Phylogenetic analyses reveal an increase in the speciation rate of raphid pennate diatoms in the Cretaceous

The raphid pennates (order Bacillariales) are a diverse group of diatoms easily recognized by having a slit in the siliceous cell wall, called the raphe, with functions in cell motility. It has been hypothesized that this morphological innovation contributed to the evolutionary success of this relatively young but species-rich group of diatoms. However, owing to the incompleteness of the fossil record this hypothesis remains untested. Using the 18S ribosomal RNA gene, fossil calibrations, and Bayesian phylogenetic and diversification frameworks, we detect a shift in the speciation rate of marine raphid pennate diatoms in the Cretaceous, not detected in other diatom lineages nor previously recognized in the microfossil record. Our results suggest a positive link between the speciation of raphid pennate diatoms and the benefits derived from evolving motility skills, which could account for their outstanding present-day global diversity. The coincidence between the advent of the raphe and the increase in the speciation rate of raphid pennates supports the idea that simple morphological novelties can have important consequences on the evolutionary history of eukaryotic microorganisms.