WHERE THE FOSSIL RECORD FALLS FLAT: USING A BAYESIAN BIOGEOGRAPHIC MODEL TO DATE PARASITIC FLATWORM PHYLOGENIES

Fossil evidence has long been used to calibrate phylogenetic divergence times. While this approach is the most widely accepted and arguably the most intuitive, it is limited by the actual fossil record available. Large gaps or completely absent fossil records make such approaches impossible for certain groups. As such, the development of fossil-free calibration models is essential. The utilization of geological evidence constraining biogeographical events has the potential to accurately date divergence times where fossil information is sparse or absent. These models rely on geological time constraints on paleogeographical events to explain the divergence of taxa. One group that would benefit from such an approach are parasitic flatworms (platyhelminthes). Due to their small size and soft body these parasites have an extremely limited fossil record. Their divergence times are still poorly resolved and often based on the host fossil record. This requires making prior assumptions about host parasite interactions, which can affect the resulting times and prevents testing hypotheses about these relationships. However, the early evolution of some clades of platyhelminths has been linked to the breakup of Gondwana, making them ideal for biogeographic dating. Based on a Bayesian biogeographic model, we calibrate the divergence times of platyhelminthes and test hypotheses of host-parasite interactions and biogeography through time. The results are not consistent with earlier studies based on host or fossil calibrations and have implications for our understanding of flatworm evolution. While fossil evidence may be the preferred approach of dating, where that is not possible models like the one used in this study provide a promising opportunity to estimate divergence times.