Multilocus phylogeny and historical biogeography of the pill millipede genus Rhopalomeris (Diplopoda, Glomerida) in Thailand

Abstract Southeast Asia is widely recognized as a global biodiversity hotspot with a complex geological history and high species richness across multiple taxa. The pill millipede genus Rhopalomeris Verhoeff, 1906 has received increasing taxonomic attention in recent years, with several new species described, particularly from western Southeast Asia, including Thailand and Myanmar. However, its historical biogeography has remained insufficiently understood. In this study, we used an integrative analysis combining morphology, comprehensive molecular phylogeny based on mitochondrial and nuclear genes, and biogeographic approaches to provide new insights into the evolutionary history of Rhopalomeris in Thailand. The multilocus phylogeny indicated the presence of 21 species-level subclades, with nine previously described species and 12 putative species. Divergence time estimation and historical biogeography analysis revealed that Rhopalomeris likely originated in the Tenasserim Range and the inner area of Indochina around the Late Eocene to Early Oligocene (41.39–28.89 Mya, mean age = 34.78 Mya), followed by southward expansion into the Thai-Malay Peninsula and a series of subsequent diversifications shaped by recurrent sea-level fluctuations and multidirectional dispersal. Our findings highlight the dynamic interplay between Earth’s history and biogeographic processes driving the diversity of these Southeast Asian pill millipedes.