Historical biogeography of the ancient lycophyte genus Selaginella: early adaptation to xeric habitats on Pangea.

The ancient and cosmopolitan lycophyte genus Selaginella has living representatives around the world, but their historical biogeography has not been assessed with modern methods. We estimated a time-calibrated phylogeny using DNA marker regions rbcL and ITS1-5.8S-ITS2 from 200 species. Node density analyses revealed that Selaginellaceae has significantly older median and mean node ages than other putative "ancient" families. We used statistical model comparison to assess different biogeographical models on our dated tree, and to estimate ancestral ranges. These revealed that Selaginella originated on Euramerica around 383 Ma in the Devonian period, while its peak diversification began with the formation of Pangea. The divergence of the two main species-rich Selaginella lineages occurred approximately 318 Ma on the supercontinent. The major divergences within these main lineages of Selaginella took place in the Late Permian and Early Triassic, along with lineages highly adapted for xeric habitats on Pangea.