Divergence dating
From the divergence dated phylogenies, it appears that the genusStasimopus radiated in the Karoo region approximately 64.30 MYA (CI: 48.30 - 84.15), towards the beginning of the Paleocene. The beginning of the Paleocene is marked by the Cretaceous - Paleogene extinction event approximately 66MYA . The new niche gaps, along with high temperatures contributed to many radiation events, which would have aided the origin of the Stasimopus genus .
Diversification of the genus ramps up towards the end of the Eocene and beginning of the Oligocene (seen by the rise of clades A, B, C, G and F). This Eocene - Oligocene boundary has been noted as ‘the great divide’ due to large shifts in climate (increasing temperatures in Southern Africa), leading to changes in fauna globally . This trend of mygalomorph diversification and geological desertification was mirrored in Australia, which saw the radiation of the Idiopidae and Halonoproctidae families at this time .
The genus continued to radiate in the Miocene, with the rise of the rest of the clades, except K. The late Miocene coincides with an increase in temperature and onset of aridification once again . This is also mirrored in South Western Australia which experienced similar environmental conditions and the speciation of many plants and animals . The late Miocene and early Pliocene are noted as vitally important in the establishment of arid zone diversity in Australia in mygalomorph taxa such as Idiopidae (Rix et al., 2017). This same time period appears to be important for the diversification of the arid adaptedStasimopus species.
Stasimopus leipoldti (Clade K) may have arisen in the Pleistocene, this is however a tentative conclusion due to the small sample size of the species in this study. The Pleistocene saw Southern Africa becoming more arid. It is also marked by alternating glacial and interglacial cycles . These cycles are known to have had impacts on various species ranges . There is however, evidence to suggest that Fynbos and Succulent Karoo biomes were decoupled from these effects and experienced climate stability . This may have led to the high levels of diversity and endemism present in these biomes today . Stasimopus leipoldti is originally described from Clanwilliam (Western Cape), over 150 km away, falling into the Succulent Karoo Biome, the samples collected for this study were from the Tankwa Karoo, which is a small, isolated enclave of this biome nestled in the harsher Nama Karoo Biome . This Tankwa Karoo area may thus be a Pleistocene glacial cycle refugia for the species. A larger sample size as well as genetic sampling of the type locality is needed to show this more definitively. The same pattern of a Pleistocene glacial refugia was reported for the Aname genus in Pilbara, Western Australia, which experienced a similar geological history to the Karoo region .
A similar relationship is seen between the type of S. erythrognathus (Worster, Eastern Cape) and the specimens occurring over 700 km away. This species could not be accurately dated in isolation due to forming part of a genetic species complex. The distance between the type locality and the specimens assessed here may indicate another species complex, but sampling the type locality would be required to test this.
Aridification has played a large role in shaping the evolutionary history of Stasimopus in the Karoo. The region is however, experiencing rapid desertification due to climate change and habitat degradation . This change may thus continue to drive the diversification of the genus, or the conditions could become too harsh over too short a time span leading to the extinction of these arid adapted species.