BioGeoBEARS analyses indicate that the crown of Coptis and Xanthorhiza most likely occurred in a widespread area comprising North America and Japan (Fig. 3; node 1), although other somewhat less likely ARE are possible (Additional file 2: Figure S1). The estimated age for the split of these two genera highly coincides with the mid-Miocene Climatic Optimum (MMCO; ~ 15–17 Ma; Fig. 3) . During this period, exchange of temperate plants between East Asia and North America could occur via the Bering land bridge . Paleobotanical data indicate that the mixed mesophytic forest of the early and middle Miocene was continuous from Japan through Alaska and into conterminous North America [47, 48].