Phylogenetic Analysis. Bayesian analysis of divergence times among Thalassiosira spp. based on rDNA ITS1 sequence alignment. Time estimates are derived from a relaxed molecular clock calibrated using Sorhannus . Branch numbers represent time of divergence (Mya). Chronogram shows that T. rotula split from T. gravida approximately 3.28 Mya. Within T. rotula, Lineages 1 and 2 split from Lineage 3 approximately 0.68 Mya. Lineages 1 and 2 diverged from one another approximately 0.22 Mya. The tree topology matches that of Sorhannus , with the divergence of T. pseudonana from all other Thalassiosira spp at 30 Mya. Placement of T. weissflogii and T. guillardii differ from Sorhannus , which may be due to differences in ITS1 and 18S mutation rates.