Phylogenetics and Diversification of Acontinae
The evolution of the legless skinks of the subfamily Acontinae is an interesting area of study for herpetologists and evolutionary biologists. These skinks are a group of burrowing reptiles that have lost the use of their limbs over time, leading to a more serpentine body shape.
The evolution of leglessness in skinks is believed to have been driven by several factors, including an increase in burrowing behavior and a decrease in predation pressure in underground environments. Over time, skinks with reduced or absent limbs were better adapted to burrowing and were able to occupy new niches and habitats, further leading to the diversification of this group.
The loss of limbs in the Acontinae skinks is an example of convergent evolution, where similar adaptations have evolved independently in different groups of reptiles. This phenomenon highlights the role of natural selection and adaptation in shaping the diversity of life on Earth.
The subfamily Acontinae has a long history if taxonomic and systematic issues. A recent paper that I was fortunate to co-author, published in Molecular Phylogenetics and Evolution, aimed to resolve some of these issues through several means. For those of you that know me, you should be well aware that I have a somewhat unhealthy obsession with fossorial skinks. Having spent many years looking for these reptiles, and generally, when in a new area, focusing much of my herping efforts into finding them.
Zhao Z, Conradie W, Pietersen DW, Jordaan A, Nicolau G, Edwards S, Riekert S, Heideman N. 2023. Diversification of the African legless skinks in the subfamily Acontinae (Family Scincidae). Mol. Phylogenet. Evol. 26;182:107747. https://doi.org/10.1016/j.ympev.2023.107747
Below are some of the species that were investigated and a brief breakdown of the results.
Acontias rieppeli - Woodbush Legless Skink from Haenertsburg, Limpopo, South Africa. This population was found to be closely related to Acontias cregoi - Cregoi’s Legless Skink. However, there are clear morphological differences and geographical isolation between the two species.
Acontias aff. cregoi - Cregoi’s Legless Skink a candidate species from an the eastern Soutpansberg, Limpopo, South Africa.
Acontias cregoi - Cregoi’s Legless Skink from an isolated granite outcrop near Polokwane, Limpopo, South Africa.
Acontias cregoi - Cregoi’s Legless Skink from montane regions of the western Soutpansberg, Limpopo, South Africa.
Currently known as Acontias orientalis - Eastern Cape Legless Skink from Kommadagga, Eastern Cape, South Africa. This species will be synonymised with Acontias meleagris - Cape Legless Skink.
Currently known as Acontias orientalis - Eastern Cape Legless Skink from Makhanda, Eastern Cape, South Africa. This species will be synonymised with Acontias meleagris - Cape Legless Skink.
An undescribed Acontias sp. from the West Coast, Western Cape, South Africa
Acontias lineatus - Striped Legless Skink
Acontias lineatus - Striped Legless Skink
The dwarf legless skinks from the West Coast, namely Acotnias grayi, Acontias tristis, Acontias litoralis, and Acontias lineatus showed very little divergence at a species level. The three species should be synonymised with Acontias lineatus.
Localities of all species and samples used in the study.
“Cladogenic diversification is often explained by referring to climatic oscillations and geomorphic shifts that cause allopatric speciation. In this regard, southern Africa retains a high level of landscape heterogeneity in vegetation, geology, and rainfall patterns. The legless skink subfamily Acontinae occurs broadly across the southern African subcontinent and therefore provides an ideal model group for investigating biogeographic patterns associated with the region. A robust phylogenetic study of the Acontinae with comprehensive coverage and adequate sampling of each taxon has been lacking up until now, resulting in unresolved questions regarding the subfamily's biogeography and evolution. In this study, we used multi-locus genetic markers (three mitochondrial and two nuclear) with comprehensive taxon coverage (all currently recognized Acontinae species) and adequate sampling (multiple specimens for most taxa) of each taxon to infer a phylogeny for the subfamily. The phylogeny retrieved four well-supported clades in Acontias and supported the monophyly of Typhlosaurus. Following the General Lineage Concept (GLC), many long-standing phylogenetic enigmas within Acontias occidentalis and the A. kgalagadi, A. lineatus and A. meleagris species complexes, and within Typhlosaurus were resolved. Our species delimitation analyses suggest the existence of hidden taxa in the A. occidentalis, A. cregoi and A. meleagris species groups, but also suggest that some currently recognized species in the A. lineatus and A. meleagris species groups, and within Typhlosaurus, should be synonymised. We also possibly encountered "ghost introgression" in A. occidentalis. Our inferred species tree revealed a signal of gene flow, which implies possible cross-over in some groups. Fossil evidence calibration dating results showed that the divergence between Typhlosaurus and Acontias was likely influenced by cooling and increasing aridity along the southwest coast in the mid-Oligocene caused by the opening of the Drake Passage. Further cladogenesis observed in Typhlosaurus and Acontias was likely influenced by Miocene cooling, expansion of open habitat, uplifting of the eastern Great Escarpment (GE), and variation in rainfall patterns, together with the effect of the warm Agulhas Current since the early Miocene, the development of the cold Benguela Current since the late Miocene, and their co-effects. The biogeographic pattern of the Acontinae bears close resemblance to that of other herpetofauna (e.g., rain frogs and African vipers) in southern Africa.”
Typhlosaurus caecus - Southern Blind Legless Skink
Typhlosaurus vermis - Pink Blind Legless Skink
Acontias richardi - Richard’s Legless Skink, a poorly known skink from the eastern Soutpansberg, Limpopo, South Africa.
Acontias subtaeniatus - Stripe-bellied Legless Skink, a poorly known skink from the western Soutpansberg, Limpopo, South Africa.
Acontias occidentalis - Savanna Legless Skink, a common species widespread consisting of multiple clades throughout Southern Africa.
Acontias breviceps - Short-headed Legless Skink, a common species widespread throughout the Eastern Cape province, South Africa.
Acontias gracilicauda - Thin-tailed Legless Skink is a widely distributed species that is closely related to Acontias albigularis - White Throated Legless Skink and Acontias wakkerstroomensis - Wakkerstroom Legless Skink.
Acontias albigularis - White Throated Legless Skink, a data deficient species known only from a few localities in Mpumalanga, South Africa.
Key Findings
Acontias kglalagadi subtaeniatus elevated to species status (A. subtaeniatus)
Acontias richardi and Acontias subtainiatus are sister species
Synonymise Acontias orientalis with main Acontias meleagris clade
Synonymise small west coast species Acotnias grayi, Acontias tristis, Acontias litoralis into Acontias lineata
Synonymise Typhlosaurus lomeia with Typhlosaurus vermis
Acontias occidentalis contains several cryptic species
Poor genetic diversity among morphologically distinct species Acontias riepelli and Acontias cregoi
Potentially cryptic species related to Acontias cregoi
Cryptic species from the west coast of South Africa
Zhao Z, Conradie W, Pietersen DW, Jordaan A, Nicolau G, Edwards S, Riekert S, Heideman N. 2023. Diversification of the African legless skinks in the subfamily Acontinae (Family Scincidae). Mol. Phylogenet. Evol. 26;182:107747. https://doi.org/10.1016/j.ympev.2023.107747