Timing deep divergence events in calcareous dinoflagellates

Based on morphological and molecular data, calcareous dinoflagellates (Thoracosphaeraceae, Peridiniales) are a monophyletic group comprising the three major clades Ensiculifera/Pentapharsodinium, Thoracosphaera/Pfiesteria, and Scrippsiella sensu lato. We used stratigraphically well-documented first occurrences of particular archeopyle types to constrain relaxed Bayesian molecular clocks applied to nuclear rRNA sequences of 18 representatives of the three main clades. By comparing divergence estimates obtained in differently calibrated clocks with first stratigraphic occurrences of taxa not themselves used as constraints, we identified plausible divergence times for several subclades of calcareous dinoflagellates. The initial diversification of extant calcareous dinoflagellates probably took place in the Late Jurassic, with the three main clades all established by the Cretaceous. The two mesoepicystal operculum types observed in calcareous dinoflagellates probably evolved independently from simple apical archeopyles. Based on our taxon sample, the K/T boundary had relatively little effect on the diversity of the group, with several lineages dating to before 65 mya (million years ago). The first stratigraphic occurrences of key taxa, such as Thoracosphaera and Calciodinellum (not themselves used as constraints), are in agreement with the molecular time estimates. Conflicts that involve "Calciodinellum" levantinum, Leonella, Pentapharsodinium, Pernambugia, and the Scrippsiella trochoidea species complex may be due to inaccurate assignment of fossils because of high morphological homoplasy and insufficient knowledge of the extant diversity of calcareous dinoflagellates.