Supplementary data and files

Southern Africa has undergone geomorphological evolution, including uplift and reorganisation of drainage during the Neogene. These geomorphological changes have been hypothesised to have influenced diversification in arthropod taxa in the region. However, the roles of geological uplift and drainage reorganisation in the radiation of millipedes in southern Africa remain poorly understood. As such, this study examined the historical biogeography of the millipede genus Bicoxidens to assess whether the diversification patterns are consistent with vicariance driven by the landscape evolution. A multi-locus data set (COI, 16S rRNA, and ITS2) was used to infer a time-calibrated phylogeny. Ancestral ranges were reconstructed, incorporating five geomorphologically defined regions based on the historical cratonic plateaus, escarpments and major drainage basins in Zimbabwe. The divergences and ancestral-range estimations suggest that Bicoxidens likely originated in the late Oligocene (~26 Mya) across a widespread ancestral range spanning the Southern African Interior Plateau, the Eastern Highlands, and the Proto-Zambezi Basin. Furthermore, biogeographic inference supported the hypothesis that vicariance-dominated diversification, shaped by major Neogene landscape transformations, including mid-Miocene drainage reorganisations, was the primary driver of speciation in the genus Bicoxidens.