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The molecular phylogeny of brown algae was examined using concatenated  DNA  sequences of seven chloroplast and mitochondrial genes ( atp B,  psa A,  psa B,  psb A,  psb C,  rbc L, and  cox 1). The study was carried out mostly from unialgal cultures; we included  Phaeostrophion irregulare  and  Platysiphon glacialis  because their ordinal taxonomic positions were unclear. Overall, the molecular phylogeny agreed with previously published studies, however,  Platysiphon  clustered with  Halosiphon  and  Stschapovia  and was paraphyletic with the Tilopteridales.  Platysiphon  resembled  Stschapovia  in showing remarkable morphological changes between young and mature thalli.  Platysiphon ,  Halosiphon  and  Stschapovia  also shared parenchymatous, terete, erect thalli with assimilatory filaments in whorls or on the distal end. Based on these results, we proposed a new order Stschapoviales and a new family Platysiphonaceae. We proposed to include  Phaeostrophion  in the Sphacelariales, and we emended the order to include this foliose member. Finally, using basal taxa not included in earlier studies, the origin and divergence times for brown algae were re‐investigated. Results showed that the Phaeophyceae branched from Schizocladiophyceae ~260 Ma during the Permian Period. The early diverging brown algae had isomorphic life histories, whereas the derived taxa with heteromorphic life histories evolved 155–110 Ma when they branched from the basal taxa. Based on these results, we propose that the development of heteromorphic life histories and their success in the temperate and cold‐water regions was induced by the development of the remarkable seasonality caused by the breakup of Pangaea. Most brown algal orders had diverged by roughly 60 Ma, around the last mass extinction event during the Cretaceous Period, and therefore a drastic climate change might have triggered the divergence of brown algae.

journal of phycology

Molecular phylogeny of two unusual brown algae,  Phaeostrophion irregulare  and  Platysiphon glacialis , proposal of the Stschapoviales ord. nov. and Platysiphonaceae fam. nov., and a re‐examination of divergence times for brown algal orders