ORIGIN AND EVOLUTION OF PARASITIC FLATWORMS WITH FOSSIL EVIDENCE

Novel fossil discoveries have contributed to our understanding of the evolutionary appearance of parasitism in flatworms. Furthermore, genetic analyses with greater coverage have shifted our views on the co-evolution of parasitic flatworms and their hosts. The oldest fossil evidence for the presence of helminths falls in the Middle Devonian in the form of hooks, some of which are most reminiscent of extant Monogenea with simple life cycles, although some could also belong to Acanthocephala or more derived flatworms (Cestoda). The oldest secure record of parasitic flatworms with complex parasite life cycles lies in the Permian, which can be confidently assigned to cestodes. The presence of igloo-shaped concretions reminiscent of those caused by gymnophallid trematodes in extant bivalves might already indicate the presence of derived parasitic flatworms with complex parasite life cycles in the Late Silurian. These Silurian occurrences are, however, not consistent with evolutionary history of current gymnophallid hosts (shorebirds), which are believed to have appeared somewhere between the Cretaceous and Eocene. The body fossil record of parasitic flatworms is patchy, but consistent with extant host associations, so that it can be used to put constraints on the evolutionary origin of the parasites themselves. The future lies in new molecular clock analyses combined with additional discoveries of exceptionally preserved flatworms associated with hosts and coprolites. Besides direct evidence, the host fossil record and biogeography have the potential to constrain their evolutionary history, albeit with caution needed to avoid circularity, and a need for calibrations to be implemented in the most conservative way. This might result in imprecise, but accurate divergence estimates for the evolution of parasitic flatworms.