Biogeographical and diversification analyses of Indian pseudoscorpions reveal the Western Ghats as museums of ancient biodiversity

Abstract

The tropical forests of the Western Ghats (WG) of India are considered ‘refugia’ harbouring highly diverse and endemic taxa but these refugia are under immense anthropogenic pressure. Most phylogenetic studies have explained diversity patterns across the WG using vertebrates, however, the processes impacting the highly endemic invertebrate fauna are still poorly understood. Here we investigate the evolutionary history of an ancient and widespread arachnid lineage (Pseudoscorpiones: Chthoniidae: Tyrannochthoniini) in the WG through a variety of biogeographical and diversification analyses, including sequence data from three markers for 45 terminals from the WG and 22 from Africa, East and Southeast Asia, Australasia and the Neotropics. Our results show that WG Tyrannochthoniini are rendered paraphyletic by African taxa, a result consistent with continental drift. WG Tyrannochthoniini are further split into two major clades, a southern WG clade and an African-central WG clade, due to vicariance along a major biogeographical barrier, the Palghat Gap. Central WG pseudoscorpions diverged from their African relatives at the Jurassic-Cretaceous boundary, coinciding with the separation of India from Africa. Despite ancient vicariance, six dispersal events occurred across the Palghat Gap, beginning in the Early Cretaceous as India was rafting towards Southeast Asia and ending by the early Miocene, implying that the Palghat Gap became impermeable to dispersal for these humid-adapted organisms during Miocene aridification. Although the ancestor of WG Tyrannochthoniini originated in lowland WG forests, three dispersal events into high montane Shola forests occurred most likely as a result of Late Cretaceous orogenesis and Neogene uplift, allowing lineages to occupy niches at higher elevations. An exponentially declining diversification rate, typical of older lineages, supports the ‘museum’ model of diversification for WG Tyrannochthoniini. Our study sheds light on the historical biogeography of relictual soil-arthropods in the WG and emphasises the role of WG forest refugia in preserving ancient invertebrate biodiversity.