Not that young: Combining plastid phylogenomic, plate tectonic and fossil evidence indicates a Palaeogene diversification of Cycadaceae

Abstract

Background and Aims: Previous molecular dating studies revealed historical mass extinctions and recent radiations of extant cycads, but debates still exist between palaeobotanists and evolutionary biologists regarding the origin and evolution of Cycadaceae. Methods: Using whole plastomic data, we revisited the phylogeny of this family and found the Palawan endemic Cycas clade was strongly related to all lineages from Southeast Eurasia, coinciding with a plate drift event occurring in the Early Oligocene. By integrating fossil and biogeographical calibrations as well as molecular data from protein-coding genes, we established different calibration schemes and tested competing evolutionary timelines of Cycadaceae. Key Results: We found recent dispersal cannot explain the distribution of Palawan Cycas, yet the scenario including the tectonic calibration yielded a mean crown age of extant Cycadaceae of ~69-43 million years ago by different tree priors, consistent with multiple Palaeogene fossils assigned to this family. Biogeographical analyses incorporating fossil distributions revealed East Asia as the ancestral area of Cycadaceae. Conclusions: Our findings challenge the previously proposed Middle-Late Miocene diversification of cycads and an Indochina origin for Cycadaceae and highlight the importance of combining phylogenetic clades, tectonic events and fossils for rebuilding the evolutionary history of lineages that have undergone massive extinctions.