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.