Unexpected phylogenetic relationships within the world's largest limbless skink species (Acontias plumbeus) highlight the need for a review of the taxonomic status of Acontias poecilus.

Acontias plumbeus has traditionally been considered a monotypic, invariable species, a fact that highly contrasts with documented examples of high phylogenetic complexity and phenotypic diversity in other members of the Acontinae. We employed mitochondrial and nuclear DNA markers to investigate genetic structuring among A. plumbeus populations and the relationship between A. plumbeus and the closely related A. poecilus. Molecular genetic analyses revealed three clades with non‐overlapping distributions: an Eastern clade, a widely distributed Northern clade, and a Southern clade that includes topotypical A. poecilus. Morphometric analyses of preserved specimens showed that Southern clade populations are comprised of individuals with absolute and proportionally smaller body sizes than their Northern and Eastern relatives. Phylogenetic affinities within A. plumbeus indicate a complex biogeographic scenario within South Africa and suggest that A. poecilus should be considered a junior synonym of A. plumbeus instead of a truly valid species. Acontias plumbeus consists of allopatric Northern, Eastern, and Southern clades, with the Northern and Southern groups being sister taxa. Populations of the Southern clade, which includes Acontias poecilus, are formed by smaller individuals than those of the Northern and Eastern clades. Both phylogenetic and morphological analyses suggest that A. poecilus should not be considered a valid species, but a junior synonym of A. plumbeus instead. [ABSTRACT FROM AUTHOR]