Phylogeny of the diatom order Thalassiosirales based on a Bayesian analysis of four concatenated genes. Pie graphs at internal nodes show the relative maximum likelihood support for the inferred ancestral habitat type, and asterisks show that the more strongly supported ancestral habitat type is significantly better than the alternative. Arrows represent inferred marine-to-freshwater (blue) and freshwater-to-marine (white) colonization events. Dashed lines identify nodes with <0.95 Bayesian posterior probability support, and lineages containing potentially strong candidates for alternative model marine diatoms are marked with "M." Cyclotella tecta and Thalassiosira nordenskioeldii are the nomenclatural types ("type") for their respective genera. Genus abbreviations are: Bacterosira (B), Cyclotella (C), Cyclostephanos (Cs), Detonula (D), Discostella (Di), Lauderia (L), Minidiscus (M), Porosira (P), Shionodiscus (Sh), Skeletonema (S), Stephanodiscus (St), and Thalassiosira (T). The figure is modified from ref. .