Brown seaweeds (Phaeophyceae) are ubiquitous in the global coastal environment and often dominate shallow benthic marine habitats . However , we know little about the macroevolutionary and biogeographic processes explaining present - day patterns in diversity and biogeography. The aim of this study is to seek evolutionary explanations for the geographic distributions of the Dictyotales , one of the most diverse brown algal orders,. Taking a phylogenetic approach w e compiled the largest and most comprehensive dat aset of this order to date (>4000 specimens belonging to >300 species) and present the first macroevolutionary study on brown seaweeds. Dictyotales reach their highest diversity in the tropical Indo - Pacific and display a clear latitudinal diversity gradien t on the species level. Ancestral range reconstruction is pointing to a southern hemisphere origin of the Dictyotales corresponding most plausibly to the Southern Tethys (in ‘tropical’ environments). Remarkably, only two clades experienced major burst s of diversification in the Cenozoic as indicated by diversification analyses . Radiation within these two clades, which are displaying contrasting present day latitudinal distribution, seemingly resulted from different evolutionary processes. Dictyota diversifi ed by adapting to cooler temperatures and moving out of the tropics, while Lobophora retained a predominantly tropical niche . In the latter case biotic co mpetition in coral reef environments may have acted as a driver for diversification .