Reconstructing the age and historical biogeography of the ancient flowering-plant family Hydatellaceae (Nymphaeales)

Background: The aquatic flowering-plant family Hydatellaceae has a classic Gondwanan distribution, as it is found in Australia, India and New Zealand. To shed light on the biogeographic history of this apparently ancient branch of angiosperm phylogeny, we dated the family in the context of other seed-plant divergences, and evaluated its biogeography using parsimony and likelihood methods. We also explicitly tested the effect of different extinction rates on biogeographic inferences. Results: We infer that the stem lineage of Hydatellaceae originated in the Lower Cretaceous; in contrast, its crown originated much more recently, in the early Miocene, with the bulk of its diversification after the onset of the Pliocene. Biogeographic reconstructions predict a mix of dispersal and vicariance events, but considerations of geological history preclude most vicariance events, besides a split at the root of the family between southern and northern clades. High extinction rates are plausible in the family, and when these are taken into account there is greater uncertainty in biogeographic inferences. Conclusions: A stem origin for Hydatellaceae in the Lower Cretaceous is consistent with the initial appearance of fossils attributed to its sister clade, the water lilies. In contrast, the crown clade is young, indicating that vicariant explanations for species outside Australia are improbable. Although long-distance dispersal is likely the primary driver of biogeographic distribution in Hydatellaceae, we infer that the recent drying out of central Australia divided the family into tropical vs. subtropical/temperate clades around the beginning of the Miocene.